Yup. It's common to go through four to six replacement generators on GM products built after they redesigned them for the '87 model year. Two failures so close together though suggests a problem that should be addressed by the part's warranty. You already did the number one thing that reduces the number of repeat failures. That is replacing the perfectly good battery unless it is less than about two years old. These generators produce huge voltage spikes and the battery is key in controlling them. As it ages, the battery loses its ability to dampen and absorb those spikes even though it cranks the engine just fine. Those spikes destroy the internal diodes and the internal voltage regulator. That's what happened here. The only way to get 18 volts is from a shorted voltage regulator. BUT, ... Since the battery is new, I suspect the voltage regulator may have just failed on its own. There can also be a bad batch of them that were supplied to the rebuilder so to be safe, you might request a replacement generator of a different brand if the parts store has them.
Please consider a to help us answer more questions.
Monday, October 22nd, 2012 AT 11:31 PM