98 Pontiac Montana Wont idle with Alternator harness connected, will run fine off battery till it dies.

Replace the battery. Since the '87 model year, GM went from the second best generator design to the world's worst pile ever. They develop huge voltage spikes that take out the internal diodes and voltage regulator, and those spikes interfere with computer sensor signals, and the computers, which are very sensitive to system voltage. As the battery ages, it can still crank the engine but they lose their ability to dampen and absorb those spikes. It's real common to go through four to six generators in the life of the vehicle, but many professionals are finding out that to reduce the number of repeat failures, just replace the perfectly good battery at the same time.

I don't know how you figured out that unplugging the generator would provide a clue, but that IS a common test. On the off chance the battery doesn't correct the problem, look for any wiring harness that are not routed in their original orientation. Also be sure there's no undesired resistance in the fat wire between the back of the generator and the battery or fuse box. That added resistance makes it harder for the battery to dampen those spikes.

In technical terms, as the lead flakes off the plates in the battery over time, it's "internal resistance" goes up. That's a term usually only heard by electronic designers and technicians. In practical terms, the old battery can still provide the current needed by the starter, but for a much shorter period of time compared to a new battery.

Check for AC voltage at battery with alternator connected. I've heard of up to 30v AC which means bad alternator.

14.8 is just above the industry-accepted limit of 14.75 volts but it is not excessive. Did you try a new battery?