Be aware too that generators can not be fully-tested off the car. Generator test benches typically use a one horsepower motor to run the generator. On the car they can easily take over five horsepower to get maximum output. The best the seller can assure you is the unit is putting out something, but not how much it is capable of.
Once it is installed on your car, have the charging system tested with a professional load tester for maximum output current and "ripple" voltage. I found three generators listed for your car. They're 90 amp, 120 amp, and 130 amp units. During the full-load test, which only takes a few seconds, the generator will deliver very close to its rated value, or exactly one third of it. With one failed diode of the six, you'll lose two thirds of the output capacity. Even the smaller 90 amp generator is pretty hefty compared to years ago, but if it can only produce 30 amps, that is not enough 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.
With a failed diode, "ripple" voltage will also be very high. That is what leads to harmful voltage spikes, especially so on '87 and newer GM generators. Those spikes can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. They are a big cause of elusive engine running problems that defy diagnosis, AND those repeat generator failures I mentioned.
The battery is the key component in damping and absorbing those voltage spikes, but as the lead flakes off the plates as they age, they lose their ability to do that. That's why I wanted to mention this. If your replacement generator is capable of developing its rated current, please be sure to look at the battery and replace it if it's over two years old. If you don't, and it causes your new generator to fail, you'll be making another 130-mile trip.
The most common failure in a generator is worn brushes, and those CAN be verified as working on a test bench. You didn't list the mileage so I can't make any judgements there, but if the replacement generator came from a car with mileage similar to yours, you might expect the same failure soon. Of course you won't know the history of the part you're getting. It could be a rebuilt unit that was just installed, then he guy piled up his car a week later. You run the risk of getting a generator that's just as good as a rebuilt one from the parts store.
I understand that family and friends are happy to have you get the part because that lets them off the hook if it fails but the labor should cost less because they're your friends, not because you got the part. Their labor charge should be the same regardless where the part came from.
Saturday, March 28th, 2015 AT 10:54 PM