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Question Car: 1990 Pontiac Grand Prix 6 cylinder 3.1 liter 42,000 miles. Bought the car about 3 months ago. Ran great. This week the car started running rough. It feels like I'm running on only 4 cylinders. Does this car have the 3 coil system? And can I do a timing light check on each spark plug wire to see if it's getting the signal to fire? Can you suggest any other checks? Thanks in advance. Kevin

Reply Hi Kevin, thanks for writing. Most 1990 Grand Prix 3.1L came with DIS ignition which uses 3 ignition coils, each in series with two cylinders. To test the coil output you can use a spark tester or disconnect both spark plug wires from each suspect coil and crank the engine. The spark should be able to arc across the two output terminals per coil. If not, you either have a bad coil or a bad control module. Resistance checks on the coil will determine if the coil is OK. Switching the coils on the module will determine if the module is working. 99.9% of the time both the module and the coil need replacement because out of specification coil resistance causes a module failure. Note, if spark exists at all of the coils inspect the spark plugs, spark plug wires, compression, etc. Good luck with it, Angelo Silverado Auto Service

Followup Angelo, Thanks for the reply. I tested with the timing light and found 1 of the 3 coils not working. It was for cylinders 1 and 4. I pulled the coil out, and the resistance measures on the primary side, less than 1 ohm, and about 6000 on the secondary. Both are in spec. I then heated the coil to about 250 F and repeated the test. Both values climbed a little, but not out of spec. I guess I'm looking at a module repair. I'm going to swap coil positions to see if it follows the coil, or stays in the same module position. Thanks for the return message. Kevin

Followup Angelo, You were right on the money. I diagnosed the coils to be "in spec". Your message about the ignition modules gave me the confidence to replace it myself. The dealer wanted $60.00 per hour and a part charge of $200.00 I did the job for $150.00. My only comment is...what a terrible location for an electronic module! Down low on the engine, just above the starter, and just below the exhaust manifold. Thanks again for your help, Kevin