Computer Controlled Cranking Circuits: Part 2

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!

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