95 Deville loses power - possibly electrical?

BUSHJC

MEMBER
1995 CADILLAC DEVILLE
170,000 MILES

1995 Deville - car will sometimes "miss" or act like it is starving for gas - usually happens after it has been sitting for a while. When the car begins to miss, I can put it in Neutral, turn off engine and restart and the issue stops. - The car has had new plugs, wires, fuel filter and alternator. When I replaced the alternator, the car ran perfect for 4 days, then same problem started again - I do notice that when driving at night and I use the brakes, when the brake lights come on, the headlights will quickly dim for just an instant. Also, when driving yesterday, I pressed one of the power window buttons and the car lurched as if losing power.

1 Reply

CARADIODOC

MECHANIC
33,873 POSTS

Chances are the new generator is bad already. A load test will confirm that. There's no way to sugar-coat it. GM went from the second best AC generator to the world's worst pile ever beginning with the '87 model year. It is real common to go through four to six of them in the life of the vehicle, but what many professionals are finding out is to reduce the number of repeat failures, replace the perfectly good battery at the same time. These generators, due to their design, produce huge voltage spikes just like an ignition coil is supposed to do. Those spikes destroy the internal voltage regulator and diodes, and the spikes can "induce" voltage spikes into other wires. If those other wires are for computer sensors, the computers will get confused and do strange things. As the battery ages, it loses its ability to dampen and absorb those spikes. The old battery will work fine in an '86 or older car.

During the load test, if one of the six diodes is bad, you will only be able to get one third of the rated capacity from the generator. That's around 30 amps for a common 90 - 100 amp unit. 30 amps isn't enough to meet all the demands from the electric fuel pump, heater fan, head lights and tail lights, and ignition system and numerous computers. In addition, with a bad diode, one of the three output phases will be missing resulting in very high "ripple". Most professional load testers display ripple as a bar graph, not as an actual number. The goal is to have very low ripple. With high ripple, suspect a damaged diode. The fix is to replace the generator again and replace the battery.