Hi SCOTT FRIEDMAN. Welcome to the forum. It is real common for these generators to be intermittent so you have to catch it while it's acting up to do a test on it. When it acts up next time, use a cheap digital voltmeter to measure the voltage on the small red wire plugged into the side of the generator. If it is around 2.0 volts, it means the internal voltage regulator is grounding that wire to turn on the red warning light on the dash. If you find full battery voltage, measure the voltage on the large wire bolted to the back of the generator. If it's around 12.0 to 12.6 volts, the generator is not working. (The engine must be running, of course).
If you find 0 volts on the small red wire, suspect a break in the wire between that plug and the warning light on the dash. Two common places for the break to occur are at the firewall connector and on the flexible printed circuit on the back of the instrument cluster. Those breaks on the cluster can be hard to see. You might need an ohm meter to narrow it down.
99 percent chance the generator is defective. There's no kind way to say it, this unit is a pile. GM had the second best generator in the world up through 1986. With the new design that started in 1987, it is common to go through four or more generators in the life of the vehicle. Since yours appears to be intermittent, I would suspect worn brushes. There are other more common failures that involve the special "zener" diodes needed to dampen the voltage spikes these generators produce. What more and more mechanics are finding is they can reduce the number of repeat failures by replacing the battery at the same time as they replace the generator. Your old battery will work fine in older cars, but as they get older, the battery's "internal resistance" goes up which reduces its ability to absorb those voltage spikes. Since you've already replaced the battery, you should have no trouble with a new generator.
Tuesday, May 11th, 2010 AT 2:16 PM