First you have to be aware that starting with the '87 model year, GM went from, in my opinion, the world's second best generator design to by far the worst. It is very common to go through four to six replacement generators in the life of the vehicle, however, that number can be reduced by replacing the battery at the same time, as you already did. Because of their design, these generators develop huge voltage spikes that can destroy its internal diodes, voltage regulator, and interfere with computer sensor signals. The battery is the main component in damping and absorbing those voltage spikes, but as they age, they lose their ability to do that. That's why I asked if you had a used battery installed. Typically when you have to replace the generator you do not need to replace the battery if it is less than about two years old.
Since you DO have a new battery already, it is more likely the new generator just failed. You confused me though with, "It ran good for 3wks & all had started back up again. All diagnostic test that were done, all came back fine". First of all, I'm pretty sure you can ignore the Air Bag light and the ABS light. Both of those systems are run by computers that know it can take substantial current and correct system voltage to make everything work properly. Those computers recognize the low system voltage could prevent them from working right, so they shut down and turn the warning lights on to tell you. To say that another way, solve the charging system problem and the other symptoms will take care of themselves.
Next, there has to be a cause for the "Battery" light to be on. If testing with a professional load tester didn't indicate a problem existed, that suggests the cause is intermittent and the problem wasn't acting up during the testing. The mechanic needs to be made aware of that so he will look for potential causes and he will try to make the problem occur. Most of the things that fail in these generators do not cause intermittent problems. When they fail, they stay that way and they can be identified. Intermittent problems are more likely to be caused by wiring and connection problems.
The things the mechanic needs to specifically look for are "full-load output current" and "ripple voltage". A lot of inexperienced mechanics just measure battery voltage while the engine is running. That must be between 13.75 and 14.75 volts, and while that is a good first test that any car owner can do, it is only the first part of the test that tells you if it pays to do the more-involved tests.
Specifically what I need to see is if full-load output current is normal or exactly one third of normal. With one failed internal diode of the six, you will only get one third of the generator's maximum design value. 30 amps from the common 90 amp generator is not sufficient 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. Ripple voltage will also be very high. Even when these tests fail, that basic battery voltage test I mentioned will usually appear to be okay. That's why it is just the preliminary test but that's all some mechanics perform.
When they do the full test, most shops provide you with the test results, either on a small printed form or on the repair order receipt. You're helping to pay for very expensive test equipment and the shop owners are rather proud to prove they have this tester and used it on your vehicle. If you have those numbers, post them in your next reply and I'll interpret them for you.
Tuesday, March 10th, 2015 AT 5:19 PM