Sorry for the delay.
Not a good idea. We need to know charging voltage, full-load output current, and "ripple" voltage. You can start the testing with an inexpensive digital voltmeter. Measure the battery's voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is okay, that only means it is okay to perform the rest of the tests, but that requires a professional load tester, and it requires the generator to be on the engine. Most bench-testers in auto parts stores use a half or three quarter horsepower motor to drive the generator. It can take well over five horsepower to run a good generator at full load. On-car testing includes the entire system in the tests, not just the one part.
Since the engineers redesigned their generators for the 1987 model year, GM has had a real big problem with voltage spikes. Those spikes can damage the generator's internal diodes and voltage regulator, and interfere with computer sensor signals. When one diode fails, the most current you will be able to get is exactly one third of the rated maximum current. 30 amps from the common 90-amp generator is not sufficient to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks. This is where the full-load output current test comes in, and is only accurate when tested on the car. A bad diode will also cause "ripple" voltage to be very high. That is not measured on test benches. Professional load testers usually show that on a bar chart, between "no ripple" and "high ripple". A few print it out as a voltage.
All generators are very inefficient at slow speeds. Your dandy observation that the charging system does appear to be working but only at higher speeds is the biggest clue there is a failed diode. The in-store testers basically show whether the generator is doing something or nothing. We need to have the system professionally tested to see exactly how much current can be developed.
If testing shows you are only getting one third of the maximum rated current, the generator will need to be replaced. These were designed to be extremely difficult to get apart without damaging some of the parts, and once you do get it apart, there is no way to test some of the individual pieces. Most importantly, it is common to go through four to six replacement generators in the life of the car. To reduce the number of repeat failures, always replace the battery at the same time, unless it is less than about two years old. The battery is the key component in absorbing and damping those harmful voltage spikes, but as they age and the lead flakes off the plates, they lose their ability to do that.
Wednesday, February 15th, 2017 AT 6:15 PM