Diagnotics – ICM

Symptoms Of A Bad Ignition Module And Ignition Coil

Generally, when the ICM or the ignition coil fails… your Corvair will Crank but NOT START. Specifically, the ignition coil will not spark.

When in Tuner Pro, all things look normal and you are receiving RPM information when the engine is Cranked (about 300-600 RPM).

For this particular reason, you need to verify that the ignition coil IS NOT sparking by connecting a spark plug directly to the coil output,  grounding the plug and cranking the engine.

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 Corvair 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 (your Laptop important to have) Here is what you need to effectively use the information in this article:

  1. A multimeter
  2. A Laptop w/ TunerPro
  3. A 12 Volt test light.
  4. A helper to crank the engine.
  5. A Wire Piercing probe.

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 Distributor Pickup sensor, the ignition control module (ICM), the ignition coil.  Power is supplied by the #1 relay
  2. The Distributor Pickup Sensor sensor starts to create its VR Signal, which it sends to the ECM.
  3. The ECM, upon receiving the crank signal (along with other sensor input signals) 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

If everything is working properly, the engine will start. Now, the important thing to know is that the ECM controls the creation of spark from the get-go (crank up) and at all engine speeds thru’ the ignition control module (ICM). The good 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 Wire:
    1. Engine Ground Circuit.
  4. Circuit labeled D -White with Black stripe Wire:
    1. Switching Signal Circuit.