I have a 97 gmc jimmy,ive replaced the alternator twice,it starts to squeal and alternator will get very hot after 5 minutes,squeal stops if i unplug connector and ive checked it for a short with a test light between neg wire and neg battery terminal and i does not light c

The post on the back of the alternator should`read 14 volts when running. You should get very close to the same reading at the battery posts (when the rear alternator post is hooked up to the harness).If you get any significantly different reading at either location, disconnect the harness from the back of the alternator and run a 4 gauge wire from the alternator to the positive post of the battery and see if you still have a 'hot'. noisy alternator. My guess is that the wiring in the harness is shorted somewhere.

Alternators should get harder to turn when they are producing more current.
They should also get hotter when they are producing more current.
So it is hard to tell exactly what is wrong.
One possibility is that you could have a short or some sort of drain that is taking too much current.
Another is that you just have never tightened the belt enough, and the slipping is causing friction heat.

To test belt tension, at the longest belt run, try to deflect the belt in the middle, by pushing on it.
It should not deflect more than half an inch. If it does, tighten it.

If the problem is high draw, then try it with everything possible off.