Hi guys. Let me offer a suggestion. This is a REAL common problem with GM vehicles and might be what's happening here. Many voltage regulators are a "switch-mode" design similar to what's used in a vcr. The switching transistor is either fully turned on or fully turned off and it switches rapidly between those two states, just like a digital computer signal. If you calculate Ohm' Law, power (heat) equal volts times amps. Depending on whether the transistor is on or off, there is either zero current through it or zero volts across it, so there is always zero watts dissipated, and they can use a tiny, inexpensive transistor to do the job. It should never get hot.
Because that current is abruptly switched off, the field coil in the generator develops a huge voltage spike just like the ignition coil primary winding. The battery absorbs and dampens those spikes. As the battery ages, its "internal resistance" goes up due to the lead flaking off the plates. When that gets bad enough, it can't supply enough current to crank the engine. Before it gets that bad, it loses its ability to absorb those spikes. The voltage spikes can interfere with sensor signals for the various computers, but they can also destroy diodes inside the generator and the voltage regulator. From your description, it sounds like you have a bad diode now. The easiest way to prove that is to have a load test performed with a tester that measures voltage ripple. AC generators put out a three-phase output. You'll lose one of those phases with one bad diode, ( that causes the really high ripple), and the unit will only be able to supply exactly one third of its rated capacity. That means you will only get about 30 amps from a 90 amp generator.
It's real common to go through four to six generators in the life of a GM vehicle. Replacing the perfectly good battery at the same time as the generator takes care of most of the repeat failures. In the case of the GMs, the old battery will work fine in a 1986 or older model that doesn't have the current design generator.
I don't know of a test that identifies a tired battery. If you have the opportunity, you might do a load test on your new generator and see how much ripple it is producing, then check it again after replacing the battery. If it's much lower with the new battery, I suspect the new generator will last a lot longer.
I'm not aware of this being a common problem on any car other than GM products, but your description of the symptoms and voltages fits exactly. Most other manufacturers also use switch-mode voltage regulators and they don't have trouble.
Sunday, July 3rd, 2011 AT 8:14 AM