GM cars since the '87 models have had a very poor generator design that causes a lot of trouble. It is common to go through four to six of them in the life of the car, but you already did the best thing to reduce the number of those repeat failures. That is to replace the battery at the same time unless it is less than about two years old. In this case though, you have something else going on. These generators produce large voltage spikes that destroy the internal diodes and voltage regulator, (a good battery will dampen and absorb those spikes but they lose their ability to do that as they age), but those failures are almost always permanent.
You have an intermittent problem and any testing must be done while the problem is occurring. First of all look if the battery warning light turns on when the voltage drops. If it does, the voltage regulator knows there's a problem. If the light doesn't turn on, stop the engine, then turn the ignition switch back on. If the battery light does not turn on, there is a break in the wire going from the instrument cluster to it.
Restart the engine. If the warning light goes out but the gauge still shows low voltage, most likely the generator is working but the current isn't getting back to the battery. Measure the voltage at the battery and at the large bolted-on output terminal on the back of the generator. The two must be the same, and must be between 13.75 and 14.75 volts. If the generator voltage is higher, there is a break in that wire. Look for a large bolted-in fuse in the under-hood fuse box. Be sure those nuts are tight.
Another way to do the same thing is to stop the engine, then measure the voltage at the generator's output terminal. If you find 0 volts or something lower than the battery's voltage, there's a break in that wire or that fuse has a bad connection.
Sunday, November 4th, 2012 AT 1:37 AM