1990 GMC Sierra

Use a digital multimeter across the battery posts, engine running, what is the reading? Some of the national brand autostores will check the charging system for you.

Since the '87 model year, GM has had a really poor design generator. It is common to go through four to six of them in the life of the truck. If it proves to be defective, what many professionals are finding out is to reduce the repeat failures, replace the perfectly good battery at the same time. As they age, they lose their ability to dampen and absorb the huge killer voltage spikes these generators produce due to their design. Your old battery will work fine in a pre-1987 vehicle.