First of all, dash gauges are notoriously inaccurate. All they're good for is for you to notice when something is not normal. The place to start is by measuring battery voltage when the engine is running. It must be between 13.75 and 14.75 volts. (In particular, I need to know if it goes outside that range when the lights go bright or dim.
Second, there's no way to sugar-coat this. Your generator is the world's worst design. GM had probably the world's second best generator from the early '70s through 1986. For the '87 model year they redesigned them and have had a huge problem. Without going into lots of electrical theory, they develop large voltage spikes, just like an ignition coil does, and those spikes can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. It is common to go through four to six replacement generators in the life of the vehicle, but to reduce the number of repeat failures, you must replace the battery at the same time unless it is less than about two years old. The battery is the key component in damping and absorbing those voltage spikes, but as they age and the lead flakes off the plates, they lose their ability to do that.
Given that you have an uncommon battery, even though it's not that old, it sounds like you're running into this same problem. I suspect there is a characteristic of your battery that is not in the best interest of helping the generator survive. The lead, for example, could be packed onto the plates very densely to provide a longer reserve capacity, but that could reduce its ability to absorb rapidly-occurring voltage spikes. It sounds like you need another replacement generator, but if you have problems again, replace the battery with what is supposed to be in your vehicle. Don't fall for any gimmicks or advertising hype.
Measuring battery voltage with the engine running is only the first half of the test. For the next part you need a professional load tester. Under full load, you need to measure maximum current output and "ripple" voltage. Under a brief full load, you will get either the rated current capacity of the generator or very near exactly one third of that value. If one of the six diodes fails, which is real common on these generators, you will only be able to get one third of the rated current. 30 amps from the common 90 amp generator 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.
Diodes are rarely intermittent, so it's more likely your voltage regulator is failing. That will typically turn the generator on like it is supposed to be, then off, and that will cause lights to be bright, then dim.
Wednesday, April 15th, 2015 AT 11:30 PM