This is so common that normally I paste a copy of a standard reply but in this case I'm suspicious there's something going on with those rebuilt units. A new, inexperienced worker might not be soldering an internal connection properly. There might not be heat sink grease applied to the back of the voltage regulator. Things like that. This is almost always solved by switching to a more expensive or less expensive generator from a different rebuilder, but handled by the same parts store. My friend ran into this with his Suburban a few years ago. He bought a generator with a lifetime warranty for $200.00 and had three in a row fail about two weeks apart. Next, he "upgraded" to a $250.00 generator with only a one-year warranty, and hasn't had a problem in years.
The more common issue is GM had what I considered the second best generator in the world up through 1986. They redesigned them for the '87 model year and turned them into the world's worst pile, and they have no intention of fixing that design. Due to how the internal voltage regulator switches field current on and off about 400 times per second, the output circuit, which is similar in operation to an ignition coil, develops huge voltage spikes that can destroy that regulator, the internal diodes, and they can interfere with computer sensor signals. Elusive running problems on GM vehicles often clear up when the smaller plug on the generator is unplugged to disable it.
The battery is the key component in dampening and absorbing those voltage spikes, (and is related to why we must never ever remove a battery cable while the engine is running). As any battery ages, the lead starts to flake off the plates and that reduces its ability to absorb those spikes. Once it gets to be more than about two years old, those voltage spikes start to become a concern. My standard reply, when someone finds the generator needs to be replaced, is to replace the perfectly good battery at the same time unless it is less than about two years old.
To be fair, all manufacturers switch field current on and off this way because a very light-duty switching transistor can handle the job without overheating. (Vcrs and newer tvs also use these "switching power supplies" for the same reason. I don't know why, but this voltage spike problem only involves GM's generators. Their '86 and older models worked the same way but never caused a problem.
Since you beat me to the punch and already replaced the battery, you either have an intermittent connection or the generators are failing due to a mistake at the rebuilders. To check for a bad connection or corroded wire, remember that the voltage on the large output stud on the back of the generator, and the voltage between the two battery posts, must always be the same. If that fat wire has a break in it, you'll find 0.0 volts on the generator's output stud when the engine is not running, and considerably more than 14.75 volts, as in 16 to 18 volts when the engine is running.
Thursday, October 16th, 2014 AT 12:11 AM