Hi kemperdog. Welcome to the forum. There is some good news in my reply. There is no way to sugar-coat the bad news though. It is a well-known fact that GM went from the second best charging system in the world to the worst pile ever produced beginning with the 1987 model year. It is common to go through four to six generators, (aka alternator, but that term is copyrighted by Chrysler), in the life of the vehicle. They are extremely difficult to disassemble and just as difficult to identify the defective components so most people just replace the entire unit which includes the voltage regulator.
The good news is if this is the first time you have had charging system trouble in 172,000 miles, you are doing way better than average. The second bit of good news is more and more mechanics are learning that to prevent repeat failures, the battery, which might work just fine in any other car or truck, must be replaced at the same time. Since your battery was just replaced, you will likely not have a problem with your new generator.
Due to the design of this generator, they develop a lot of voltage spikes. Special "zener" diodes help to dampen most of those spikes, the ones that get through can wreak havoc on the many computers on the truck. Sensor voltages vary by a few hundredths of a volt. Voltage spikes affect sensor readings and will make the computers respond to the wrong things and do weird things. GM generators are a known cause of engine performance problems even when they are still keeping the battery fully charged.
As the battery ages, its "internal resistance" goes up which lessens its ability to absorb and dampen voltage spikes. That is why replacing the battery when the generator is replaced will make the generator last longer.
There are tests you can do on the charging system, but you already noticed the voltage reading is low intermittently. Everything is self-contained in the generator. The only other thing that can cause it to stop charging is a break in the small red or brown wire that plugs into the side of it, but that is also the wire that turns on the warning light. Since the light is working, that wire must be okay too.
Here are the wiring diagrams and a guide to help you test the wiring
Images (Click to enlarge)
Monday, May 31st, 2010 AT 1:40 PM