Sounds like the charging system needs to be checked. Charge the battery at a slow rate for about two hours, then try to start the engine. You can start the testing by measuring the battery voltage with the engine running. It must be between 13.75 to 14.75 volts. If that is what you find, it is okay to have your mechanic perform the rest of the tests, but that requires a professional load tester.
GM generators starting with 1987 models have an extremely high failure rate. When you need to replace the generator, to reduce the number of repeat failures, always replace the battery at the same time unless it is less than about two years old. These generators develop a lot of harmful voltage spikes that can damage the internal voltage regulator and diodes, and interfere with computer sensor signals. The battery is the key component that dampens and absorbs those spikes, but it loses its ability to do that as it ages and the lead flakes off the plates.
Monday, September 19th, 2016 AT 7:20 PM