Voltage drop

GM likes to use self-contained systems. This generator has the voltage regulator built in and is nearly impossible to replace without destroying other parts. This is one generator that is not practical to try to disassemble and repair. Also, this design was first used in 1987 models and has an extremely high failure rate. They develop huge harmful voltage spikes that destroy the voltage regulator, internal diodes, and can interfere with computer sensor signals. The battery is the key component in damping and absorbing those spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. Any time you have to replace a generator on an 1987 or newer GM product, to avoid numerous repeat failures, always replace the battery at the same time, unless it is less than about two years old. That battery will work fine in an 1986 or older model.

There are two wiring issues you should investigate first before condemning the generator. Typically it is a small brown wire that goes from the warning light on the dash to the small plug on the back of the generator that provides the turn-on signal. If you see the "battery" warning light turn on when the problem occurs, that circuit is working. If you see the warning light turn on when you turn on the ignition switch, but before the engine is running, again, that circuit is working, but if it does not turn on again when the problem occurs, a good suspect is a corroded terminal in the plug at the generator. On models that don't have a "battery" warning light, the same dash circuit is used, but there is a resistor in place where the light bulb would have been. Turn-on circuit operation is the same. You just do not have the warning light to look at for clues.

The other wiring issue is the fat output wire bolted to the back of the generator. The voltage on that wire must always match battery voltage. Today most cars have a bolted-in fuse in that circuit, but those were just starting to show up by 2000. If your car has that, be sure those nuts are clean and tight. The clue is when the problem occurs and you find as little as 12.6 volts at the battery with the engine running, you will find considerably more at the generator's output terminal, perhaps as much as 18.0 volts. A few models, particularly those with digital dashes, tap off a remote place to monitor system voltage, but on most models, system voltage is monitored right inside the generator, at the output terminal. You may never see voltage there go above the acceptable 13.75 to 14.75 volts, but the clue is still that it will be higher than battery voltage.

If those two wiring problems do not seem to be the cause of intermittent charging, it is being caused by a failing voltage regulator. To say that more accurately, it is being caused by a voltage regulator that is failing to work properly. Remember those huge voltage spikes? Before you jump on the generator, look at the age of the battery. If it is going to need to be replaced, do that first. If the problem still occurs, then replace the generator. If the problem appears to be solved, the regulator may have been responding to those spikes. It sees them as system voltage is too high, so it tries to reduce output voltage. While it may continue to see those spikes, and respond to them, you see the results as a failure to charge.

Worn brushes can also cause intermittent charging. That will get worse over the next weeks and months if you ignore it that long. At the mileage you listed, this is not a good suspect yet, but here too, replacing them is a real tedious and time-consuming job. If your time is worth five bucks per hour, you are better off just replacing the generator with a professionally-rebuilt unit with a warranty. The other problem is this is one of the few designs where there is no way to make continuity tests to identify worn brushes. To disassembly the unit, there is a real good chance you will break off the flimsy tabs on the diode block, so that will have to be replaced, then there is no practical way to test the voltage regulator. The regulator and the brushes can cause intermittent charging. With the very high difficulty in doing this repair once, you do not want to gamble that you are replacing the right part, so you need to replace everything. Just buy the replacement generator and let the professionals fight through all the grief and frustration.

I'm leaning toward worn brushes causing intermittent operation. If I'm right, the problem will act up more and more frequently over the next weeks or months, but a total failure could be months away.