Boy, I hear you about older cars. For me it's the unnecessary use of computers for things we used to have but didn't need computers for, like power locks and windows. I drive an old rusty, trusty '88 Grand Caravan as a daily driver in Wisconsin, the road salt capital of the world. My newer cars sit because I don't trust them as much.
Anyway, when Chrysler sticks the battery in a really stupid place, they provide jump-start terminals under the hood. Those are connected right to the battery so testing can be done there. You will also have an under-hood fuse box with a smaller positive battery wire connected to it. You can use that, and you can use the larger output wire bolted to the back of the generator. A fully-charged battery will read 12.6 volts. If you find around 12.2 volts, the battery is good but it's discharged. If it has a shorted cell, you'll find closer to 11 volts. For ground you can use the engine block or a paint-free point on the body, like a ground screw.
The next test is the same, but with the engine running. You must find between 13.75 and 14.75 volts. If it's low, suspect the generator. If it's a little high, suspect the generator too. Normally when it's low you won't see any change between engine off and running. When it's high, it's usually because there's one bad diode of the six. That will result in the unit producing a maximum of exactly one third of its maximum rated current. 30 amps from the common 90 amp generator isn't enough to run the entire electrical system under all conditions, so the battery will have to make up the difference, until it runs down. These are three-phase generators, and with a bad diode, you'll be missing one phase. That makes "ripple" voltage very high. The voltage regulator may respond to those dips resulting the average output voltage being a little higher than 14.75 volts.
I don't know what's different about your generator that they can't test it, but GM did have a huge problem with them starting with the '87 models. They develop harmful voltage spikes due to their design. Those spikes can destroy the internal voltage regulator or those diodes. It's real common to go through four to six replacements in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. As they age they lose their ability to dampen and absorb those spikes.
The voltage test is just the first part in testing a generator. They also should be tested for maximum current output and that ripple voltage I mentioned. You need a professional load-tester for that.
Monday, November 25th, 2013 AT 6:34 PM