How To Test The GM Distributor Mounted Ignition Module

The GM distributor mounted ignition control module (ICM), can be tested on the car or truck easily. Not only that, you don't need any expensive tools to do it. Now, AutoZone can test it for you (if you remove it and take it to them), but for those of you that can't afford the time this involves or for those who want to add another diagnostic technique to their ‘toolbox’ of know-how, this article is for you.

This article will walk you step by step thru' the testing and diagnostic of a MISFIRE or NO START Condition. You'll test the following components: ignition control module, spark plug wires, distributor cap and rotor and ignition coil and pick up coil of the GM 4.3L, 5.0L, 5.7L and 7.4L distributor type ignition system.

Before we start, just want to remind you that since this is an On Car test, do not remove the ignition control module from the distributor or the ignition coil. Some of the images in this article show them off of the vehicle just to make it easier to explain their testing process.

The following tutorials will also help you:

  1. How To Test The ‘Spider’ Fuel Injector Assembly (GM 4.3L, 5.0L, 5.7L) (at:
  2. How To Diagnose Misfire Codes (GM 4.3L, 5.0L, 5.7L) (at:
  3. GM Engine Compression Test (GM 4.3L, 5.0L, 5.7L) (at:
  4. How To Test A Misfire / No Spark-No Start Condition (4.3L, 5.0L, 5.7L 96-04).

You can find this tutorial in Spanish here: Cómo Probar el Sistema de Encendido (GM 4.3L, 5.0L, 5.7L) (at:

If you need to test the 7 pin (older) ignition module, the following tutorial will help: How To Test The Ignition Control Module (2.8L V6 GM).

The typical ignition system circuit diagram for the 1992-1995 4.3L, 5.0L, and 5.7L 1500, 2500, 3500 Pick Up and Suburban can be found here: Ignition System Circuit Diagram (1992-1995 Chevy/GMC Pick Up And SUV).

Basic Operating Theory

Here is a little background information (and I stress ‘little’) explained in plain english, to help you diagnose this NO START/NO SPARK Condition of the distributor. In a nutshell, when you crank up the engine (and the system is working properly):

  1. The distributor shaft starts to rotate, inducing the pick up coil to start generating its magnetic signal.
  2. This pick up coil signal is sent directly to the ignition control module.
  3. The ignition module, upon receiving this pick up coil signal (for all intended purposes it's a Crankshaft Position Sensor signal) converts it to a digital signal that is now sent to the fuel injection computer. This digital signal is called the: Distributor Reference Hi Signal in the majority of the service literature.
  4. Also, after receiving the pick up coil signal, the ignition control module starts to switch the Primary Current (of the ignition coil) On and Off. As you might already know, it's this ‘Switching Signal’ that makes the ignition coil start sparking away.
  5. OK, once the fuel injection computer receives the Reference Hi Signal, it starts activating the fuel injectors and above 400 RPM, starts to send a 5 V Bypass Signal to the ignition control module. It's with the Bypass Signal that the computer starts to retard and advance ignition timing with the IC Signal.
  6. So, then above 400 RPM (any RPM above this and the ECM considers the engine as having started) the fuel injection computer starts to control the ignition timing.

The tests that you're gonna' learn in this article only deal with steps 1 thru' 4, among several tests. But whether your car or truck DOES NOT START or STARTS but runs with a MISFIRE, this is the article for you!

Symptoms Of A BAD Ignition Control Module

The following are usually the most common symptoms of a BAD ignition control module on this type of GM distributor mounted ignition control module:

  1. The car (or truck, or mini-van, or van) will Crank but NO START.
  2. No spark coming from any of the spark plug wires.
  3. The Throttle Body Fuel Injectors do not spray gasoline.

The following are usually the most common symptoms of a BAD spark plug wires, or a BAD distributor cap and rotor on this type of GM distributor mounted ignition control module:

  1. The car (or truck, or mini-van, or van) STARTS and RUNS, but with a misfire.
  2. The check engine light is on.
  3. Lack of power.
  4. Rough idle.
  5. BAD gas mileage.
  6. Rotten egg smell coming out of the tailpipe.
  7. Black smoke coming out of the tailpipe.
  8. Won't pass the mandatory state emissions' test.

The following is usually the most common (and only) symptom of a BAD ignition coil on this type of GM distributor mounted ignition control module:

  1. The car (or truck, or mini-van, or van) CRANKS but does NOT START.