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

TEST 3: Testing For The Ignition Coil Switching Signal

In this test step, you're gonna' test to see if the ignition module is feeding the ignition coil with a Switching signal.

If the switching signal is present, then you can accurately conclude that the ignition coil is bad and needs to be replaced.

If the switching signal is NOT present, the your next step is to check that the ICM is being activated by the PCM (TEST 4).

How do you test for the switching signal? With an LED light and in the test steps below I'll tell you how to connect it.

NOTE: After connecting the LED light, it may (or may not) come ON as soon as you turn the key on (but without cranking the engine). This is nothing to be concerned about because the LED light result you're looking for is a flashing ON and OFF (of the LED) while your helper is cranking the engine only.

You'll be testing this signal at the ignition control module (ICM) itself:

1. 1

   This test is performed with the ignition coil connector disconnected.

2. 2

   Connect the red wire of the LED to the battery positive (+) terminal.

3. 3

   Connect the BLACK wire of the LED to the D circuit of the ignition control module. This is the WHITE with BLACK stripe wire of the connector. Make sure that you're installing everything in it's proper place.

4. 4

   Have an assistant crank the engine.

Let's take a look at what your test results mean:

CASE 1: The LED light flashed ON and OFF the whole time the engine was cranking. This result lets you know that the ignition coil is bad. Replacing it will solve your no-start condition.

To be more specific, this test result tells you that the ignition coil is bad because:

• You confirmed that the ignition coil and ignition control module (ICM) are getting power (TEST 1).
• That the ICM is getting Ground (TEST 2).
• That the ignition coil is being fed the switching signal (confirmed in this test).

Since all the signals are present (power, Ground, and the switching signal), you can confidently conclude that the ignition coil is bad and needs to be replaced.

CASE 2: The LED Light DID NOT flash ON and OFF the whole time the engine was cranking. Recheck all of your connections and repeat the test.

If still no flashes from the LED Light (usually because the LED light just stayed lit or just stayed OFF), then this result eliminates the ignition coil as the source of the NO SPARK, NO START problem.

To be more specific, this test result tells you that:

• The ignition coil is OK (as in not defective).
• That the ignition coil is not sparking due to the fact that the ICM is not activating it.

Your next step is to check that the ICM is getting a triggering signal from the PCM. For this test go to: TEST 4: Testing For The ICM Trigger Signal From The PCM.

TEST 4: Testing For The ICM Trigger Signal From The PCM

In this test step, your mission is to test for the Triggering signal that the PCM (Powertrain Control Module=Fuel Injection Computer) provides to the ignition control module (ICM) using the same LED light that you used in TEST 3.

This Triggering signal is called the IC (Ignition Control) signal in the GM service literature.

Now, if the PCM is NOT providing the triggering signal the ICM needs to activate the ignition coil (to spark), then the LED light will only stay lit and not flash ON and OFF as you crank the engine.

NOTE: The LED light may light up as soon as you turn the key ON and this is normal. The specific action you're looking for, from the LED light, is an ON and OFF flashing as your helper cranks the engine.

This test is accomplished with the ICM connected to its connector:

1. 1

   With key in the OFF position.

2. 2

   Reconnect all connectors.

3. 3

   With a suitable tool, pierce the WHITE wire of the ignition control module connector. This is the B circuit of the ignition control module connector. Pierce as far back as possible from the connector itself.

4. 4

   Connect black wire of the LED to the tool that is piercing thru' the wire.

5. 5

   Connect red wire of the LED to the battery positive (+) terminal.

6. 6

   Have an assistant crank the engine while you observe the LED.

Let's take a look at what your test results mean:

CASE 1: The LED light flashed ON and OFF the whole time the engine was cranking. This result lets you know that the ignition control module (ICM) is bad. Replacing it will solve your no-start condition.

To be more specific, this test result tells you that the ignition control module (ICM) is bad because:

• You confirmed that the ignition coil and ignition control module (ICM) are getting power (TEST 1).
• That the ICM is getting Ground (TEST 2).
• That the ignition coil IS NOT being fed the switching signal (TEST 3).
• That the ICM is getting the triggering signal from the PCM (confirmed in this test).

Since power, Ground, and the triggering signal from the PCM are present but NOT the switching signal, you can confidently conclude that the ignition control module (ICM) is bad and needs to be replaced.

CASE 2: The LED light DID NOT flash ON and OFF the whole time the engine was cranking. Recheck all of your connections.

If, after re-checking your connections and repeating the test, the LED light just stays lit and DOES NOT flash ON and OFF as your helper cranks the engine, then this result eliminates the ICM and the ignition coil as the source of the no-spark no-start problem.

To be more specific, this test result tells you that:

• The PCM is not sending the triggering signal to the ICM.
• That the ICM is OK. In other words: the ICM is not bad (since without this triggering signal it cannot activate the ignition coil to spark).

The most likely causes are going to be: 1) A bad crankshaft position sensor or 2) an open in this circuit between the ignition module and the PCM or 3) a bad PCM, although this is rare.

Your next step is to test the crankshaft position sensor. You can find the tutorial (that'll show you how to test the crank sensor) at troubleshootmyvehicle.com by clicking here: How To Test The Crank Sensor (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).

Related Articles/Tutorials

The following articles/tutorials will be a great source of information to further help you diagnose/troubleshoot your vehicle:

• Troubleshooting the fuel pump (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).
• How To Test The ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).
• Troubleshooting A Blown Head Gasket (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).
• Coolant Leaking From Intake Gaskets (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).
• How To Test The Crank Sensor (GM 4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).