Computer Controlled Cranking Circuits: Part 1

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?

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.

Fig. 1-1 Computer Controlled Cranking Circuit
Learn how to read schematic diagrams

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.

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.

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 andelectron 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.

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.

Continue Reading...


For Technicians

Learn about our technician training programs in electrical systems troubleshooting and electronics repair.

Learn More ›

For Shop Owners

Our training programs can train your technicians to be more proficient in electrical repair - saving labor hours, down time and fewer good parts replaced.

Learn More ›

For Teachers

We have the electrical-electronics curriculum that your students will need when they enter the job market. Be "THAT" teacher that showed them how!

Learn More ›