How to Test PCM with Multimeter (8 Steps)

In this 8-step guide, I will teach you how to test your PCM with a multimeter.

If you suddenly experience increased fuel consumption, engine misfiring, or excessive exhaust in your vehicle, it might indicate a bad PCM unit. But how can you confirm this? While working as a mechanic, I’ve learned a few tricks and tips that might help you. I’m going to share that knowledge for my PCM testing process.

Quick Summary, steps to test a PCM with a multimeter:

  • Gather all the necessary things.
  • Locate the PCM.
  • Inspect the diagram and find the power and ground circuit pins.
  • Set the multimeter to DC Volts settings.
  • Measure the battery voltage.
  • Measure the voltage drop on the power circuit.
  • Measure the voltage drop on the ground circuit.
  • Determine the condition of the PCM by using voltage drop values.

Follow the below-detailed guide for a better understanding.

Before We Start

PCM, aka the Powertrain Control Module, is a combined controller of the ECU (Engine Control Unit) and TCU (Transmission Control Unit.

Some people may recognize this unit as the ICM or ECM. Whatever the name, the PCM is the brains of your vehicle. It manages the engine, transmission, and other essential systems by tracking information using different sensors.

You can guess how important it is to keep your PCM unit functioning perfectly. In this test, I’ll show you how to test both power and ground circuits using a multimeter. There are recommended voltage drop values for both circuits. So, I will use them to determine the condition of the PCM. You’ll get a good grasp of it through this step-by-step guide.

8-Step Guide on How to Test PCM with Multimeter

Step 1 – Gather Required Items

First, you’ll need a few things for this testing process. Here is the list.

  • Digital Multimeter
  • PCM diagram chart for your vehicle
  • Vehicle’s instruction manual
  • Screwdriver
  • Flashlight
  • Safety glasses
  • Notebook and pencil

Step 2 – Locate the PCM

Locate the PCM unit. Open the front hood of your vehicle. The PCM is located in the engine bay close to the fuse box. Sometimes the PCM might be located inside the car near the fuse box. And for some vehicle models, the PCM will be under the front windshield.

Tip of the Day: Check the vehicle’s instruction manual to find the PCM’s location. Also, don’t forget to wear safety glasses during this testing process.

Step 3 – Inspect the PCM Diagram

After successfully finding the PCM, you should locate the power and ground circuit pins.

Typically, a PCM includes over 50 pins. You’ll need the PCM diagram associated with your vehicle model to identify power and ground pins and connectors. (1)

On the PCM diagram, there should be a numbered list. Find the corresponding numbers for power and ground pins from the list and locate them in the diagram.

Now, disconnect the corresponding connectors on the PCM unit to expose the pins.

Step 4 – Set the Multimeter to DC Volts Settings

Next, take the multimeter and connect the red and black jacks to the Voltage and COM ports. Then, turn the dial to the DC volt settings.

Quick Tip: Here, you are measuring the voltage from the battery. Therefore, you should choose DC volts.

Step 5 – Measure the Battery Voltage

measure the battery voltage
Video | ChrisFix

This is the step most people ignore. But you’re better than that! Before start testing the PCM, you need to test the battery. The PCM unit won’t work properly if you run your vehicle on a weak battery. Therefore, check the battery first.

  1. Connect the multimeter’s red probe to the positive battery terminal.
  2. Connect the multimeter’s black probe to the negative battery terminal.
  3. Check the multimeter reading.

You should test the battery voltage under two circumstances.

While the Ignition Switch is On

You should get a reading close to 12.6V

While the Engine is Running

You should get a reading close to 13.7V

If the voltage reading drops below the above-recommended levels, you’ll have to charge the battery. Or you might have to replace the battery.

Step 6 – Measure the Voltage Drop in the Power Circuit

Important: During steps 6 and 7, the vehicle’s ignition key should be ON. But don’t start the engine.

To measure the voltage drop in the power circuit:

  1. Connect the multimeter’s red probe to the power circuit’s pin.
  2. Connect the multimeter’s black probe to the positive battery terminal.
  3. Note down the multimeter reading correctly.

As you can see, I’m getting a reading of 0.0615V (61.5 millivolts).

Step 7 – Measure the Voltage Drop in the Ground Circuit

Now, start the ground circuit testing process.

  1. Connect the multimeter’s red probe to the ground circuit’s pin.
  2. Connect the multimeter’s black probe to the negative battery terminal.
  3. Note down the multimeter reading correctly.

Here, I’m getting a reading of 0.0032V (3.2 millivolts).

Step 8 – Determine the Condition of the PCM

After you measure the voltage drops for the power and ground circuits, you can use those results to identify the condition of the PCM.

The Recommended Voltage Drop for Power and Ground Circuits

The voltage drop should not go up above 0.05V (50 millivolts). So, let’s compare it with the test results.

For the ground circuit, I got a reading of 0.0032V. This value is below the recommended 0.05V. Therefore, the PCM is working fine.

However, for the power circuit, I got a reading of 0.0615V. This value is above the recommended 0.05V value, and this is an indication of a bad PCM unit. So, it’s time for a repair or replacement appointment.

Reasons for Bad Powertrain Control Module

There can be many reasons for a bad PCM, and here are some of them.

  • Loose connections
  • Excessive rust or corrosion
  • Weak battery (that is why you should check the battery first)

Remember, a high voltage drop can affect the performance of the PCM, and eventually, it affects the other circuits connected to the PCM. So, you’ll have to do some visual inspection to figure out the problem. Here are a few simple steps for that.

Check all the wire connections. If you detect any loose ends, remember to connect them properly. After that, rerun the PCM test.

Or inspect the PCM unit closely for any corrosion or rust. Excessive corrosion or rust will create a high resistance level in the circuits. With higher resistance, the voltage drop gets increased. (2)

Repair or Replace?

If you are lucky, you’ll be able to repair the control module PCM unit without much effort. However, sometimes you might have to replace the whole unit. If that is the case, remember to seek professional help. Trying to replace the PCM by yourself can lead to many dangerous issues in the rest of your car.

Take a look at some of our related articles below.




References
(1) vehicle model – https://www.valuepenguin.com/auto-insurance/car-make-model
(2) high resistance level – https://www.investopedia.com/terms/r/
resistance.asp

Video References

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HVCCAutomotive

Automotive electronics from schematics by Joseph

ChrisFix

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.