When I turn on the ignition without starting the engine, the in-dash voltmeter reads about 10 volts. Then I start it, and after maybe five seconds the voltage goes up to say 14.5 V.

I believe the voltage is bouncing suddenly from about 10 volts to about 14.5 volts. The headlights and panel lights go bright suddenly and dim suddenly, staying one way or the other for a half second or a second or two seconds.

I think there is some sluggishness designed into the needle of the voltmeter, and I see it moving slowly between 10 and 14.5 rather than jumping suddenly, as the lights do.

After the car warms up, the voltage doesn't change as often. The needle stays near either 10 or 14.5 for longer periods. After it gets even warmer, it is near 10 volts most of the time. Eventually it goes into the red zone below 10 and the red warning light comes on.

When the car is parked, the alarm goes off spontaneously sometimes, with the lights flashing and the horn honking.

The battery is only a year old, but the car was laid up with other problems and was driven very little during that year. So I am not sure the battery is in top shape.

I had a mechanic test the thing in a hurry. He did not disconnect the battery cables. He said the battery was okay, but that there was some problem with the alternator.

Can you say what is probably wrong with the alternator? If I just have to open it and replace something inside, I would rather do that than spend $160 for a new or reman unit.

Certainly I would rather spend a few dollars on brushes if that is the only problem.

Please advise.
_____________________________________________
1995 Buick Park Avenue Ultra, L67, VIN 1, 160K miles

I would start with checking the battery cables. I'm willing to bet you have corrosion. A bad connection would explain the alarm going off, as well as the voltage changes. Check both ends of both cables. Clean them good and use some dielectric grease.

The cause was a flaky exciter signal from the computer. Terminal "L" of the voltage regulator is an input to the alternator that causes the alternator to turn on and charge the system. The voltage regulator gets turned on when there is 12V at terminal "L". Without that signal, the alternator gives no output and the battery eventually goes dead. I suppose this is a computer problem, but I don't know.

Fixed by attaching a jumper between the fuel pump fuse (in the fuse block beneath the glove compartment) and terminal 'L". So the alternator gets turned on whenever the ignition switch is on. But "L" gets no power when the ignition is off, which is important, since the alternator would draw 1.5 amps through fuse when the alternator is not being turned by the engine and would drain the battery.

did it only fluctuate when u were at red light or stop sign with the brakes press and the engine idling????

No, it would fluctuate after say ten minutes of highway driving, starting from a cold start. The main pattern was temperature sensitivity. The voltage would drop after warming the car during cold weather.

I would say the voltage was more likely to drop when suddenly taking load off the engine--say when driving with the cruise control on, then tapping the brake pedal. In that way, maybe a slightly-lowered voltage (weakened exciter signal due to decreased RPM) would lead to a sudden voltage drop. So I would say that it had something to do with RPM, but the most obvious effect was from temperature.

I wonder whether this was due simply to a bad connection from corrosion or physical wear of the the ECU connector. Sometime I am going to unplug the ECU to see about that.

I found that the anti-theft alarm was going off due to the trunk lid being out of adjustment, nothing to do with the charging problem.

Notice also that my fix causes a drain on the battery when the ignition is on but the engine is not running. There is considerable current (on the order of an amp if I recall correctly) through the VR when the exciter circuit is on and the alternator is not turning. It counts for a little extra load when starting, probably negligible compared to the cranking amps. The computer is supposed to keep the exciter off when the engine is not running. Anyway I am sure to turn off the ignition switch when the engine is not running.