Technical Article Series

Computer Control Cranking Circuits
+ Reading Schematic Diagrams . . .
+ Essential Electrical-Electronics Circuit Analysis
 & Troubleshooting Circuits

Introduction . . . . Getting Started
A new electrical troubleshooting training series begins with this introduction. I don't know how many articles will be necessary to cover the subject. We will know when we get to the end. I am not even sure what the title should be or will be. The title may change as we plow through the series. We start at the beginning of electrical circuits then plow through the growth of automotive electronics technology with this series of articles using the computer controlled cranking circuit as our automotive electronic circuit to study recent automotive electronic developments.

The last series of articles covered the essentials of ground electron current and ground voltage because a lot of mystery in the minds of technicians surrounds ground circuits. We addressed a few misconceptions about the ground circuit and explained how ground electron current flowed through the ground circuit. We also explained how ground circuits could be tested to prove if the ground circuit was good or bad. Hopefully, by now you understand the ground circuit, what is going on in the ground circuit and how to test ground circuits. Hopefully, the mystery has been resolved.

In this new series we intend to cover some essential electrical troubleshooting topics along the way in the context of this complex cranking circuit that used to be so simple in the old days. Now the cranking circuit is complex with the gradual introduction of electronic control. It seems like electronic technology in the automobile has gone berserk. The automotive cranking circuit is no exception and has gone through a major technological evolution. In the early days of the development of automotive electrical circuits, cranking circuits were simple and very effective at cranking the engine. Then automotive technology got out of control eventually resulting in computers controlling most vehicle functions including cranking the engine. What used to be a simple electrical cranking circuit has been complicated with electronics. As a result, our job has become more difficult and the cost to the customer for repairs has gone sky high.

Figure 1-1 below is a composite and generic schematic diagram of a modern cranking circuit based on computer control technology.

Fig. 1-1 Computer Controlled Cranking Circuit

I call this a composite schematic diagram because in this one page schematic diagram all components responsible to operate the Starter Motor are shown. By having all components on the same schematic diagram provides some clarity on what components are included, how they are interconnected and what components may affect the operation of the circuit. Besides the Starter Motor and Starter Solenoid we discover a mechanical Starter Relay. The Relay is controlled by two independent on-board computers which will be discussed later on in this series of articles.

Take some time reviewing this generic schematic diagram in Figure 1-1. We have purposely left out the color code of wires, wire diameter markers and numbered connectors completing a wire between two points on the schematic. This was done to keep the diagram from being too cluttered so we can focus on the components in the circuit and discuss how each contributes to the operation of the DC Starter Motor.

These schematic items are present in shop manual schematic diagrams and of course are essential when tracing a circuit on an actual vehicle but not necessary to explain how this circuit works and principles of circuit troubleshooting.

In a shop manual this circuit may require several pages to depict the circuit shown in Figure 1-1.  For example, the battery and DC Starter Motor may appear on one or two different pages in the shop manual. This makes understanding the connections between these two important components a little ambiguous rather than show the clarity that exists between them.

The PCM will likely appear on another page in the shop manual and may or may not show the fuel pump Relay on the same page. The IPM (Integrated Power Module as we call it) will be on another page of the shop manual and so on. To follow the flow of circuit operation using the schematic diagram may require flipping through several pages in the shop manual. These pages are often separated by pages highlighting other circuits on the vehicle.

In my experience I have found the shop manual method of laying out vehicle circuits through several pages interrupts my thinking and hinders my attempt to understand and analyze the circuit. Components spread out over multiple pages makes it difficult to see the circuit in its entirety.  A composite schematic diagram helps to develop a coherent troubleshooting procedure. When a circuit on a vehicle has a problem, and you are not familiar with this circuit on this vehicle, a composite schematic diagram helps the circuit stands out from all the other diagrams. A composite schematic diagram like Figure 1 also makes it easier to explain the circuit as it evolved into the complex electronic circuit it is today.

Reading Schematic Diagrams
Over the years many technicians have told me nobody ever taught them how to read a schematic diagram. They picked up what they know from trial and error. I can’t tell you how many times technicians have called us and asked if we had training that would teach them how to read electrical schematic diagrams. My answer is always “Yes” and “No.” Yes! We can teach you how to read a schematic diagram. No!, because reading the schematic diagram with understanding requires that you understand the parameters of voltage and electron current in a circuit, the effect resistance has at various points in the circuit and how series and parallel circuits work. Without this essential electrical knowledge, you cannot comprehend what you are reading in a schematic diagram nor understand what to look for.  And we can teach you how to do that.

It seems many technicians have drawn the conclusion that if they could “read a schematic diagram” like their shop buddy Joe does, they would be able to troubleshoot and repair an electrical circuit like Joe does. They don’t realize that Joe probably understands a little about voltage and current in a circuit, the effect resistance has at various points in the circuit and how series and parallel circuits work which Joe applies to the circuit as Joe reads the schematic diagram to conclude what to do next.

It is very likely that Joe also has some understanding of using a DMM and current clamp to test and measure voltage and electron current in the circuit and from these readings determine what’s wrong in the circuit. We call this electrical troubleshooting training. This happens to be the focus of all of our training programs offered by Veejer Enterprises Inc.


How To Read a Schematic Diagram
I realize many of you reading these articles will say I already know how to read a schematic diagram but many do not. Maybe we can help all do a more professional job reading a schematic diagram. So, let’s begin at the beginning and see if we can add something more to your ability to read a schematic diagram.

Step 1: Identify all major components in the schematic diagram. Get the big picture of what is in the circuit. This is where a composite schematic diagram can be a great asset. It clearly shows all components comprising the circuit to operate the DC Starter Motor. If using a shop manual, I would suggest you draw your own simple composite block diagram of the circuit as you plow through the pages of the shop manual. It doesn't’t have to have a lot of detail like Figure 1 but at least it will give you the big picture.

Step 2:
Identify the main components that the circuit uses to operate or control the DC Starter Motor. In this case it is the DC Starter Motor control by the Starter Solenoid. We determine the primary device the circuit controls is the DC Starter Motor, load in the circuit.

Step 3:
Trace the wiring connected to the load terminals. There is a wire connected to the voltage side of the load and a wire connected to the ground side of the load. It doesn't’t matter if you decide to trace the voltage side first or the ground side first. The important point is that you trace wiring on both sides of the load. If you are tracing the voltage side wire continue until you reach the B+ terminal (battery positive post +BATT or generator B+ terminal). We decide to trace the voltage side of the circuit first. We begin at the + cable terminal on the Starter Motor, the voltage side of the Starter Motor. Then trace back to the positive terminal of the battery through the Starter Solenoid heavy-duty contacts.

Next, trace the ground side of the Starter Motor which is bolted to the engine block which is connected to the battery negative terminal (B-) by the engine ground cable. Yes, this appears simple because this is a simple electrical circuit. But you have already identified components, cables and connections that will have to be checked (tested) if there is a cranking problem. You have also learned there are no surprises in this part of the circuit which appears to be a common everyday cranking circuit.

If the circuit were very complex these two steps would provide you with insight into the circuit. Even if the circuit is more complex it is easy to begin to understand the schematic diagram following these two simple procedures of tracing B+ and B- from the load in the circuit.  Now you have a picture of how the Starter Motor is wired into the circuit and have learned the DC Starter Motor is controlled by Solenoid contacts on the voltage side of the Starter Motor, not the ground side. From our previous articles on electron ground current you should find it easy to trace the electron flow from B- through the Starter Motor, through the Solenoid contacts which are connected to B+.

Step 4:
Trace the circuit for the Starter Solenoid to see how it is engaged to operate the Starter Motor. It doesn't’t matter if you trace the voltage side or the ground side first. Let’s trace the ground side wire which connects to the Starter Motor case. Of course, on some vehicles the Starter Solenoid is physically mounted on the Starter Motor which is the ground circuit for the Starter Solenoid.

(In Figure 1-1, the Starter Solenoid is mounted independent of the Starter Motor housing. This requires a ground wire which we show grounded to the Starter Motor housing. On some vehicles the mounting bolt of the Starter Solenoid to the sheet metal or case of the DC Cranking Motor serves as the Starter Solenoid ground circuit and no Starter Solenoid ground wire is needed. But it will still need to be tested with a voltage drop test as we will see later.)

The B+ supply to the Starter Solenoid is traced back to Pin 87 on the Starter Relay.

Step 5:
At this point, you encounter a previously unknown fact from tracing the schematic diagram. This vehicle has a dedicated Starter Relay. You probably would never have known this without tracing the schematic diagram from the Starter Solenoid to the Starter Relay. Pin 87 on the Starter Relay gets B+ when Pin 30 (connected to B+) connects to Pin 87 through the closed Relay contacts. The Relay contacts close (Relay “clicks”) when Starter Relay Pin 86 has B+ from the IPM Pin 22 and Starter Relay Pin 85 has B- provided by the PCM Pin 8. The topic of Relay operation by these two onboard computers will be explored in more detail in future articles.

You also discovered that there are two onboard computers that each must contribute something to operate the Starter Relay, to operate the Starter Solenoid to power up the DC Starter Motor. The IPM provides (B+) voltage to pin 86 of the Relay. The PCM provides a ground (B-) to pin 85. This is where diagnosing the circuit gets a bit more complicated. Many questions arise. What do I do if the IPM doesn't’t provide B+ to pin 86? What do I do if the PCM does not provide B- to pin 85? Before we answer these questions, we have to spend a few articles covering testing and troubleshooting the cranking circuit as it developed without the IPM and the PCM which are rather recent developments in operating the cranking circuit. They have made life a lot more difficult for us and repairs a lot more expensive for your customer.

What we have accomplished at this point is trace wires to discover all the components in the computer controlled cranking circuit and see how components are connected together. A trained electrical troubleshooting technician also begins to understand what electrical tests he may have to perform on various components while reading the schematic diagram. Understanding voltage and electron current in a circuit, how circuits work, how circuits fail, what voltage readings should appear at various points in the circuit if it is a good circuit, he can quickly eliminate different component problems with a few simple voltage or electron current tests using a current clamp.

Understanding voltage and electron current in circuits, how to test voltage and electron current in a circuit based on how the circuit is wired together and what to do next if the DMM and current clamp readings are good, too high or too low, goes along at the same time reading a schematic diagram. Technicians who can do this love schematic diagrams and do not want to get along without them. So you can see, if you want to understand how to read schematic diagrams and apply that to this computer controlled cranking circuit you have to understand electrical circuits and how to test them. You need to understand good voltage and electron current readings, what to do next if the reading is good, too high or too low. I call this electrical troubleshooting training and will be the focus of this series of articles.

For training information click on one of these 3 links!

Computer Control Cranking Circuits
Part 1 of (8) Parts

Getting into The Nitty-Gritty
As the title of this new series indicates, we are involving several electrical skills at the same time because they all work together to properly complete electrical repairs. What good does it do to trace a schematic diagram to connect all the components together but not understand how the components work and how to test and troubleshoot each component in the circuit? Our focus is not only how to read a schematic diagram, but cover some concepts of electrical and electronic circuit analysis. We will also discuss principles and highlights of troubleshooting automotive electrical-electronic circuits.

What Is an Electrical Circuit?
What Is an Electronic Circuit?
An electrical circuit contains electrical components but does NOT contain electronic components such as transistors and integrated circuits. Whereas an electronic circuit DOES contain transistors and integrated circuits (ICs) which are thousands of microscopic electronic circuits encapsulated into one small integrated package with numerous pins extending out to solder into a circuit board.

The complex circuit in Figure 1-1 above, appears in many vehicles today. It is a full-blown electrical-electronic circuit because the electrical components are the battery, starter motor, starter solenoid and starter relay. The electronic components consist of the IPM and the PCM which both contain transistors and integrated circuits.

There can be different configurations of the circuit between vehicle make and model. For example, the starter relay could be mounted inside the IPM depending on make and model of the vehicle. We chose to place the starter relay externally in a fuse panel box so it can be easily removed and replaced. Later we will discuss some tips on relay circuit troubleshooting.

The function of operating the starter motor to crank the engine is embedded in this complex electrical-electronic circuit with software programming.

The battery, starter solenoid and starter motor comprise the same reliable electrical cranking circuit from many years ago. What the manufacturers have decided to do is to control the cranking function with electronics. Why? If you were to ask 5 automotive engineers this question you would likely get five different answers. Rather than worry about the reasons why they did this, since they exist, we have to focus our training on how to troubleshoot and repair these electrical-electronic circuits.

What Did You Think as You Traced the Schematic Diagram?
From the first article, “Introduction to C.C.C.C.,” as you began to trace the circuit in Step 1 and identified the major components, the starter motor, the circuit load, had to stand out as the reason for this circuit. In your mind did you have thoughts about how you would test the starter motor? Did it occur to you how you would test the starter solenoid and the battery as you noticed their presence in the circuit? If you know how to test a battery and starter motor you invariably would recall these test procedures in the back of your mind just noticing the presence of these components in the circuit. The recognition of testing procedures in the back of your mind while scanning a schematic diagram is the mark of a knowledgeable electrical troubleshooter.

On the other hand, if while tracing a schematic diagram and testing procedures did not begin to come to mind as you noticed their presence in the schematic, then that is an indication you could use some training in how to test and troubleshoot these components. You can use this mental recognition concept while tracing a schematic to discover where you need training on a particular circuit or component as you trace a schematic diagram.

Electrical Troubleshooting “Back in The Day”
Figure 1-2 below illustrates a pure “electrical” cranking circuit from years gone by. There are no transistors and integrated circuits. This cranking circuit could be from a car built in the 1930s or earlier. You may notice the battery, starter motor and starter solenoid circuit is the identical circuit from Figure 1-1 but with two mechanical switches to control the starter solenoid.

Fig. 1-2 Mechanical cranking circuit, switch operated

Many old timers will look at this electrical cranking circuit and say: “I could troubleshoot this cranking circuit with a test light and I don’t need no stinking DMM.” He would be correct up to a point. It would be possible to check the fuse link, whether or not the two switches are working correctly and test if voltage is present at the starter solenoid with a test light when both switches are CLOSED.

Looking back to our original circuit of an electrical-electronic computer controlled cranking circuit in Figure 1-1, you would be in deep trouble if all you had was a test light to troubleshoot the electronic portion of the computer controlled circuit.

Need To Know Stuff A Test Light Cannot Provide
A test light cannot test the state of charge of a battery at rest and under load. A test light cannot measure the decrease in battery voltage while cranking to evaluate battery performance under load. Also, and very important, a test light is not very effective in measuring voltage drops across cables and connections like a DMM can do! A test light cannot measure the electron current flowing through the starter motor while cranking the engine to evaluate the condition of the starter motor. You need a high amp current clamp for that. All these bits of circuit information are crucial to properly troubleshoot the electrical cranking circuit.

I believe most technicians would agree a test light doers’t allow professional level electrical troubleshooting that is necessary with today’s advanced battery designs in hi-tech vehicles with sophisticated electronic circuits. A DMM and a current clamp are essential troubleshooting tools to address the electrical-electronic problems we have with computer control circuits.

Since our cranking circuit has now begun to incorporate computer control, a DMM and current clamp become more essential tools than ever. Imagine if an old-timer had a DMM and a current clamp back in the day to test this simple electrical cranking circuit. How much more effective electrical troubleshooting could have been accomplished and how battery reliability would have been learned in this electrical cranking circuit.

The Art of Testing an Electrical or Electronic Circuit
Any time you are testing or troubleshooting an electrical circuit, an electronic circuit or an electrical-electronic circuit, the two parameters that give you the most information about a circuit’s condition and performance are, voltage readings and electron current readings. Since these circuit parameters are greatly affected by electrical problems as they develop and affect a circuit’s operation, we need as much voltage and electron current information as we can get for accurate diagnosis. How do you acquire these parameters and what do you do with voltage and electron current readings once you have them? Let’s see!

There are (at least) 5 things to do and/or know when troubleshooting a circuit.
1) Where in the circuit is it important to measure voltage and electron current in the circuit?
2) When is it important to measure voltage and electron current during a circuit’s operation?
3) What are the correct readings in a good circuit when the circuit is (OFF)?
4) What are the correct readings in a good circuit when the circuit is operating (ON)?
5) What do you do next if any voltage or electron current reading is too high or too low?

The where, when and what electrical issues are what effective electrical-electronics troubleshooting training is all about. Helpful electrical training should include explanations of good readings and what to do when good readings become bad readings that are too high or too low and affect circuit operation. The best way to illustrate some of these concepts is to demonstrate using Figure 1-3, our electrical cranking circuit without the electronics but DMM’s attached.

Fig. 1-3 (vehicle is parked - ignition key OFF)

This is the simple electrical cranking circuit from yesteryear. The starter solenoid is controlled by two mechanical switches which are added to our schematic diagram; P/N (Park/Neutral) AND START (ignition key in CRANK).

Both switches must be CLOSED at the same time to engage the starter solenoid and provide battery voltage to the starter motor through the heavy-duty solenoid contacts. But before we turn the circuit ON and crank the engine we need to understand the purpose of the two DMMs and their placement in the circuit as shown.

A DMM and DMM with current clamp significantly increases our understanding of the circuit’s voltage and electron current conditions at rest and when in operation cranking the engine than can be accomplished with a test light. These DMM readings will verify the circuit is functioning correctly and that there are no hidden problems that could surface in the near future causing another failure to crank. These electrical testing techniques in the electrical cranking circuit follows the five steps mentioned above. They are universal electrical testing techniques and are the same whether the circuit is a purely electrical circuit or contains electronics.

DMM #1 Measures Battery Terminal Post Voltage.
The vehicle is parked so the battery voltage being measured at this time indicates the battery’s state of charge as long as the vehicle has not been driven in the last 1 to 2 hours. This time frame with the engine off allows the battery voltage to calm down. The battery terminal post voltage becomes stable and an important indicator of battery condition that cannot be done with a test light.

If the battery is fully charged, the DMM will read in the range of 12.6x to 12.8x volts. (The “x” means the digit could be any number from”0” to”9” and not significantly affect the validity of the reading.)

If the battery is not fully charged, the terminal post voltage will be lower than 12.6x indicating a lower state of charge due to three of the most common negative factors.
(1) The vehicle may not have been driven very much recently resulting in the battery not being charged long enough by the generator to fully re-charge.
(2) The generator is not producing proper charging voltage.
(3) There could be a key off drain problem which would require additional tests beyond the scope of this article.

If you need more of an in-depth analysis of the electrical issues just discussed, I recommend obtaining a copy of our book “Vehicle Electrical Troubleshooting SHORTCUTS” (250 pages in seven sections). Battery testing is covered in Section 4, testing cranking circuits in Section 5 and testing charging systems in Section 6.

Each section contains a comprehensive analysis of electrical circuit operation with illustrations and testing procedures that you can perform with just a DMM and a current clamp.

CLICK HERE to buy a copy of "SHORTCUTS"

DMM #2 Is Connected to A Current Clamp
The current clamp senses electron current passing through the battery cable inside its jaws and sends a small DC voltage to the DMM directly proportional to the electron current flowing. The DMM indicates the voltage produced by the current clamp that allows us to interpret how many amps are flowing through the cable. This current clamp is a high amp current clamp and very accurate. It reads from 1.0 amp up to 600 amps. It cannot read below 1.0 amp so it is not effective for measuring key off drain. We discussed current clamps extensively in the previous series of articles relating to ground electron current.

At this time the vehicle is parked and the starter motor is not engaged so there is no electron current flow from the battery through the starter motor. DMM #2 therefore reads .000 for zero amps at rest. Before the cranking action is attempted we have determined the state of charge (condition) of the battery with DMM #1. I expect to see the battery voltage no lower than 12.4x volts for normal cranking. If the battery voltage is less than 12.4x volts it may still crank the engine provided the engine doesn't’t have to crank too long to start. Once the engine begins running, a properly operating generator begins recharging the battery. The charging voltage at the battery terminals should increase to the normal charging voltage of the vehicle’s charging system. If the vehicle is allowed to run long enough, the generator would recharge a low battery.

From our previous series of articles discussing ground electron current you should have a good grasp on how electron current flows through the circuit. In our next article we will turn the circuit ON and crank the engine to explain how the DMM and current clamp readings change. Will also discuss what to do if the readings are too high or too low.

For training information click on one of these 3 links!

Computer Control Cranking Circuits
Part 2 of (8) Parts

What to Test, How and When?
The two most important parameters in any electrical and electronic circuit are voltage values, including voltage drop tests and electron current readings performed at the right time and at the right place in the circuit. This vital information is the heart and soul of troubleshooting ANY circuit. Some DMM readings are important when the circuit is at rest as well at various stages during circuit operation. Effective electrical-electronic circuit troubleshooting training must specify what to check, when to check it and what the proper reading should be. Then, what to do if a reading is too high or too low.

In Part 1 we discussed and illustrated a cranking circuit controlled by computer. Now we begin to study troubleshooting this circuit by first adding mechanical switches as we would find in a typical earlier version cranking circuit and focus our discussion on troubleshooting the battery and the starter motor first. These troubleshooting procedures will be used throughout the entire circuit as we proceed into the electronics.

DMM #1 measures battery voltage and DMM #2/current clamp measures starter motor draw. Below in Figure 2-1, the circuit is placed in operation by closing two switches, P/N and START. The starter solenoid is energized and heavy-duty contacts close to connect the battery to the starter motor. The battery supplies B+ voltage which drives starter motor electron current through the circuit.

This gives us our first voltage and electron current measurements to define circuit performance. Battery voltage is measured with the circuit at rest to determine battery state of charge. Then the circuit is turned ON and battery voltage continues to be measured to see how the battery performs under load. While the starter motor is engaged a current clamp measures electron current flowing to the starter motor as some would call “starter draw.”

Fig. 2-1 Measuring Cranking Voltage and Cranking Amps

This circuit seems simple and easily diagnosed with a test light. However as was pointed out in Part 1, there are critical circuit component performance issues during circuit operation that cannot be evaluated with a test light. For example, how can battery voltage state of charge be measured with a test light? How much does the battery voltage drop under load while cranking? What do these voltage readings tell us? How do you measure starter motor draw with a test light? How much electron current does the starter motor draw? How does that reading help us determine starter motor efficiency? These issues can only be determined by voltage and electron current measurements during circuit operation and in the appropriate location in the circuit. Comparing battery cranking voltage and starter motor cranking amps will quickly confirm the circuit is functioning properly or identify problems if they exist.

If we can understand these electrical tests for voltage and electron current we will have a better understanding of what these readings tell us about the electronic circuits controlling the starter solenoid if they are applied.

DMM #1 in Figure 2-2, below, is measuring battery voltage under load and this voltage measurement is called the “Cranking Voltage Test.” This is a critical voltage value that helps to evaluate the entire circuit. Understand that the battery voltage while cranking the engine decreases and at the same time must maintain sufficient voltage to continue to operate the electronic circuit at the same time it is supplying electron current to the starter motor.

Fig. 2-2 DMM #1 Measures Cranking Voltage

If the battery (cranking) voltage drops too low it could affect the PCM’s ability to pulse the fuel injectors resulting in a no RUN condition. Of course, if the battery cranking voltage dropped that low, you would expect to hear a suspicious dragging starter motor. And seeing the battery voltage drop below 9.2 V under load would reveal why the fuel injectors aren’t delivering fuel. Most PCM’s will shut off the fuel injectors when battery voltage drops below 9.2 V or whatever voltage threshold a particular manufacturer specifies.

You may not easily find a specification from some vehicle manufacturer for how low battery voltage can drop when cranking the engine. There are too many variables to consider, especially ambient temperature. Let’s say we have a good battery and the cranking voltage drops to 10.8 V when the ambient temperature is 90° F (32.2° C). Now let’s say that the ambient temperature overnight drops to 30° F (-1.1° C) and the same vehicle is cranked after sitting overnight. In the cold of the morning this same battery and cranking circuit with a cold engine might see the cranking voltage drop down to 10.21 V as shown in Figure 2-2. Ambient temperature has a major impact on the cranking voltage.

Other factors which affect the cranking voltage are state of charge of the battery, age of the battery, condition of the starter motor and many others but I think you get the point that the cranking voltage can vary over a wide range depending on numerous factors. We now have a dilemma. What it is an acceptable cranking voltage in the real world?

Solution to The Battery Cranking Voltage Dilemma
The question of how low battery voltage can drop while cranking the engine and the battery be considered good or bad often would come up in my electrical classes. I needed an answer! I lived and trained technicians in the real world. The answer is in the real world, not in books or in a battery manufacturer specification chart.

When I started electrical training classes full time in 1985, the only answer I could offer to test the battery was to use a carbon pile load tester. That proved to be more trouble than it was worth because it created more questions needing answers. Besides, many shops did not have a carbon pile load tester. The following question kept coming up and I needed to find an answer.

“Why does the battery voltage drop below 9.6 V (indicating bad battery) during the carbon pile load test but the battery still cranks the engine? How can I tell the customer he needs a new battery because it failed the carbon pile load test when it still cranks the engine?

I needed a better answer than a carbon pile load tester. As the years went by and I presented one electrical class after another all over the USA and Canada, in all kinds of fleets, in all kinds of weather, containing all types of vehicles, personal cars, domestic and imports, sedans, pickups, SUVs and even some big rigs and heavy equipment, I tested the cranking voltage on thousands of vehicles all kinds of weather.

Here is what I found. A “good” functioning battery during engine cranking should stay above 10.00 V. I can confidently say that after testing hundreds of vehicles over 30 years, the 10 V rule is a reliable indicator of battery condition under load operating the starter motor of the vehicle. Keep in mind the battery is cranking the engine while measuring the cranking voltage. Above 10.00 V the battery stays in service. If battery voltage drops under 10.00 V it indicates a weak or marginal battery. You must make a decision whether or not to replace the battery at this time. Such a decision is based on many factors which are discussed in our book SHORTCUTS.

I recommend performing a cranking voltage test every time a new battery is installed for practice. A new, fully charged battery has a cranking voltage as high as 11.50 V. The cranking voltage gradually decreases as the battery ages and that’s what we can use to evaluate battery condition. “The Battery Voltage Rule Under Load” makes it sound official and is stated below. It will help arriving at a decision to remain in service or replace.

A “good” battery will maintain a cranking voltage above 10.00 V.
A “marginal” battery will drop below 10.00 V during cranking.

A marginal battery will drop below 10.00 V, say 9.90 V. The lower battery cranking voltage drops below 10.00 V, the weaker the battery. Extremely cold weather could contribute to battery cranking voltage dropping below 10.00 V but be perfectly good in warmer weather and not drop below 10.00 V. Ambient temperature must be factored in to your final decision to recommend the battery should be replaced because it drops below 10.00 V.

For a deeper analysis and discussion of this concept consult our book Vehicle Electrical Troubleshooting SHORTCUTS.

DMM #2/Current Clamp, shown below in Figure 2-3, is measuring starter motor electron current draw.

Fig. 2-3 DMM #2/Current Clamp Measures Starter Motor Cranking Amps

The DMM indicates .195 which represents 195 cranking amps. This reading is a clear indication of normal starter motor performance. But first we have to discuss what normal readings we should expect.

The problem is vehicle manufacturers are very hesitant to specify the correct cranking amps for a particular vehicle because there are too many factors that can affect the cranking amp reading. The major factor is the cranking motor RPM which has a dramatic impact on starter motor cranking amps. DC starter motors are notorious for drawing higher than normal electron current when they operate lower than their normal RPM, as in a “dragging” starter motor.

There are two “electric” forces bucking each other in a DC electric motor. EMF is the electromotive force (called voltage) that pushes electrons through the motor winding. CEMF is a counter electromotive force (a form of resistance) that “resists” the flow of electrons through the motor winding.

As DC motor RPM increases the counter electromotive force also increases to oppose the electron current flowing through the motor winding resulting in a lower cranking amp reading which we come to recognize as a “normal” starter motor draw.

A high cranking amp reading does not necessarily indicate a bad starter motor. It could be nothing more than the extra load of a cold engine on a cold morning that is harder to crank so the cranking amps are higher in the cold morning then they would be in the warmer afternoon. You would also hear the starter motor laboring “dragging” on a cold morning as it cranks at a lower RPM.

However, if the same vehicle is tested in the warmer afternoon the cranking amps will be lower and at a more normal reading, whatever that is for a particular make and model and engine size (4-6-8 cylinders). You will also notice a more robust cranking sound as the starter motor cranks at a higher RPM.

We could go on and on discussing battery age and state of charge, battery cable condition, ambient temperature and engine compression efficiency to name just a few to try and come up with a formula for what the cranking amps should be for particular engine. But we live in the real world, don’t we? And yes, the answer is in the real world. All the years that I tested battery cranking voltage in the real world, at the same time I also tested starter cranking amps or as some call it “starter motor draw.”

Over time I was able to conclude from the cranking amp reading, considering ambient temperature at test time, number of cylinders size and the make and model of vehicle, what I should expect for a normal cranking amp reading. I highly recommend that you begin to test cranking amps every chance you get and acquire your own mental database for the cranking amps that normally appear in the vehicles you work on every day. And don’t forget you will also hear cranking performance which will give you additional audible information to help you decide why the cranking amps are too high or too low and what that means.

I say again, if you don’t have a current clamp GET ONE! There is no better way to test a DC cranking motor than measuring starter motor draw and knowing battery cranking voltage at the same time to pinpoint the cause of a cranking problem.

The starter motor circuit troubleshooting scenario has not changed since the first starter motors appeared on cars back in the early days till today with computer controlled cranking circuits. Some things never change but get better with the electrical tools we have like DMMs and Current Clamps if we know how to use them.

Next time in Part 3 we discuss using voltage drop measurements to find difficult electrical problems. It's easy when you know how!

For training information click on one of these 3 links!

Computer Control Cranking Circuits
Part 3 of (8) Parts

What Do You Test and When?
With all the new technology that captures a lot of our imagination and increases our desire to learn more advanced electronics, essential electrical troubleshooting techniques often fall by the wayside. That creates a lot of frustration in technicians when they encounter an electrical problem that could be found with the same type of electrical troubleshooting procedures that have proven to be successful through the years. We simply cannot ignore these electrical troubleshooting techniques. They will always be valid.

Below in figure 3-1 there is a cranking problem. For purposes of this training, let’s say the starter motor doesn’t turn the engine over fast enough to allow the engine to start running. Back in Part 2 we discussed how to measure the cranking voltage and the cranking amps. Let’s also say that the cranking voltage did not drop as much as expected and the cranking amps reading was lower than expected. The starter motor is not drawing enough cranking amps to work properly and crank the engine fast enough.

There is a problem with the battery supplying sufficient current to the starter motor. This would be due to a bad connection or a corroded cable between the battery and the starter motor which is best diagnosed by a voltage drop measurement. You should remember that voltage drop tests must be performed when electron current is flowing, that is, the circuit is turned ON and operating to some extent or not operating at all.

Fig. 3-1 Testing the voltage drop of the B+ (voltage) side of the starter motor circuit.

In Figure 3-1 the starter motor is engaged and electron current is flowing. The meter is shown measuring the Voltage drop of the voltage side (Vdvs) of the cranking circuit. The red test lead connects to the battery positive post the black lead touches the connection of the battery cable to the starter motor. This test places the entire voltage side of the cranking circuit between the DMM test leads and the reading obtained is the voltage dropped by all the cable and connections on the voltage side of the cranking circuit.

A normal reading should be around 0.50V. The reading will be higher if the battery positive cable is corroded or if the heavy-duty contacts in the starter solenoid are corroded a significant voltage drop reading will be noted.

The greater the corrosion or deterioration in the starter solenoid heavy-duty contacts the higher the voltage drop reading. If the voltage drop reading is battery voltage (12V) then you have an OPEN (broken) connection between the battery positive post and the positive terminal on the starter motor. Voltage drop testing is the fastest way to find any electrical circuit corrosion on the voltage side of the cranking circuit.

(If you are interested in more training like this in voltage drop (Vd) testing we suggest our book “Vehicle Electrical Troubleshooting SHORTCUTS,” 250 pages of electrical training with how-to-do-it exercises and work sheets.)

Fig. 3-2 Testing the voltage drop of the B- (ground) side of the starter motor circuit.

Move the DMM test leads by placing the black test lead on the battery negative post in the red test lead touches the case of the starter motor.

This test places the entire ground side of the cranking circuit between the DMM test leads and the reading obtained is the voltage dropped by all the cable and connections on the ground side of the cranking circuit. A normal reading should be around 0.10V. The reading will be higher if the battery negative cable and connection is corroded or if the starter motor ground connection is corroded a significant voltage drop reading will be noted. The greater the corrosion or deterioration in the battery ground cable or the starter motor ground connection the higher the voltage drop reading. This is the fastest way to find any corrosion or electrical connection on the ground side of the cranking circuit.

If the voltage drop reading is battery voltage (12V) then you have an OPEN (broken) connection between the battery negative post and the ground terminal on the starter motor. Voltage drop testing is the fastest way to find any electrical circuit corrosion on the ground side of the cranking circuit.

These two voltage drop measurements are simple to perform but remember the starter motor has to be engaged for the bad connection or corroded cable to produce a higher than normal voltage drop. The two voltage drop readings given are what you can expect to find on a new vehicle. As vehicles get older, these “perfect” voltage drop readings will begin to rise slightly as cables and connections begin to develop a little corrosion. You will find that even as these two voltage drop readings rise slightly that the vehicle cranks with no problem. With practice you will come to learn how high the voltage drops can be before they affect the cranking circuit.

Problems With The Starter Solenoid
There are two simple voltage tests to perform if you have a situation where the starter solenoid does not engage fully or is not engaged at all.

Fig. 3-3 Testing B+ voltage to the starter solenoid.

In Figure 3-3 the DMM measures the operating voltage (B+) to the starter solenoid which is the cranking voltage, 10.21V. We are performing this test on a good starter solenoid circuit to illustrate that a good DMM B+ reading to the solenoid is the cranking voltage.

If DMM Reading Is Low
The starter solenoid will be sluggish and may not close the heavy-duty contacts. This could be due to a corroded connection tracing back through the START and P/N switches and wiring through the Fuse F1. It could also be corroded contacts in one or both of the START or P/N switches.

If DMM Reading Is 0.00V
The starter solenoid does not operate. There is an OPEN connection (broken) wire going back through the START and P/N switches. Fuse F1 could be blown or is not connected back to B+.

(If you are interested in more training like this to test voltages in a circuit and tracing through the circuit to find the location of an OPEN circuit or a voltage drop we suggest our hands-on electrical training program, starting with The Starter Kit, H-111A. You can watch a You Tube video. Type “H-111A vince” in the You Tube search bar.)

Fig. 3-4 Testing the starter solenoid ground connection.

In Figure 3-4, the starter solenoid ground wire is connected to the outer case of the starter motor. Electron current flowing through the starter solenoid passes through the case of the starter motor to energize the starter solenoid. This ground wire could also be connected directly to sheet metal ground.

The DMM is measuring the ground voltage at the starter solenoid. The DMM reading indicates .012V. The solenoid ground terminal should have a ground voltage drop of 0.10V or less. We are performing this test on a good starter solenoid circuit to illustrate what a good DMM reading should be.

If The Reading Is Higher Than 0.10V
The solenoid ground could be corroded or the wire is damage but not completely broken.

If The Reading Is 12V

The solenoid ground wire is broken or not connected to a good ground connection.

(Interested in more training like this to test voltage in a circuit and tracing through the circuit to find the location of an OPEN circuit or a voltage drop? We suggest our hands-on electrical training program, starting with The Starter Kit, H-111A.

You can watch a You Tube video.

For training information click on one of these 3 links!

Next time we will discuss how a mechanical relay is used to control a starter solenoid.

Computer Control Cranking Circuits
Part 4 of (8) Parts

+ Reading Schematic Diagrams . . .
+ Essential Electrical-Electronics Circuit Analysis
 & Troubleshooting

Starter Control Circuit Activated with Relay Control

There is a lot to be said about electromechanical relays which is covered extensively in our 60 lesson electronics course on-line at THIS LINK. Lessons 38, 39, 40, 41 cover relays in depth for a total of 31 pages. For purposes of this brief training program, we will limit our discussion to major points about relay operation to continue this series of articles. Below in Figure 4-1 a Starter (mechanical) Relay is used to control the Starter Solenoid.

Figure 4-1

The relay used in the circuit, illustrated in Figure 4-1, is a standard 5 pin relay. Pins 86 and Pin 85 connect to the relay coil. Across the relay coil a semiconductor diode is mounted inside the relay to provide spike voltage suppression when the diode is turned OFF (deactivated). Spike voltage suppression will be discussed a little later.

In this relay circuit Pin 86 connects to B+. Pin 85 connects to B- through the closed PARK NEUTRAL (P/N) switch and the START switch which are drawn in the conventional way switches are drawn in schematic diagrams. They are always shown as OPEN (not CLOSED). In this way Pin 85 is not connected to ground or B- until both switches are CLOSED at the same time. When reading a schematic diagram, the technician mentally closes the two switches to activate the relay.

As soon as both switches are CLOSED, electron current passes through the relay coil creating an electromagnet, as shown in Figure 4-2 below indicated by the dotted lines around the relay coil. Trace the electron current through the relay coil circuit starting at -BATT (B-) since the battery is the voltage source during engine cranking and ending at +BATT.

Fig 4-2

Mentally CLOSE the Ign Sw to enable the cranking circuit. To activate the relay the P/N and START switches must both be closed. When both are CLOSED Pin 85 is now grounded (connected to -BATT or B-) through the switch contacts. Electron current flows up from ground through the relay coil, through fuse F2 and on to B+. The electron current through the relay coil changes the relay coil into an electromagnet which attracts the relay’s movable contact at Pin 30 and pulls it inward to contact Pin 87. The B+ at Pin 30, through fuse F11 now appears at relay Pin 87 through the closed relay contacts as shown.

Pin 30 and Pin 87 connect to the relay contacts which act as a mechanical switch to operate a circuit, such as the Starter Solenoid. A relay at rest is said to be deactivated (or de-energized). When a relay is at rest, Pin 87A is the normally closed (N/C) contact while Pin 87 is the normally OPEN (N/O) contact. When the relay is turned ON or activated, Pin 30 moves from Pin 87A and moves to Pin 87 as shown in Figure 4-2.

The wire from Pin 87 connects to the B+ terminal on the starter solenoid. Since the solenoid is permanently grounded by a wire from Pin G to the starter motor housing, electrons flow up from ground through the grounded outer housing of the starter motor to supply electrons through the starter solenoid winding which becomes another electromagnet. The starter solenoid plunger is attracted into the center of the starter solenoid winding closing the circuit between the B and M terminals of the starter solenoid heavy-duty contacts. This applies battery voltage (B+) directly to the starter motor.

Since the starter motor housing is grounded by mounting bolts, electrons flow through the starter motor winding as long as the starter solenoid’s heavy-duty contacts remain closed and the engine cranks.

The cranking action ceases when either the P/N or START switch are opened which causes the Starter Relay to deactivate. As soon as electron current through the relay coil stops, the electromagnetic field around the coil quickly collapses and dumps its energy back into the circuit. This action is often referred to as an “energy dump.”

The diode placed across the relay coil is called a spike suppression diode. It allows the energy dump to remain in the relay and not cause arcing across the contacts of the P/N and START switches as they OPEN.

Understanding Spike Suppression Diodes

Spike suppression diodes serve a vital purpose protecting electronic circuits. What follows is a brief explanation of how a spike suppression diode protects a circuit by preventing "energy dumps" and the surging electron current that can damage a solid-state component (such as a PCM, transistor and/or an integrated circuit.) when a coil powers down and the electromagnetic field collapses.

Fig 4-3

The illustration above, Figure 4-3, shows the schematic of a coil connected to B+ and a control switch on the ground side of the coil. Think of this coil as the coil inside a relay.

Think of the switch performing the function of the P/N and START switches. When the switch closes, as shown, electron current flows through the coil creating an electromagnetic field indicated by the dotted lines and the two arrows pointing outward to show the electromagnetic field’s lines of force build up around the coil.

Notice the polarity of the voltage drop across the coil while electrons pass through the coil. Electrons enter the bottom of the coil, flow through the coil and exit the coil at the top to go to B+. This creates a measurable voltage drop across the coil which is negative (-) at the bottom and positive (+) at the top of the coil. A DMM can measure the voltage drop across the coil with the red test lead at the top of the coil and the black test lead at the bottom of the coil. Reading should be close to B+.

The electromagnetic field represents electrical energy stored (held) around the coil. This energy is taken from the circuit during the time the relay is activated to create an electromagnetic field which quickly builds up as electrons flow through the coil. Maximum intensity is reached shortly after coil electron current begins to flow.

The electromagnetic field is sustained and the polarity of the voltage drop remains constant as long as electron current flows through the coil. During this time, the relay is said to be “ON” or energized and the relay contacts are closed. At this point in the circuit’s operation there is no problem in the circuit which is performing precisely as it should. The relay contacts remain in the closed position as long as current flows through the coil. The problem occurs when the switch OPENs and the electron current through the coil stops. The problem that arises is called an “energy dump.”

In the illustration below, Figure 4-4, the switch is shown in the OPEN condition to deactivate the relay or turn the relay ”OFF” which also serves to open the relay contacts (not shown).

Fig. 4-4

The two arrows indicating the electromagnetic field are shown pointing inward to illustrate the electromagnetic field immediately collapses as soon as the electron current through the coil stops.

At the exact instantaneous split-second moment the switch is flipped OFF, the electron current through the coil stops and at the same exact time the electromagnetic field IMMEDIATELY collapses. All the energy stored in the electromagnetic field is dumped back into the circuit at that moment creating a significant energy dump that produces high electron current surge back into the circuit.

A voltage spike also briefly appears which can be viewed with an oscilloscope. In an electronics class I used to teach I had a 30 ohm coil connected to 14 volt B+ source. When the coil was turned OFF an oscilloscope briefly displayed the voltage spike which was as high as 135 volts.

Notice that during the time the electromagnetic field is collapsing the voltage drop across the coil reverses polarity because the lines of force are now moving inward, the opposite direction. Remember electrons always flow from the negative (-) to the positive (+).

Surge electrons leave the top of the coil which is a negative (-) voltage while the field is collapsing and are forced through the power source by the power of the energy dump and travel through the ground circuit and up through the switch OPEN contacts. This energy dump occurs so powerful (high voltage) that electrons jump across the gap of the open switch contacts. Over time this causes erosion of the switch contacts. Look closely at the illustration above and noticed the little arc appearing across the open switch contacts as electrons seek to get to the high positive (+) voltage at the bottom of the coil. This is the magnitude of the force of the energy dump causing electrons to jump across the gap of the open switch contacts. (The same principle of a collapsing electromagnetic field is used to create a spark across a spark plug gap.)

Once the electrons induced into the circuit by the energy dump travel through the circuit and jump across the gap of the OPEN switch contacts, electrons are supplied to the bottom of the coil. The circuit comes to rest again once the energy dump is dissipated in the circuit, .

Fig. 4-5

In the schematic above, Figure 4-5, there is a diode connected the relay coil. The negative (-) voltage at the top of the coil indicates the field is collapsing. The electron surge of the energy dump travels through the diode to arrive at the positive side the coil without traveling through the external circuit. There is no arcing across the switch contacts. The spike suppression diode allows the energy dump to pass to the positive side of the coil. The total time period of the energy dump is shorter than the blink of an eye. But it must be controlled so that it does not pass through the external circuit

If the electron current induced by the energy dump is allowed to pass through electronic circuits they could be permanently damaged. In our next article, Part 5, we get into more electronics as we use an onboard computer to control the relay. Stay tuned for more when we control the relay with a computer next time.

Computer Control Cranking Circuits
Part 5 of (8) Parts

+ Reading Schematic Diagrams . . .
+ Essential Electrical-Electronics Circuit Analysis
 & Troubleshooting

A New Automotive Era Began in Model Year 1982
Every GM car produced in model year 1982 contained a new electronic device called an Engine Control Module or ECM for short. The ECM controlled fuel mixture, spark timing, EGR, TCC and other functions to reduce emissions and maximize fuel economy. The author of this document, yours truly, set up the ECM remanufacturing facility in Dallas Texas and began to rebuild ECM’s for General Motors on January 2, 1982 following three weeks of school covering ECM remanufacturing processes.

A Sharp Contrast
A new host of problems for automotive and truck technicians began to emerge as ECM “electronics” became the controller of some of the automobile’s mechanical/engine systems. Up to this point, technicians evaluated mechanical system failures by “smell, touch, shake, rattle and roll” and observing the wear and tear on mechanical components.

Electronic circuit failures are different. There is seldom anything to observe with an electronic circuit other than the fact a circuit controlled by the ECM either works or does not work. Meanwhile the ECM “looks” fine. Then suddenly for no apparent reason it stops working then starts working again. Meanwhile the outward appearance of the circuit and components look perfectly fine. In the world of electronics, we call this an “intermittent problem” and many factors come into play causing electronics circuits to work then not work. We will discuss some of these factors as we continue.

Electronic technicians are used to the frustration caused by intermittent electronic circuit problems and have devised a few troubleshooting techniques over the years to pass on. Automotive and truck technicians must learn to deal with intermittent problems. Everyone who works with electronic circuits (radios, TVs, computers) has the same problem and there is no easy solution to this dilemma. Intermittent problems come with the territory of electronics in the vehicle.

The ECM became a whole new world for automotive and truck technicians to consider. The ECM contains thousands of semiconductor components, such as various types of diodes, transistors of every type and those mysterious components called integrated circuits. All these solid-state components are arranged and soldered to a circuit board housed in a metal container. Numerous wires exit the metal container and connect the ECM/PCM circuits to mechanical systems on the vehicle. Electronic components can be grouped into three general semiconductor categories.

Diodes in a circuit allow electron current to flow in only one direction, not both directions. When a solid-state diode becomes “OPEN” in a circuit no electron current flows through the diode. When a diode becomes “SHORTED” electron current can flow in both directions. In either failure mode, “OPEN” or “SHORTED,” the bad diode causes the electronic circuit to not function properly. Diode failures in electronic circuits can also cause the failure of a perfectly good transistor or integrated circuit from unsuppressed voltage spikes.

Transistors are used to control the amount of electron current or switch current ON or OFF. A “Load,” could be a lamp, DC motor, solenoid, or relay. When a transistor is “ON” it controls the amount of electron current through the load causing the load to operate. When a transistor is “OFF,” electron current through the load is stopped and the load does not work. These devices are called “switching transistors” because they switch electron current ON or OFF. High current switching transistors can fail “OPEN” so the circuit does not work at all or they can fail “SHORTED” and the circuit works all the time.

The most difficult problem to diagnose with a switching transistor occurs when the transistor is intermittent. Sometimes the transistor works and sometimes it doesn’t. It may look fine to the naked eye. If that transistor can be identified by testing, it must be replaced. Other types of low power transistors are used to create and pass digital electronic signals through circuitry. They also can be “OPEN,” or “SHORTED” or operate intermittently.

Integrated Circuits:
Integrated Circuits, abbreviated as IC or ICs (plural) may contain only a few transistors or millions of transistors in memory ICs that store digital information as a series of “1” (one) or “0” (zero). Integrated circuits are mounted on a substrate and encased in an epoxy type material with numerous pins sticking out from the sides of the case. The IC pins are soldered to circuit components on the circuit board to complete a circuit. If one of the millions of transistors in an IC goes bad the integrated circuit may fail to function properly and must be replaced. If IC replacement is not practical or possible (most of the time) the entire computer must be replaced. Integrated circuits can also become “OPEN,” or “SHORTED” or operate intermittently which causes great frustration. In this day and age of “throw-away computers” a new computer must be installed to restore circuit operation.

Although in the 1980s automotive computers could be repaired by hand (failed parts replaced). The present small size of electronic components is almost impossible to replace on a circuit board by human hand. Robots are programmed to place these tiny components on a circuit board with precision when the circuit board is first produced. In most cases, it’s cheaper to build a new computer with a robot than to attempt to repair a defective computer by troubleshooting the circuits and replacing defective components by hand.

There is seldom anything to observe with the naked eye when an electronic circuit has failed other than the fact a circuit controlled by the ECM either works or does not work. Then suddenly for no apparent reason it starts working again. In the world of electronics, we call this “OFF-again/ON-again” syndrome an “intermittent problem.” Electronic technicians are used to the frustration caused by intermittent problems. It’s part of life in the world of electronics. Automotive and truck technicians are learning this the hard way like everyone else. We all have the same intermittent problem and there is no easy solution to this dilemma.

An intermittent problem can be caused by a crack in a circuit board developed from vehicle vibration or a “cold solder joint” that intermittently makes and breaks contact with the loose leads of an electronic component intermittently contacting the copper trace on the circuit board. Sometimes this intermittent failure mode can be duplicated by a gentle “Tap Test” of the ECM case. A control unit sensitive to vibration must be replaced.

Another cause of an intermittent problem can be excessive heat or excessive cold and then the circuit sometimes returns to normal operation when the temperature extreme is removed. Both temperature extremes can cause a semiconductor to permanently or intermittently fail. During the days of humans repairing ECMs, a technician would struggle to duplicate an intermittent problem by applying external heat to the circuit board with an infrared lamp to warm up the semiconductor components. When the failure occurred the semi-conductor components were sprayed with “Freeze Spray” designed to quickly cool down the heated electronic components. An intermittent semiconductor component would immediately begin to operate again when hit with the coolant spray thus identifying a component with an intermittent problem.

A Not So Obvious ECM Problem
In the early days of computer control (early 1980s) I remember a luxury car that had a history of ECM failures. Each time we went into the newly failed ECM to see what was wrong we found cold solder joints on the circuit board. Why did just this car have this problem? The dealership sent us the car to diagnose the problem. What we found was something quite surprising.

The generator charging voltage was running about 17.5x V instead of a more normal 13.8xV-14.xxV. In the hot Texas summer the ECM was being exposed to higher than normal operating temperatures and a higher B+ voltage causing circuit components to get so hot that the solder connections on the circuit board would soften during vehicle operation then re-solidify when the engine was turned off and the circuit board cooled down to ambient temperature. Over a short period of time the heating up and cooling down created intermittent solder connections on the circuit board.

Ever since then I have been adamant in teaching technicians to test and monitor the charging voltage during engine run. I also have written several training programs with details on how to check the charging voltage on a vehicle. You may have heard about this test in FIRST THINGS FIRST flip charts. The test only takes 60 seconds.

Below in figure 5-01 shows our by now well-known cranking circuit where the starter relay is now controlled by a PCM (Powertrain Control Module). The PCM, formally known as an ECM, performs the function of the P/N and Start switches in “B.C.” days (Before Computers). The relay coil connects between Pins 86 and 85. An electron current through the relay coil energizes the relay and Pin 30 connects to Pin 87 applying B+ to the Starter Solenoid.

Fig. 5-01 PCM Controls Starter Relay

The PCM contains many circuits. In Figure 5-01 the role of the PCM is shown and only the circuit in the PCM that controls the starter relay is shown. In some schematic diagram formats the lines surrounding the PCM might be shown as a dotted line to indicate there are other circuits in the computer not shown or used in the starter relay circuit.

The PCM circuit in our illustration contains two diodes, an NPN transistor - Q1 and a circuit designated as U1.

Diode D1 is a Polarity Protection Diode that prevents damage to PCM circuitry should a jump battery be connected in reverse polarity by accident of course. The polarity diode does not allow reverse current to flow through the PCM circuits which could smoke all solid-state components.

The diode across the transistor Q1 is not numbered but protects the transistor from the energy dump created when the relay powers down. It is called a “spike suppression diode.”

Crank Input is a DC command voltage (5 volts or 12 volts) from the Ignition Switch in the CRANK position. The voltage present tells the computer to “please” activate (turn ON) transistor Q1 to energize the relay to power the Starter Solenoid.

When Q1 turns on, electrons flow up from ground and enter Q1 emitter, (Q1 emitter connects to ground) then flow through the transistor and exit Q1 collector (the collector connects to PCM-Pin 8). Electrons leave the PCM at Pin 8 and travel by wire to Pin 85 of the relay.

Q1 electron current flows through the relay coil exiting the coil at pin 86 and flowing to B+. The relay is now activated and closes the relay contacts. Relay pin 30 moves from relay Pin 87A to Pin 87 to provide B+ voltage to activate the Starter Solenoid. The Starter Solenoid Pin G being hard wired to ground allows current to flow through the Starter Solenoid which closes the heavy-duty contacts and activates the starter motor.

Circuit U1 represents all internal PCM circuitry. Think of U1 as a portion of the computer’s memory and brain circuits that control various functions. This is where the computer’s program is stored and used to control, in this case, the cranking process by turning Q1 ON providing electron current through the relay coil.

U1 can be programmed to limit the time the starter solenoid can crank the engine should the engine not begin to run. Other inputs to the PCM (not shown) will contribute information to the computer’s brain in the event of a long crank.

U1 can also be programmed to prevent cranking when battery voltage is below a threshold because injectors are not pulsed when battery voltage drops below 9.0V.

Herein Lies A Problem
An onboard computer is programmed to do certain things by computer programmers. What has the PCM “brain” been programmed to do and how does it respond to its program? What I came to learn in the early days of repairing ECMs/PCMs is that the computer program dictates how the onboard computer thinks and works which I like to describe as the computer’s “personality.” All vehicles of the same make and model will have the same personality because all run on the same software program. If a brand of car has a different engine it will be programmed differently resulting in a unique personality for that engine computer.

What is Computer Personality?
In the early days of computer control I would hook up a lab scope to the O2 sensor and watch the O2 sensor voltage swing through rich up to 0.9 V and lean down to 0.1 V. Any adjustments on the vehicle that could affect fuel mixture could be adjusted to their proper setting by observing the reaction of the waveform as the mixture trace line on the scope went to rich (trace swings high) to lean (trace swings low) and from lean back to rich. Make and model of cars had a different action of swinging from rich to lean or lean to rich. The differences were not dramatic but you could see similar vehicles all behaved the same way while a different make or model of a vehicle might have its own slightly different lean/rich waveform. These subtle differences are a reflection of a computer’s personality.

Any function an onboard computer is programmed to perform will contribute to the overall personality of that particular make and model vehicle. The more the circuitry the more complex the personality of the control unit. The more you work on the same type of vehicle with the same engine the more you become aware of each computer’s personality. Many of you already recognize this phenomenon in the vehicles you service the most but you may not have called it “computer personality.” The more programming that is involved in the operation of a control unit and the more functions on the vehicle it controls will contribute a great deal to that control unit’s unique and sometimes complex personality.

This particular ECM/PCM generation of ECM, Figure 5-01 controlling a starter relay, is an example of computer control and it is still in use today. This function by itself is not nearly as sophisticated as president onboard control units are today with the complex circuitry and vehicle functions they control. Expect significant differences in today’s onboard computers personality. Watch for it!

The more you know about each computer’s personality the better you are able to diagnose various driveability problems. If you work on whatever comes in the door it takes a little longer to recognize each vehicle’s unique computer personality.

Next time we will look at various ways this computer-controlled cranking circuit can fail in the electronics of the circuitry and offer some electronic testing techniques to diagnose the problem.

Computer Control Cranking Circuits
Part 6 of (8) Parts

+ Reading Schematic Diagrams . . .
+ Essential Electrical-Electronics Circuit Analysis
 & Troubleshooting

What Do You Test and When?
An electrical circuit operates with voltage applied, electron current flowing and circuit components that have the correct resistance to keep the electron current moving at a safe level.

When a circuit is performing correctly, voltage readings, electron current measurements and resistance values around the circuit are normal. Most technicians never bother to check these values in a circuit when it is working correctly.

“Why should I do that, you ask?” It is practice using your DMM and at the same time learning what the normal voltage readings are throughout the circuit and what is the normal level of electron current. Resistance measurements also indicate the normal resistance value of critical components in a circuit. This is all vital information in troubleshooting a faulty circuit. An electrical problem in a circuit will certainly affect voltage readings around the circuit and the amount of electron current flowing. When faulty readings are compared to known good readings, you are well on your way to determining the problem in the circuit.

When a circuit develops a significant problem that affects circuit performance is the time most technicians begin, for the first time, to check voltages, check electron current and maybe the resistance value of a critical component without any knowledge of what the normal readings should be for each of these exercises. As a result, technicians cannot be certain a voltage value, or electron current measurement found in a faulty circuit is normal or abnormal indicating the problem in the circuit.

The Solution
When you have repaired an electrical circuit, that is the time to take a few critical voltage readings to establish what normal voltage readings exist in a good circuit.

In the case of DC motor circuits, it would certainly be advisable to check the amount of electron current flowing through the DC motor circuit when the DC motor is operating properly. Use your current clamp to quickly measure the electron current. What? You don’t have a current clamp. GET ONE! Current clamps can quickly be connected to a circuit to measure the electron current flowing during normal operation.

Sampling voltage and electron current readings in repaired circuits will begin to educate you about what good readings look like. This will build up your store of electrical knowledge that will be useful when testing another faulty circuit for the first time. You will begin to accumulate knowledge about good readings in a circuit – you are learning valuable information needed to troubleshoot electrical problems.

Understanding Computer Control
The addition of the PCM circuit in Figure 6-01 to control the starter relay introduces numerous issues that must be considered in the diagnostic process. Take a few minutes to look over the schematic diagram.

Fig. 6-01

First notice the addition of the PCM and its place in the cranking circuit. We have the luxury of showing the components in the PCM relating to starter relay control.

Transistor Q1 in the PCM is a NPN (solid-state) transistor switch and connected to the relay Pin 85 to control the relay on the ground side of the relay coil. The voltage side of the relay coil is hard wired to B+ through Fuse F12. The PCM provides the same function as the P/N and Crank Switches.

Q1 turns ON to operate the relay and drives an electron current through the relay coil. Electrons travel up from “ground” (the source of all electrons) and enter PCM Pin 55, travel through the turned “ON” transistor and exit the PCM at Pin 8, travel to the Relay Pin 85, flow through the relay coil and exit the Relay at Pin 86 as electrons race to  B+.

1. What is required for the PCM to operate the Starter Relay?
First and foremost, the PCM must have good B+ voltage on all of its B+ wires in the PCM connector (only Pin 6 in this schematic, others not shown). The PCM must also have a good ground circuit voltage drop of 0.05V on all PCM ground Pins 48 and 55. When these critical PCM circuit inputs (good B+ voltage and good B- ground) are present, the PCM is ready to operate the cranking circuit on command. These electrical values are determined by measuring the voltages (back probing) at the PCM connector pins. Make sure your DMM is grounded at -BATT to obtain the most accurate voltage readings, especially on the ground pins.

2. When does the PCM operate the starter relay?
Immediately upon receiving the proper voltage input on the three INPUT pins (1) Crank Request, (2) P/N and (3) BRAKE (pedal depressed), a DC voltage appears at each pin at the same time. The DC voltage could be a +5 volt or a +12 V input. You can’t be sure until you test this voltage in a known good circuit if the manufacturer doesn’t provide this information.

Internal circuitry in the PCM combines these three inputs (using an AND Logic Gate) which allows the PCM to activate circuit U1 to put a DC voltage on the base of Q1. This small voltage is called “forward bias” and Q1 allows sufficient electron current through the relay coil to energize the relay. The B+ voltage at pin 30, from fuse F11, is connected to Pin 87 through the CLOSED relay contacts.

It any one of these 3 inputs are not present the PCM will not activate Q1. The PCM requires all three to appear on their pins at the same time.

3. How does the PCM operate the starter relay?
Transistor Q1 supplies enough electron current to energize the starter relay when told to do so. If transistor Q1 is good the voltage on PCM Pin 85 will drop to about 0.8 V.

If transistor Q1 is OPEN the starter relay never energizes. Pin 8 of the PCM remains at 12 V.

4. What do you do first when the PCM does not operate the starter relay?
First place the electrical system in CRANK request, Gear shift in P/N position and the depress BRAKE the brake pedal.

Does the relay click? YES – NO. Put your finger on the relay case to feel the vibration when the relay clicks. I like to use a big screwdriver placed on the case of the relay and hold the handle of the screwdriver to my ear so I can hear the relay clicking.

YES - Relay Clicks
This means the PCM is doing the job to energize the starter relay.
Check the voltage at starter relay pin 30 and then at starter relay Pin 87. Both pins should indicate B+ is present.


If Pin 30 does not have B+ check Fuse F11 and the voltage on both sides of the fuse.

If no or low B+ at Pin 30 look for a voltage drop on the wire feeding fuse F11 and the wire going to Pin 30.

If Pin 87 does not have B+ immediately recheck Pin 30 again to verify B+ is still present at Pin 30 with the relay contacts CLOSED.

If Pin 87 does not have B+ (but Pin 30 has B+) then the relay contacts are bad. Replace the relay with an identical relay.

NO - Relay Does Not Click
Verify that the PCM has good B+ voltage on all of its B+ wires by measuring the voltage at each B+ pin. Next verify that all ground circuit pins have a voltage drop of 0.05V on all PCM ground pins. Then measure the CRANK request pin, P/N pin and BRAKE pins for the proper voltage.  You should measure at least 5 V on some vehicles and it may be as high as 12 V depending on the voltage level used in this vehicle.

Replacing a Relay - CAUTION
Do not try any old relay you have laying around to see if it clicks. It may not be configured (pin arrangement) the same as the original relay. A spare relay that has the same pin arrangement may have the wrong relay coil resistance.

If the relay coil resistance is too low, transistor Q1 could be damaged which would require a new PCM. A second reason do not use a spare relay laying around is the component across the relay coil for spike voltage suppression could be missing in your spare relay. That would cause a voltage spike when the relay powers down which would damage the driver transistor in the computer.

Some relays use a resistor for spike suppression and some relays use a solid-state diode. A replacement relay with the same part number as the original relay avoids all these potential problems that could damage a control unit. Just don’t use a spare relay that isn’t the same part number.

For safety sake always use an original replacement relay part number any time you are replacing a relay you suspect is defective. There are several reasons for using an exact replacement relay which would require much more explanation than is possible to provide in this brief article.

Notice the schematic diagram below in Figure 6-02. The internal circuitry of the PCM is not shown. The PCM is represented by an empty box.

Fig. 6-02

This can lead to some confusion because it’s hard to determine how the PCM is controlling the relay. From our understanding of relays, it should become obvious that the PCM is controlling the ground side of the relay coil.

Relay Coil Pin Voltages
When the relay is commanded “OFF” the voltage on Pin 86 and Pin 85 is B+. It is very important to remember that taking voltage measurements in a computer control circuit requires the DMM to be grounded at -BATT, the negative terminal of the battery which is the best ground for the DMM and results in the most accurate voltage readings.

When the PCM turns the relay ON, the voltage at Pin 86 remains at B+. But the voltage on Pin 85 drops to about 0.8 V. This is the voltage drop of the driver transistor inside the PCM. This reading is very important to analyze. When the transistor driver begins to deteriorate, the voltage on Pin 8 begins to rise slightly when the relay is “ON.” If the voltage exceeds 1.0 V on Pin 8 the driver transistor is going bad and may not last much longer. There’s no way to know how much longer the transistor driver will function properly. It may last for years because it has a very short “ON” time since it’s only “ON” when cranking the vehicle.

Recent developments in solid-state driver transistors has progressed over the years so that the normal voltage drop of a solid-state transistor driver at 0.8 V has been reduced to as low as 0.3 V.

It is a good idea to check pin voltages on a relay’s coil circuit when the relay is “OFF” (not energized) and when the relay is “ON” (energized) and notice how the voltage on the pins change. Pay particular attention to the voltage on the side of the relay coil that goes to the control unit to learn what the voltage drop is of the transistor driver circuit in the PCM.

I would also recommend and emphasize that you measure the voltage on Pin 30 when the relay is “ON” and “OFF.” When the relay is off the voltage on Pin 30 should be B+. When the relay is energized the voltage on Pin 30 should not drop more than 0.5 V. A decrease in voltage exceeding 1.0 V indicates there is a bad connection in the wiring to Pin 30 causing an excessive voltage drop when the relay contacts close and electron current is flowing through the load, in this case the starter solenoid. Trace the wire back to B+ and repair the bad connection to establish good B+ voltage on Pin 30 when the relay is energized.

In the next segment we will see another control module added to the circuit that increases the complexity of this relatively simple function to crank an engine.

Computer Control Cranking Circuits
Part 7 of (8) Parts

+ Reading Schematic Diagrams . . .
+ Essential Electrical-Electronics Circuit Analysis
 & Troubleshooting

+ What is going on inside the PCM?

Ever wonder what is going on inside an on-board control unit? How do they do what they do? How does a tech troubleshoot these circuits? In Part 7 a sample of some of the digital circuitry from our on-board control unit controlled cranking circuit, utilizing some PCM circuitry is shown. This discussion will help in understanding what a technician should do when the decision must be made to replace a control unit. Figure 7-1 below is a schematic of some of the inner workings of the PCM circuitry that controls the Starter Relay. The partial schematic of PCM circuitry shows three electronic circuit components.

Transistor Q1:
Q1 Driver transistor provides electron current through the relay coil to turn the relay ON. Q1 is a NPN transistor that controls the relay coil from the ground side. Notice the small arrow drawn on Q1’s “e” (emitter) wire lead is pointing outward in the transistor symbol indicating the transistor is a NPN (Not-Pointing-iN).
(A PNP transistor is used to control the relay coil from the voltage side at Pin 86 and the arrow is Pointing iN.)

Fig. 7-1

To the left of Q1 is a circuit labeled “U1” which controls Q1. It is part of the PCM’s “CPU” (Central Processing Unit), or the “Brain” of the PCM that makes all the important decisions according to the PCM’s software program. The stored operating program is what you change or edit when you “flash” a control unit.

AND Gate:
To the left of U1 is a digital logic circuit called an “AND Gate” that provides an output on Pin 4 when all inputs are present. For (3-input) AND Gate U2 to provide an output signal at Pin 4 (which connects to U1’s input), requires 3 inputs to appear at the same time to AND Gate Pins, 5, 6 and 7. That’s why it is called an AND Gate. It takes an input to Pin 5 and an input to Pin 6 and an input to Pin 7 at the same time before an output appears on Pin 4.

Logic Legend
There are two categories of logic circuits, Positive Logic or Negative logic. The logic level of a “1” or “0” must be defined according the logic circuits used. Our Logic Legend shown in the upper left of Figure 1 describes Positive Logic for our circuit because a “1” logic level is a positive voltage level of either 5V or 12V and a “0” logic level is zero volt. Our circuit works with positive logic so let’s continue with that understanding.

Digital logic circuits work with two conditions simply called a “1” (say one) and a “0” (say zero). It is common to write the logic level of a “1” or “0” on a digital schematic diagram to express the “logic level” which is the voltage present at that point in the circuit at a particular moment in time. Then a technician measures the “logic level” (voltage) present at each point in the digital circuit to verify the logic circuit is working correctly (has the correct voltage at each point) or has a problem (voltage doesn’t match the logic level at each point). Pins on an “IC” (integrated circuit) or the wire connected to the pin are labeled with a “1” should have 5V or 12V present and wires labeled with a “0” should be 0V (zero volt) present.

Logic levels present at the connector pins of an on-board control unit can be 12V for a “1” and zero volt for a “0.” An exception could be a sensor that provides a 5V signal rather than a 12V signal to a computer, as we show in Figure 1. Inside the on-board control unit a logic “1” is most often 5V and 0V for a “0.” An exception would be the logic level designation for driver transistors such as the voltage on Pin 8 of the PCM which is the collector of Q1. When Q1 is OFF the collector is a logic HI 12V (a “1”) and drops a logic LO (“0”) when Q1 is ON.

Logic circuits are very logical if you think logically.

PCM Inputs
An on-board
control unit must have input information from sensors to make decisions. The PCM has three required inputs to activate the Starter Relay in Figure 7-1 above.

P/N switch input is a logic level “1” or 5V indicating the vehicle is in PARK or Neutral. A “1” appears at Pin 6 of U2. U2 Pin 4 output is “0” and Q1 is OFF.

BRAKE switch input is a logic level “0” indicating the brake pedal is not depressed. A “0” appears at Pin 7 of U2. U2 Pin 4 output is still “0” and Q1 is OFF.

START switch input is a logic level “0” indicating the START button is not depressed. A “0” appears at Pin 5 of U2. U2 Pin 4 output is still “0” and Q1 is OFF.

Q1 remains OFF and the Starter Relay is not ON (not energized). The vehicle is parked.

Fig. 7-2

In Figure 7-2 above, the vehicle operator initiates the cranking process by depressing the brake pedal.

BRAKE switch input is now a logic level “1” indicating the brake pedal is depressed. A “1” appears at Pin 7 of U2. U2 Pin 4 output is still “0” and Q1 is OFF.

Now AND Gate U2 has two “1” inputs on Pins 6 and 7. Since U2 Pin 5 is still a logic level “0” there is no output at U2 Pin 4. A 3 input AND Gate requires all three inputs at the same time to generate an output at Pin 4. Q1 remains OFF and the Starter Relay is OFF (not ON).

In Figure 7-3 below, the vehicle operator presses the START button.

START switch input becomes a “1” indicating the START button is depressed. A “1” appears at Pin 5 of U2.

U2 Pin 4 output becomes a “1” since all three inputs are now a “1.” This causes U1 to turn ON and place a small DC voltage of approximately 2V on the base (b) of relay driver transistor Q1 which turns ON and allows an electron current to flow through the relay coil which becomes an electromagnet.

Notice the electromagnetic field created around the relay coil and the movement of the movable contact creates an audible CLICK.

Fig. 7-03

The movable contact connected to Relay Pin 30 with B+ becomes magnetized and moves under the influence of the electromagnetic field to contact Pin 87. The B+ voltage on Pin 87 supplies B+ to the Starter Solenoid (not shown in this schematic). The engine cranks.

As soon as the engine begins to RUN the vehicle operator releases the START Button which removes the “1” from U2 Pin 5 as shown in Figure 7-4.

The AND Gate U2 no longer has three inputs at a logic “1” so it changes the “1” at U2 Pin 4 from a “1” to a  “0.”

Fig. 7-4

U1 sees a “0” at its input and turns OFF which removes the 2V from the base of Q1. Transistor Q1 turns OFF and stops electron current through the relay coil.

The engine is running.

The Aftermath
The engine is running but shutting down the Cranking Solenoid and the Starter Relay causes powerful electrical energy dumps as the electromagnetic fields across both coils collapse.

The Starter Relay has a spike suppression diode connected between Pins 86 and 85. The diode absorbs the collapsing electromagnetic field to protect transistor Q1 from being destroyed which would knock out the PCM.

The starter solenoid also has a diode connected across the coil to absorb the collapsing electromagnetic field. If this energy dump were not suppressed by the diode there is a good chance that the energy dump would travel back into the electrical system and possibly damage sensitive computer memory circuits. It has been known to happen causing the adaptive memory information inside the PCM to be erased. The vehicle would eventually relearn the adaptive strategy after about 25 miles of driving with poor engine performance until relearned.

Identifying and Confirming a Defective Control Unit
Technicians are not required to open up a control unit and repair internal circuitry such as logic circuits and transistors. Yet these components do fail requiring a new computer to get the vehicle going again. Before a technician should condemn a control unit follow some simple troubleshooting steps to confirm a control unit must be replaced.

1. Confirm the control unit has B+ on all pins connected to B+ power with the engine running.

2. Confirm the control unit has a good B- ground on all ground pins with the engine running. Always remember computer grounds need to be an exceptionally good ground. Whereas electrical circuits are considered to have a good ground when the ground voltage drop is 0.10V (100 mV), computer grounds should have a voltage drop of 0.05V (50 mV.)

3. Confirm the proper voltage or signal is present at the control unit’s appropriate input pins that are necessary to control the circuit in question.

4. Measure the voltage or signal on the control unit’s output pin (PCM Pin 8 in our example) to determine if the computer is functioning properly by the proper change in voltage that should occur.

Let’s say the problem with this vehicle is the Starter Relay does not click and the vehicle does not crank. Measure the voltage on PCM Pin 8 or at the Starter Relay Pin 85 which ever is easiest to access. If the relay is not energized, the voltage should be B+. (If the voltage on Pin 85 is 0.0V at this time the relay coil is open or there is no B+ on pin 86.) That’s not a PCM problem. It is a problem in the wire harness.

When the PCM gets all three inputs the voltage on Pin 8 should drop to a low voltage of about 0.80V as the PCM circuitry does its job to control the Starter Relay. The voltage drops from B+ (battery voltage) to about 0.80V because that is the voltage drop of transistor Q1 when Q1 is supplying electron current through the relay coil.

If the voltage at PCM Pin 8 or Starter Relay Pin 85 stays at B+ when the PCM has all three inputs, it is a reasonable conclusion that PCM internal circuitry is defective. The PCM can now be replaced with an expectation the problem is resolved.

We will conclude this series of articles with Part 8 coming soon.

For more training information click on one of these 3 links
that applies to your job description!

Final Part 8 coming in the near future!

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