A/c compressor wont come on unles you put a jumper wire to the b

i replaced the alt on a 99 s10 i removed the right front wheel and wheel well cover about 10 bolts and you can get to it from under the fender

I would check the pressure on the system first. If you have a leak in the system the pressure will drop and keep the compressor from cycling on ( keeps you from burning up the compressor) If the system is indeed holding pressure, the problem can be a compressor relay, on GM products this is normally under the hood close to the firewall. It also can be the A/C control module not funtioning. (Not sending voltage to the compressor clutch) On many of the newer systems the components( ie: clutch, hi-pressure switch, low pressure switch, relays) are already energized and the control module just completes the circuit to ground. In other words when you check for voltage at these items it will be present, even though the unit may not be engaged.