1. Verify Power and Ground:
* Battery Voltage: Ensure the battery is fully charged and the terminals are clean and tight. A low voltage can prevent the ignition system from functioning correctly. Use a voltmeter to check battery voltage directly at the terminals.
* Ignition Switch: Check the ignition switch itself. A faulty switch might not be supplying power to the ignition system even with the key turned to "on" or "start." You might need a wiring diagram to test the power coming from the ignition switch.
* Fuse/Relay: Check all fuses and relays related to the ignition system (often located in the underhood fuse box and possibly an interior fuse panel). A blown fuse or failed relay will cut off power. Refer to your owner's manual for fuse/relay locations and ratings.
* Grounds: Look for corroded or loose ground connections in the engine bay. Poor grounds can severely disrupt the electrical system. Pay close attention to the engine block's ground straps. Clean and tighten any suspect connections.
2. Ignition Control Module (ICM):
* Even though you've replaced the ECM, a faulty ICM (often integrated into the distributor on some 3.1L engines, but some were separate) can prevent spark. If your engine has a separate ICM, this is a critical component to check or replace. They are relatively inexpensive.
3. Distributor (if applicable):
* Rotor and Cap: Inspect the distributor cap and rotor for cracks, carbon tracking (black marks), or corrosion. Replace them if necessary.
* Pick-up Coil: Inside the distributor is a pick-up coil. This is less likely but could be causing problems. Testing it usually requires an oscilloscope or a specialized ignition system tester.
* Distributor Shaft: Verify the distributor shaft is turning properly when the engine is cranked. It might be seized or have internal damage.
4. Wiring:
* Harness Inspection: Carefully examine the wiring harness for any damaged, frayed, or corroded wires, particularly those leading to and from the coil pack, crank sensor, ICM, and distributor. A simple short or break can prevent spark.
* Continuity Testing: Use a multimeter to check for continuity in the ignition system wiring. This will help identify broken wires. You'll need a wiring diagram to trace the circuits correctly.
5. Professional Diagnostic:
If you've checked all of the above and still have no spark, it's time to consider professional help. A mechanic with an oscilloscope or a sophisticated scan tool can accurately diagnose more complex problems within the ignition system. They can perform tests like checking for signal waveforms at the crank sensor and ICM outputs.
Important Note: Always disconnect the negative battery terminal before working on any electrical components in your vehicle.
Finding the cause without a wiring diagram will be challenging. You'll need a wiring diagram specific to your 1990 Chevy Lumina 3.1L to effectively test the various circuits. These can often be found online (some sites sell them) or at your local auto parts store. Good luck!
