Why it might not start despite spark:
* No Fuel: Even with spark, the engine needs fuel. The ICM *does* control the fuel pump in many GM vehicles of that era. If the ICM is faulty, the fuel pump won't run, leading to no fuel injection. This is a very likely culprit given what you've been told.
* Fuel Delivery Problems (Even if the pump runs): The fuel pump might run, but there could still be issues:
* Clogged fuel filter: Restricts fuel flow.
* Bad fuel injectors: Not spraying fuel properly.
* Low fuel pressure: The pump might be weak or have a leak.
* Crankshaft Position Sensor (CKP Sensor): This sensor tells the ICM the crankshaft's position, crucial for proper ignition timing. A faulty CKP sensor can prevent the engine from starting even with spark.
* Cam Position Sensor (CMP Sensor) (less likely on a 91): Similar to the CKP sensor, but for the camshaft. Less common to cause a no-start on a 1991 Bonneville.
* Other Ignition Components: While you have spark, the spark strength might be weak due to a failing coil, wires, distributor cap, or rotor.
Troubleshooting Steps:
1. Check for Fuel:
* Listen for the fuel pump: Turn the key to the "on" position (don't crank). You should hear a brief whirring sound from the fuel pump. If not, the ICM is a prime suspect.
* Check fuel pressure: If you have a fuel pressure gauge, connect it and check the pressure. This requires more specialized tools and knowledge, so you might need a mechanic for this step.
2. Test the Ignition Control Module (ICM): This is often best done by a professional mechanic or auto parts store that can test the module. Attempting to test this yourself without a proper understanding could lead to further damage or injury.
3. Check the Crankshaft Position Sensor (CKP): This sensor is often relatively easy to access, and you can find videos online showing how to test it with a multimeter. Again, some technical knowledge is required.
4. Inspect Ignition System Components: Visually inspect the distributor cap, rotor, spark plug wires for cracks, damage, or corrosion.
In summary: The ICM is a strong possibility, especially given your information. However, a complete diagnosis requires systematically checking each component in the fuel and ignition systems. Since testing some components requires specialized tools and knowledge, taking your Bonneville to a trusted mechanic is highly recommended to save time and potentially costly mistakes. They can properly diagnose the problem and ensure it is the ICM before you replace it.
