Intermittent no-start condition?

ECRAIG1978

MEMBER
1997 CHEVROLET CAMARO
5.7L
V8
2WD
AUTOMATIC
100,000 MILES

I can confirm I'm getting 43 lb fuel pressure, I have tested injector signal with Noid lights, I run through the "engine cranks but won't start" from the Chevrolet service manual and it leads me to check ignition system. Following the guide, it tells me to eventually replace the ICM. I have replaced it twice; however, it doesn't always fix the issue. On the ICM harness I get 12 volts at A and D, I get between 1- and 4-volts AC when cranking and testing terminal B and manual says replace ICM. Not sure where else to test. Any assistance testing the ignition (coil, ICM and Opti) would be appreciated. I have an oscilloscope and would like to test the high and low reference signal but am not sure where to test this.

Eric

1 Reply

KEN L

MASTER CERTIFIED MECHANIC
42,959 POSTS

When troubleshooting an intermittent no-start condition on a Chevrolet vehicle, it's important to carefully diagnose the ignition system components, including the ignition coil, ignition control module (ICM), and the OptiSpark distributor. Here's a step-by-step guide to help you test these components:

Ignition Coil:

Disconnect the ignition coil connector.
Using an ohmmeter, measure the resistance between the coil's primary terminals (the two smaller terminals on the connector). The resistance should typically be around 0.5 to 0.9 ohms, but consult the vehicle's service manual for the specific resistance range for your model.
If the resistance is out of range, replace the ignition coil.
Ignition Control Module (ICM):

Ensure that you have a good connection between the ICM and the distributor.
Connect your oscilloscope to the ICM terminals for testing.
Check the ICM's low reference signal by connecting the oscilloscope's negative lead to battery ground and the positive lead to the ICM terminal labeled "B" (this is the low reference signal wire).
Crank the engine and observe the waveform on the oscilloscope. The low reference signal should be a square wave pattern with a voltage of around 0.2 to 0.3 volts.
If the low reference signal is abnormal or absent, check the wiring and connections between the ICM and the ECM. Repair any faults found.
If the low reference signal is normal, proceed to the next step.
OptiSpark Distributor:

Disconnect the OptiSpark connector.
Check the voltage supply to the OptiSpark by connecting your oscilloscope's negative lead to battery ground and the positive lead to the terminal labeled "C" (this is the voltage supply wire).
Crank the engine and observe the waveform on the oscilloscope. The voltage supply signal should be a stable 5 volts.
If the voltage supply is abnormal or absent, check the wiring and connections between the ICM and the OptiSpark. Repair any faults found.
If the voltage supply is normal, reconnect the OptiSpark connector.
High and Low Reference Signals:

Connect your oscilloscope's negative lead to battery ground and the positive lead to the terminal labeled "A" (this is the high reference signal wire) on the ICM connector.
Crank the engine and observe the waveform on the oscilloscope. The high reference signal should be a square wave pattern with a voltage of around 5 volts.
If the high reference signal is abnormal or absent, check the wiring and connections between the ICM and the ECM. Repair any faults found.
If you've followed these steps and still experience intermittent no-start issues after replacing the ICM, it's possible that there may be other underlying issues affecting the ignition system.