1999 GMC C2500 Electrical Problem

That's typical of a generator problem. Charging voltage measured at the battery with the engine running must be between 13.75 and 14.75 volts. Don't rely on the dash gauge for this. They're notoriously inaccurate and are only good for you to recognize when something is not normal.

Since the '87 model year when GM redesigned their generators, they have had a real big problem with repeat failures. The solution for that is to replace the battery at the same time that you replace the generator. These generators develop huge voltage spikes that can damage the internal voltage regulator and diodes, and interfere with computer sensor signals. The battery dampens and absorbs those spikes but as they age and the lead flakes off the plates, they lose their ability to do that. That's when repeat failures become common.

Since you already replaced the batteries, we shouldn't have to worry about repeat failures. Also, failed diodes or voltage regulator are rarely intermittent. It is much more likely, due to the mileage, that the brushes inside the generator are worn. Those almost always cause intermittent failure to charge for weeks or months before the failure becomes permanent. The brushes can be replaced pretty easily on almost all generators and at a real low cost, except GM's design. The way they're put together you will have destroyed most of the parts before you get down to the brushes. The best is to just replace the assembly.

The generator is driven by the belt. There will be a belt routing diagram on a sticker under the hood. The generator may be labeled on that diagram.

Measure the battery's voltage. If it's good and fully-charged, it will be 12.6 volts. If you find it's around 12.2 volts, it's good but discharged, probably due to the failing generator. Charge it at a slow rate for an hour, then try to crank the engine.

There's a few things to consider. The neutral safety switch can be intermittent. Try shifting between "park" and "neutral" a few times, and try starting it in neutral.

Some starter solenoids will not engage far enough to turn the starter motor on if battery voltage is just a little low. When the starter doesn't engage, try to not disturb anything else that might make it crank, then connect a battery charger, then try starting it. If that works, suspect the battery or the cable connections.

We had a '98 GMC Lemon Law buyback donated to my college by GM that I used a lot to teach electrical diagnosis. One of the biggest problems we ran into that wasn't built in by me was stretched terminals in the starter relay socket in the fuse box under the hood. In spite of being warned, students would poke the voltmeter probe into those terminals and stretch them. Meter probes are much fatter than the relay's terminals. The symptom was there might or might not be a very light clicking of that relay when turning the ignition switch to crank", but the starter solenoid wouldn't engage. Having a helper press in the relay often would get the engine started.

If nothing else works, remove the starter relay, then use a stretched out paper clip or piece of wire to jump two terminals. Whether or not the starter cranks will tell us which part of the system is working. Do this with the ignition switch off. Jump two terminals together in diagonally-opposite corners of the socket. If nothing happens, use the other two terminals.