How to Test Ignition Control Modules with a Multimeter (Guide)

This article shows how to test a vehicle’s Ignition Control Module with a multimeter.

The ICM plays a central role in controlling a vehicle’s ignition system to start the vehicle. The vehicle might not start if it doesn’t operate properly. If you notice a symptom that suggests a bad ICM, you will have to test it with a multimeter. But I suggest you test other parts of the system first before suspecting the ICM.

ignition module defect error label

First, test the spark plugs for current and the ignition coils for voltage, then do a continuity test for faulty wires, as this might fix the vehicle’s starting problems.

If it’s necessary to test the ignition control modules with a multimeter, place its black probe on a grounded part of the vehicle. Then, check each terminal using the red probe while cranking the engine. A multimeter reading will confirm the ICM is functioning.

I will show how to conduct each of these four tests using a multimeter.

About The Ignition Control Module

icm
An ICM

The Ignition Control Module (ICM) is a switch that turns the ignition system on and off, as required and per a sensor inside the distributor.

The sensor sends a signal that can fire up the ignition coil inside the ICM, and the process produces enough energy for the spark plugs to operate. The energy generates a voltage for the air/fuel mixture in the correct proportions and for timed combustion. The ICM thus controls the spark timings accurately.

In short, the ICM controls the ignition timing, spark plug firing, and ignition coil pack activities.

Locating the ICM

The ICM’s exact location depends on the vehicle and model.

Also, only some vehicles are equipped with a separate ICM. Most have an ICM integrated with the Engine Control Module (ECM), also called the Engine Control Unit (ECU).

Usually, the ICM is located in the engine compartment or the distributor housing. You may find a small computer chip in these two locations, the ICM (see the images below).

ICM in the engine compartment or the distributor housing
Video | hoohoohoblin

Symptoms of a Bad Ignition Control Module

The ICM plays a significant role in a vehicle’s ignition system.

But, it can cause issues occasionally, so identifying a bad ICM is crucial. Here are some signs that indicate a bad ICM:

  • Check engine light – A check engine light flashing on the dashboard indicates a bad ICM. It is the most obvious indicator.
  • Engine misfires – An engine misfire can occur due to combustion shortages. A faulty ICM is one of the main factors for incomplete combustion. [2]
  • Stalling – A faulty ICM can interfere with starting the car and stall the engine.
  • Unable to power up accessories – Usually, when your vehicle is in a running position, all other accessories will power up. But with a faulty ICM, the accessories might not after adjusting to the running position.
  • Vehicle starting issues – A bad ICM can cause some starting issues for your vehicle.

If you notice these symptoms, you may need to test the ICM with a multimeter.

Testing the ICM with a Multimeter

Items Required

You will need the following items to conduct the ICM tests:

  • A digital multimeter
  • Vehicle wiring diagram
  • A 12V test light
  • Ignition switch for replacement

Note: You will also need someone to assist you.

Estimated time: Around 10-15 minutes.

We will test 4 things in this order:

  1. The spark plugs for current
  2. The ignition coils for voltage
  3. A continuity test for faulty wires
  4. The ICM itself for resistance

ICM working process flow

Let’s get started.

Test 1: Current in the Spark Plugs

As long as current flows through the ICM, the next thing to check is the current in the spark plugs.

You will need the 12v test light for this. Place the test light onto the terminals. While holding the test light, ask your assistant to crank the engine.

If the test light flickers while the assistant cranks the engine, it confirms that the spark plugs are working properly.

Apply the same process to the other terminals. In some cases, the multimeter won’t show a reading. If you’re not getting a reading, check the wires.

Test 2: Test Ignition Coils

Temporarily disconnect the black wire attached to the battery’s negative terminal and remove the distributor cap to access the ignition coil.

Reattach the black wire to the battery, insert the spark plug, and check for signs of sparking.

A spark would confirm that the ignition coil is working fine. We can only conclude that the distributor cap isn’t working if all 4 spark plug wires don’t spark.

To confirm if the ignition coil is getting power, disconnect the battery again before removing the plastic dust cap that covers the ignition coil. This will give access to the power terminals (pointed to in the picture below).

Reconnect the battery to test for power. Ground the multimeter’s negative test probe to the battery’s negative terminal.

Then, connect the multimeter’s positive (red) probe to the ignition coil’s power terminal. You should get a reading between 10 and 12 volts with the ignition on. A reading close to zero volts could be due to a blown fuse. If so, repeat this test to check for power again.

In this case, the reading confirms that the ignition coil power is within the range.

Now, test the signal terminal with a test light.

The ignition coil is receiving the activation signal if it lights up while the engine is cranking (as shown below). Hold it in place to see if it lights up.

Test 3 – Continuity Test for Faulty Wires

You only need to check for continuity if you didn’t get a current reading in the previous tests.

We will check for continuity to identify faulty wires. Set the multimeter to check for resistance (in ohms). Isolate the wires that have visible break or burn marks to test them first. Disconnect each wire from the corresponding spark plug and test each in turn.

Since we’re checking for continuity, a wire is faulty if the multimeter shows a high or infinite reading. If the multimeter displays a low or zero reading, the wire is in good condition.

Replace all the wires you confirmed to be faulty. Ask an electrician to replace the wire if you’re not comfortable doing it yourself.

Next, set the multimeter to volts. Place one probe on each disconnected wire end and turn the ignition on. No voltage should show. If it does, it means the current is still flowing through the wire when it shouldn’t. Replace that wire.

Test 4: ICM Resistance

First, place the multimeter’s black probe on the vehicle’s metal frame (chassis).

The frame should be grounded properly; Otherwise, the test will fail. Ask your assistant to crank the engine by turning the ignition key on. Meanwhile, check the ICM’s terminals by placing the red probe on each of them.

You should get a reading on your multimeter. If you don’t, you may need to replace the entire ICM.

testing engine using black and red probe of multimeter

A Deeper Look at the ICM Circuit

Wires

During Check 3, we checked the wires coming out of the ICM.

Here’s a visual aid to help identify these wires:

car ignition parts labeled in A, B, C and D

There are four wires:

  • Pink wire (A): 12V Power Circuit
  • White wire (B): Ignition Control Signal
  • Black wire (C): Engine Ground Circuit
  • White wire with a black stripe (D): Switching Signal

Note: As mentioned earlier, the wires and locations may vary depending on the vehicle and model. Check your vehicle owner’s manual for the exact location of the ICM and wire specifications.

Block Diagram

The ICM block diagram below shows how it is connected to the battery and spark plugs. It explains why we tested the two, along with the connecting wires. [Ribbens, 2017]

electronic ignition control wiring diagram
Block diagram of the ICM

Operation

You will be able to diagnose a problem with the ICM much better if you understand how it is supposed to operate.

In a nutshell, the process is like this:

  1. The battery supplies 12-volt power to the ICM, ignition coil, and crankshaft position sensor (CPS), among other components.
  2. The CPS creates and sends a signal to the PCM.
  3. The PCM then sends a signal to the ICM called the IC (Ignition Control) signal.
  4. The IC signal activates the ignition coil to start sparking by giving a switching signal.

This process helps to start the engine.



References

William B. Ribbens. Understanding automotive electronics: An engineering perspective, p. 269. 8th edition. 2017.

Video References

Easy Auto Diagnostics

East auto Diagnostics

Hoohoohoblin

Oxmanagain

Phillips Vision

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About Alex Robertson

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Education: University Of Denver - Mechanical Engineering
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Hi, I’m Alex! I’m a co-founder, content strategist, and writer and a close friend of our co-owner, Sam Orlovsky. I received my Bachelor of Mechanical Engineering (B.M.E.) degree from Denver, where we studied together. My passion for technical and creative writing has led me to help Sam with this project.

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