Charging System problem
 

I have a problem with my charging system in that I have low voltage, 12.9 volts rather than the normal 14-15 volts, from my alternator. I had the alternator tested and it is okay. I replaced the alternator and still have the same problem. I have checked the wires from the alternator to the PCM and they have good continuity. What I do not have is a schematic of the PCM internals and where wires go from there. I need to know how to test the PCM and what else I can do to find and repair the problem. Thanks for any help.

This is pretty common voltage that you are experiencing.

What year and model to you have? There is a way to bring the voltage up. I and a few others added a pin to the alternator connector in the S or Sense position, then hooked it into a 12+V pink wire (take your choice) in the wiring loom on the back of the motor. This takes the alternator from internally sensing voltage to sensing the 12+ actually running through the car.

Thanks, Bill, but I'm not sure that I really want to do that. My Bonneville is a 1997 and is all original as purchased. It still looks, runs, and drives like it did when new. Also, I have been told I could do that as a test, but that to do so permanetly could cause an overcharge condition, putting out too many amps.

What I really would like to find is a schematic of the internal wiring for the PCM and a complete wiring schematic for the entire charging system so that I can diagnose the problem and fix it even if that means a new PCM. Now I admit that I am not the sharpest knife in the drawer, but I have had to chase down some problems on fighter jets weapons systems in the Air Force. We could not change wiring to patch a problem, we had to fix it and that's just the way I've done things ever since even though I've been out of the Air Force for over 30 years.

Again, thanks for your reply and if you know where I can get that schematic, I'd really appreciate it.

I don't have anything online that I can send you. After looking into the system extensively though I do know that the alternator field is controlled by a single wire from the PCM. It is referred to as the turn on lead. The alternator control from the pcm is either field on or field off. All the voltage regulation happens internal to the alternator.

I personally swapped about 5 alternators on my 97 trying to get the voltage up as you would like to do. In the end I found that you can correct the issue by supplying the alternator with the voltage of the vehicle and adding one wire will correct the low condition you see. I also polled about 10-15 other 97 Bonnie owners and found they all had the same condition.