How To Test The GM Ignition Control Module (1995-2005)

This is one of the easiest GM ignition control modules to test. This article will take you step by step through the whole process of diagnosing and troubleshooting a BAD ignition control module and ignition coil.

The ignition control module (ICM) and ignition coil tests in this article assume that your vehicle is Cranking but NOT STARTING due to a NO SPARK Condition. If you're GM car or truck starts and runs, this article will not help you.

Now, if you need to test a misfire condition, or test the spark plug wires, or Distributor Cap on this type of GM ignition system, click here: How To Test A Misfire / No Spark-No Start Condition GM 4.3L, 5.0L, 5.7L Distributor Ignition System (1996-2004).

Since so many things can cause your vehicle to not start (like a bad fuel pump, etc.), the following tutorial may be of help to you:

  1. How To Troubleshoot A No Start (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).

NOTE: The following ignition system circuit diagram may be of help: Ignition System Circuit Diagram (1996-1999 Chevy/GMC Pick Up And SUV).

At the end of this tutorial I've included a list of other tutorials that will help you diagnose a Cranks but Does Not Start Condition (in case it's not ignition system related).

You can find this tutorial in Spanish here: Cómo Probar el Módulo de Encendido GM 4.3L, 5.0L, 5.7L (1995-2005) (at: autotecnico-online.com).

Symptoms Of A Bad Ignition Module And Ignition Coil

Generally, when the ICM or the ignition coil fails, your GM car or truck will Crank but NOT START. Specifically, the ignition coil will not spark.

For this particular reason (and before you follow the test procedures in this tutorial), you need to verify that the ignition coil IS NOT sparking by connecting a spark tester directly on the ignition coil's tower and cranking the engine.

This is a pretty easy test, but it must be done with a spark tester. You can find the test steps here: TEST 4: Spark Test at Ignition Coil Tower (this test belong to this tutorial: How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04)).

NOTE: When the ignition coil or the ICM start to fail, they don't always cause a cranks but does NOT start condition, since sometimes these components can fail intermittently. In this scenario, your vehicle will start and run most of the time, but every now and then it won't. If this is the case, you'll have to wait till the car or truck does not start to use the tests in this article.

What Tools Do I Need?

You don't need a whole lot of stuff, heck, you don't even need a scan tool (tho' a scan tool is important to have, but not for this article)! Here is what you're gonna' need to effectively use the information in this article:

  1. A multimeter (don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing).
  2. An LED Light. To see a photo of this tool click here: Abe's LED Light.
  3. A 12 Volt test light.
  4. A helper to crank the engine.
  5. A Wire Piercing probe. This tool is a time saver. If you need to see what this tool looks like, click here: Wire Piercing Probe.

As you can see/read, these are just basic tools, nothing fancy, extravagant or EXPENSIVE.

Basic Operating Theory

Here is a little background information to help you diagnose the ignition control module (ICM) or the ignition coil. In a nutshell, when your turn the key and start cranking the engine:

  1. Power in the form of 12 volts flows into the crankshaft position (CKP) sensor, the ignition control module (ICM), the ignition coil.. among several things and...
  2. The crankshaft position (CKP) sensor starts to create its Crank Signal, which it sends to the PCM.
    1. The crankshaft position sensor is a Hall Effect type sensor and produces a digital (On/Off) signal that can be seen with an LED or an oscilloscope. On an oscilloscope, it produces a digital square waveform.
  3. The PCM, upon receiving the crank signal (along with other sensor input signals) start to do its little song and dance and sends the ignition control module (ICM) a signal called the IC Signal (IC=Ignition Control).
  4. With this IC Signal, the ignition module starts to activate the ignition coil to start sparking with a Switching Signal.
    1. As you're probably already aware, the Switching Signal is just a term that describes the switching ‘on and off’ of the Primary Current's path to ground (the Primary Current refers to the 12 volts supplied to the ignition coil).

If everything is working properly, the engine will start. Now, the important thing to know is that the PCM controls the creation of spark from the get-go (crank up) and at all engine speeds thru' the ignition control module (ICM). The really cool thing about this type of ignition system is that all of these signals (IC Signal and the Switching Signal) can be tested easily with some very simple tools.

Ignition Control Module (ICM) Circuit Descriptions

Ignition Control Module (ICM) Circuit Descriptions. How To Test The GM Ignition Control Module (1995-2005)

The ignition control module (ICM) has 4 wires coming out of it. Below are the descriptions of what each circuit does. Each circuit is identified by a letter, and this is the same letter that you will find on the ignition module's connector.

  1. Circuit labeled A -Pink Wire:
    1. Power (12 V) Circuit.
  2. Circuit labeled B -White Wire:
    1. Ignition Control (IC) Signal.
  3. Circuit labeled C -Black with White stripe Wire:
    1. Engine Ground Circuit.
  4. Circuit labeled D -White with Black stripe Wire:
    1. Switching Signal Circuit.

The following ignition system circuit diagram may be of help: Ignition System Circuit Diagram (1996-1999 Chevy/GMC Pick Up And SUV).

Ignition Coil Circuit Descriptions

Ignition Coil Circuit Descriptions. How To Test The GM Ignition Control Module (1995-2005)

The ignition coil on your car or truck may have or may not have 3 wires coming out of it. Whether it does or doesn't doesn't matter, the info in this test article still applies. Below are the descriptions of what each circuit does. Each circuit is identified by a letter, and this is the same letter that you will find on the ignition coil's connector.

  1. Circuit labeled A -Pink Wire:
    1. Power (12 V) Circuit.
  2. Circuit labeled B -White Wire:
    1. Tach Signal for the tachometer in the instrument cluster if equipped.
  3. Circuit labeled C -White with Black stripe Wire:
    1. Switching Signal Circuit. This wire (circuit) comes from the ignition module.

The following ignition system circuit diagram may be of help: Ignition System Circuit Diagram (1996-1999 Chevy/GMC Pick Up And SUV).