There's no single definitive test, but here's a breakdown of common methods and what they indicate:
1. Visual Inspection:
* Look for obvious damage: Check the ICM for any physical damage like cracks, burn marks, or loose connections. A visibly damaged module is likely faulty.
2. Testing for Power and Ground:
* Locate the ICM: This is usually mounted on the firewall or near the distributor. Consult your vehicle's wiring diagram to identify the correct ICM and its connections.
* Identify power and ground: The wiring diagram will show the power and ground wires. Typically, there's a 12V power source and a ground wire.
* Use your multimeter: Set your multimeter to DC voltage. Connect the positive lead to the power wire and the negative lead to a known good ground. You should read 12V with the ignition key ON. Repeat for the ground wire; you should read 0V relative to the battery negative. A lack of power or a poor ground indicates a wiring problem, not necessarily a bad ICM.
3. Testing the Output Signal (Most Difficult and Requires Caution):
This is the most challenging test and requires specialized tools or a good understanding of signal waveforms. This should only be attempted if you have experience working with automotive electrical systems.
* Caution: This involves working with high voltage. Wear safety glasses and ensure the ignition coil is disconnected to prevent shocks.
* You'll need a helper: One person operates the ignition switch, while the other monitors the multimeter.
* Method: Connect the multimeter to the ICM output signal wire(s) going to the ignition coil. Set the multimeter to measure AC voltage (the signal will be a pulsed signal). With the ignition switched ON, crank the engine. You should see a pulsed AC voltage signal. The absence of a signal strongly suggests a faulty ICM. The waveform's exact characteristics are specific to your vehicle and are hard to describe without the factory service manual.
* Alternative method (using a test light): Some mechanics use a test light to observe the pulsed signal. The test light will flash rapidly when the engine is cranked. Again, the absence of flashing suggests a problem.
4. Resistance Testing (Less Reliable):
* While some sources suggest resistance testing, this is generally unreliable for ICMs. The internal circuitry is complex, and resistance readings may not accurately reflect functionality.
Important Considerations:
* Wiring Diagram: A wiring diagram specific to your 1987 GMC Suburban is essential for accurate testing. You can usually find these online through forums dedicated to classic GMCs or from a repair manual.
* Other Components: A faulty ignition coil, crankshaft position sensor (CKP), or distributor can also cause symptoms similar to a bad ICM. These components should be checked before concluding the ICM is at fault.
* Professional Diagnosis: If you're uncomfortable performing these tests, it's best to take your Suburban to a qualified mechanic for diagnosis. Misdiagnosis can lead to unnecessary part replacements.
Remember, safety is paramount. If you are unsure about any step, seek professional help. Replacing the ICM is usually relatively straightforward once you have confirmed it is faulty, but improper testing could lead to more serious issues.
