That test proves the light circuit is ok. That's also the turn-on circuit for the voltage regulator. There is either a bad connection in the generator plug, or the voltage regulator is open. You can buy new regulators but disassembling GM generators is a real hassle. You're better off just replacing the whole generator. Check around on price and warranty. My friend is on the third one for his Malibu. He found one with a two year warranty for around $200.00 and a different one with a five year warranty for considerably less.
You might consider replacing the battery at the same time. It is real common to go through four or more of these generators in the life of the vehicle. What more and more professionals are finding out is they can reduce the number of repeat failures by replacing the perfectly good battery. It might work fine in older pre-1987 GM cars that used the old style generator, but this new style develops a lot of big voltage spikes that interfere with computer sensor signals and they can damage the voltage regulator and the diodes in the generator. As batteries age, they lose their ability to dampen and absorb those spikes.
Sunday, December 12th, 2010 AT 12:40 AM