Compressor clutch not engageing, ran a wire from alternator and clutch does work fine. leaving me with the impression that there's a faulty switch or sensor. don't know were to look.

Hi, when a compressor clutch does not engage on its own, it is more then like lost its freon, and is off on its low pressure switch, or low enough for it not to engage. It will start if you bypass the low pressure switch as you did. If you start your car, turn the ac on, and jump it out it will start to cool right away if it has freon in it. It will not burn out anything if you do it for only a few minutes. But, the compressor is good, and I doubt very much if its a relay to the pump. If this auto is older and has the valve depressor fittings for R-12 and not R134A, you can depress the high side, smaller line and it will have lots of pressure on it. I would go to the auto parts store and get the charging hose with gauge and can, hook them up to the low side large fitting if it is R134A, and you will know by the pressure on the gauge if its out or low. If you pierce the can to add some, the clutch will engage on its own, and that will tell you you have a leak. Depending on how long it lasts, will tell you how large the leak is.Doing this will save you lots of $$$$. Please let me know the outcome, but I am 99% sure you have lost the charge if you can jump it out.
Sincerely,
Shastalaker7
A/C, & Refrigeration Contractor