Car firing system shut down

DOUGLAS SOHNREY

MEMBER
1993 CADILLAC DEVILLE
4.9L
V8
FWD
AUTOMATIC
168,000 MILES

My idle was running rough, at times the car would shut down, I would pull over and it would start almost every time. I replaced the gas cap. Then sitting at a light the dash lights went off, no power what so ever. We pulled over, tried to start the car, the only lights that came on were oil, engine battery lght, that was all. I checked the battery conection, good, checked my fuses, good, checked all wire connections, good. No turn over, no firing, nothing. Is it the control module under the distributor again? I just replaced that about 15,000 miles ago. Is there another brain? 1993 cadillac sedan deville 4.9l

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

This isn't a computer problem. The engine isn't cranking, so computer controls aren't even in the picture yet.

You said the engine would stall at times, but you didn't say if this occurred over minutes, days, or weeks. Start by measuring the battery's voltage with a digital voltmeter. It will be 12.6 volts if it's good and fully-charged. If you find 12.2 volts, it's good but discharged. If it's closer to 11 volts, it has a shorted cell and must be replaced.

If the voltage is low, charge the battery at a slow rate for an hour, then see if the engine cranks and starts. If it does, measure the battery's voltage again with the engine running. You must find between 13.75 and 14.75 volts. If it is low, suspect the generator. If it is okay, that only means it is okay to perform the second half of the tests, and that requires a professional load tester to test for full-load output current and "ripple" voltage. If the generator has one failed diode of the six, you will only be able to get exactly one third of its rated current and ripple voltage will be "high". 30 amps from the common 90 amp generator is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference and will run down slowly over days or weeks.

Since GM redesigned their generators for the '87 model year, they have had a huge problem with repeat failures. If your generator needs to be replaced, to reduce the number of those repeat failures, replace the battery at the same time unless it is less than about two years old. These generators develop huge voltage spikes that can damage the internal diodes, internal voltage regulator, and interfere with computer sensor signals. The battery absorbs and dampens those harmful voltage spikes but as they age, they lose their ability to do that. The old battery will still crank the engine just fine, and it can be used in any other car as long as it's not a GM product.