ICM And Distributor Pickup Coil Tests (1989-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

TEST 2: Testing The Ignition Coil Control Signal

Now that you have confirmed that the ICM has power, the next step is to verify that the ICM is generating an IC control signal.

The IC control signal is simply the signal that activates the ignition coil to start sparking.

You've probably already checked for the IC control signal, but we'll do it again to make sure there aren't any loose ends to worry about.

The IC control signal is fed to the ignition coil via the white (WHT) wire of the black ignition coil connector.

In the figure above, the WHT wire connects to the male terminal labeled with the letter C.

NOTE: This test is performed on the ignition coil connector..

IMPORTANT: The ignition coil and the ignition control module must be connected to all of their connectors for this test to work.

These are the test steps:

1. 1

   Connect the test light's crocodile type connector to the battery positive (+) terminal.

2. 2

   Connect the metal point of the 12 Volt test light to the white (WHT) wire of the black connector.
   
   The WHT wire connects to the terminal labeled with the letter C in the illustration above.
   
   The WHT wire connects directly to the ignition control module and the is the one that delivers the ignition coil activation signal.

3. 3

   Have your helper crank the engine while you observe the 12 Volt test light.

4. 4

   You'll see one of two things:
   
   1.) The 12 Volt test light flashes ON and OFF.
   
   2.) The 12 Volt test light DOES NOT flash ON and OFF

Let's take a look at your test results:

CASE 1: The 12 Volt test light flashed ON and OFF. This test result confirms that the ignition control module is creating and delivering an activation signal to the ignition coil.

You can conclude that the ignition control module is working correctly and that it is not defective. If the engine is not starting, it's not because of the ignition control module.

CASE 1: The 12 Volt test light DID NOT flash ON and OFF. This test result tells you that the ignition control module is not activating the ignition coil.

Your next step is to go to: TEST 3: Testing The Pickup Coil Signal.

TEST 3: Testing The Pickup Coil Signal

For our final test, we'll check the output of the distributor pickup coil with the engine cranking.

Because the location of the distributor makes performing this test a little tricky, I recommend that you use insulated jumper wires with alligator clips to connect your multimeter test leads to the distributor pickup coil.

NOTE: You'll need to probe the female terminals of the pickup coil's connector. Be careful that the test leads you connect to the female terminals do not damage them.

IMPORTANT: The battery must be fully charged for this test.

CAUTION: You'll need to crank the engine to perform this test. Be careful and take all necessary safety precautions.

These are the test steps:

1. 1

   Place the multimeter in Volts AC mode.

2. 2

   Disconnect the pickup coil connector from the ignition control module.

3. 3

   Connect the black test lead of the multimeter to one of the female terminals of the connector (using an appropriate test lead).
   
   In case you're wondering, the polarity of the terminal doesn't matter, and you can use either of the multimeter's probes (red or black) to test the terminal.

4. 4

   Connect the red test lead of multimeter to the other female terminal of the pickup coil connector.

5. 5

   Have an assistant crank the engine while you observe the AC voltage readings on the multimeter.

6. 6

   As the engine is cranking, the AC voltage readings should fluctuate between 0.3 Volts and 1.8 Volts AC.
   
   If you're using an oscilloscope, see the photo below of the pickup coil signal waveform.

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

CASE 1: The multimeter registered the indicated AC Volts. This is the correct test result and it tells you that the pickup coil is producing its signal.

You can conclude that the ignition control module is bad and needs to be replaced if you have:

• Confirmed that none of the spark plug wires are sparking.
• Confirmed that the ignition coil tower IS NOT sparking.
• Confirmed that the ignition coil and ICM are getting power (TEST 1).
• Confirmed that the ignition coil IS NOT getting an activation signal (TEST 2).
• Confirmed that the pickup coil is producing its signal.

CASE 2: The multimeter DID NOT register the indicated AC Volts (or an analog waveform). Recheck all connections. Try again.

If you still have nothing. The pickup coil is faulty and the cause of the engine's no spark-no-start problem. Replace the pickup coil.

Distributor Pickup Coil Waveform

If you have access to an oscilloscope, this is what the pickup coil's waveform should look like.

More 2.5L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials

You can find a complete list of 2.5L Chevy S10 pickup, GMC S15 pickup, And GMC Sonoma tutorials here:

• S10, S15, Sonoma 2.5L Index of Articles.

Here's a sample of the tutorials you'll find in the index:

• How To Test Engine Compression (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
• How To Test For A Blown Head Gasket (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
• How To Test The TPS (1987-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
• How To Test The MAP Sensor (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).