It is in the generator, bolted to the rear housing and there is no way to test it. It's what the connector plugs into. There's no way to sugar-coat it. GM went from the second best generator to the world's worst pile starting with the '87 model year. Disassembling this thing is darn near impossible without breaking the tabs off the diode block. They use special zener diodes too to control the horrendous voltage spikes produced by these units. Once you finally get everything apart and the diode block is ruined, do you hope the regulator is good or do you run the extra expense of replacing it too? By the time you're done, you're better off with a rebuilt generator with a warranty.
It is common to go through four to six of them in the life of the vehicle. What more and more professionals are finding out is to reduce the repeat failures you should replace your perfectly good battery at the same time. The old battery will work fine in older cars, but as they age, their ability to absorb those voltage spikes is reduced. Those spikes also play havoc with all of the unnecessary, complicated, unreliable computers on the truck. The current pulses resulting from those spikes sets up magnetic interference that radiates into other wires in the harnesses and induces voltage spikes in wires for various computer sensors. A lot of engine running problems can be traced to a GM generator that is still charging the battery.
There isn't much else that can cause the problem you described. The voltage regulator monitors system voltage right at the output wire. The exception is on trucks that have digital dashboards. On those it is critical to maintain the voltage feeding it to prevent flickering of the display, so those will have a second wire in the plug on the back of the generator. They monitor voltage right at the cluster, understanding that the rest of the system will be close to that and isn't too critical anyway. That still shouldn't cause the output voltage to rise unexpectedly.
Besides the regulator, it's always possible the braided copper wire to one of the new brushes is rubbing against the metal housing and grounding out. Usually that won't be intermittent, but if the right one does ground out, the regulator will be bypassed and the generator will charge wide open.
Saturday, December 4th, 2010 AT 1:59 AM