System voltage is too high. That is the fault of the voltage regulator. It's a rather inexpensive part but GM builds it into their generators so you have to buy the whole thing. In a way, that's a good thing because most voltage regulator failures on GM products are caused by the design of the generator. They develop huge voltage spikes that can destroy the internal diodes or the regulator. It is common to go through four to six replacement generators in the life of the car. 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 voltage spikes.