GM has had a huge problem with their generators since they redesigned them for the '87 model year, and this is one of the common symptoms. You already did the next step. It's real common for people to go through four to six replacement generators in the life of the vehicle, but to reduce that number of repeat failures, replace the battery at the same time. Due to their design, these generators develop large voltage spikes which can damage computers, damage the generator's internal diodes and voltage regulator, and cause elusive running problems from those spikes interfering with computer sensor signals. As the battery ages, it loses its ability to dampen and absorb those spikes. You typically do not have to replace the battery if it is less than about two years old.
There are other things that can cause the lights to flicker, so the place to start is by using a professional load-tester to measure maximum output current and ripple" voltage. If one of the six diodes has failed, which is one of the two most common failures, the generator will only be able to develop exactly one third of its rated current, and that's not enough to run the entire electrical system under all conditions. Ripple voltage will be very high and the voltage regulator will have a hard time trying to hold system voltage steady. It will respond to that varying voltage and constantly bump the output voltage up and down. You'll see that as flickering lights.
Also, if you have a digital dash, those are very susceptible to voltage fluctuations. The dash brightness can flicker and be very irritating even before you see that in the rest of the car's lights. To partially address that, the voltage regulator senses voltage right at the dash on those cars instead of internally. That adds a lot more wire and connector terminals that can corrode.
If you have a replacement generator already, replacing it is the fastest test.
Monday, December 9th, 2013 AT 6:28 PM