You've gotten a lot of misinformation. The voltage must never go above 14.75 volts. It must stay between 13.75 and 14.75 volts regardless of how many loads you turn on. All the items on the vehicle are designed to run properly on that voltage, and that is where the battery will recharge fully. Above 14.75 volts the battery acid will begin to overheat and the water will boil out of it. What you're referring to is the CURRENT output must increase when more loads are turned on, and that's what's not happening. When the generator is incapable of supplying the needed current, that's when the generator's voltage, (electrical pressure) will drop. That is not the fault of the voltage regulator. It's due to one of the six diodes is defective. That will reduce any AC generator's maximum output current to exactly one third of its design value, and that's not enough to meet the needs of the electric fuel pump, heater fan, many computers, fuel injection and ignition systems, etc. The typical generator is designed to deliver around 90 amps. With one bad diode, a load test will show it can only develop around 30 amps, and "ripple" will be very high. That ripple is the variation between the highest and lowest voltage points in the output waveform. All AC generators put out three-phase output which is much smoother than that from a household battery charger. With a bad diode, one phase is missing and that's when the voltage drops a lot. The result is that very high ripple. This page will explain it better:
Your voltage regulator is indeed inside the generator and is extremely difficult to service. The entire assembly is a very poor design since they introduced it in the 1987 model year. It's predecessor, described here:
was a very nice unit. GM has always built assemblies that are expensive to replace but don't require much troubleshooting. Other manufacturers build theirs to be serviced. For example, I replaced the nine-dollar brush assembly on my '88 Grand Caravan's alternator last year in less than an hour. Many Chrysler alternators can be repaired without even removing them from the engine. A lot of Fords were the same way in the '90s. Sorry to say that's not the case with yours. To get to the voltage regulator inside the rear case housing there's a plastic plate in the way and no way to remove it without unsoldering the three heavy stator wires, (through the slots in the housing), that are attached to very flimsy sheet metal tabs on the diode block. Most of the time you'll break some of those tabs off so the diode block will be junk, (which it sounds like yours is already), AND there is no way provided by GM to test or bypass the regulator, like they did in their much better older design, so you have no way of knowing which part is bad. It takes way too much work to get it apart so you will want to replace the regulator and the diode block to insure you don't have to do the job a second time.
Additionally, they have had a lot of trouble with the front bearing seizing up and causing the serpentine belt to burn off. You'll want to replace that too. At first these generators were not designed to be repaired, just replaced, but some tool manufacturers developed tools to get them apart, then other suppliers started making replacement parts. The problem is by the time you get done replacing all the parts and hope you didn't make a mistake, you'll have almost as much in it as you would pay for a professionally-rebuilt unit with a warranty.
The next thing to be aware of is your original failure most likely is due to the battery. Because of the design of these generators, they develop huge voltage spikes that will destroy the voltage regulator, those diodes, and they can interfere with engine computer sensor signals and cause weird running problems that defy diagnosis. Part of the battery's job is to dampen and absorb those spikes, but as they age, they lose their ability to do that. If your battery is more than about two years old, replace it to reduce the chance of a repeat failure. When people don't replace the battery, it is common to go through four to six replacement generators in the life of the vehicle. If your battery is fairly new, it is most likely a diode was destroyed a while ago when the old battery was still in the truck, and you're just seeing the symptoms now.
Friday, December 28th, 2012 AT 4:14 AM