A/C Compressor will not engage
 

I have a 1995 Buick Lesabre. Today I had taken it to a shop to have the a/c compressor replaced because the original seized up. The replacement compressor was a used part from an auto salvage yard.

This is the problem: The mechanic got everything installed and hooked up/charged back up. However when the a/c button(s) are pressed to turn on the a/c on the dash climate control, the compressor won't engage. The mechanic was able to engage the compressor manually, it ran fine and the a/c came out cold. He also ran diagnostics on the a/c system and everything he checked checked out fine. He says it might be electrical, but I'm not sure. The original compressor engaged fine before it failed. Not sure what is going on here, but it's getting a little frustrating. I would like to get it working if the cost won't be too great.

Any ideas what this might be? Thanks for your help.

Your "new" one might have a bad clutch. If your mechanic applies 12v to the clutch itself he should be able to tell right away if its the clutch or not.

Yeah, he said he was able to apply voltage directly to the clutch itself and it engaged. He said when he did it that way the air came out cold.

generally, if the pressure is too low or there is moisture or air in the system the compressor will not turn on. i would get it professionally vacuumed out and have a new canister put on. if the canister has ever been exposed to air or it has been depressurized for any amount of time, it needs to be replaced. the canister is what absorbs any moisture. after almost 20 years, i would replace it regardless.

when you take your pressure reading, make sure the car is running and the compressor is on. it should be about 35-40 psi. with the compressor off it should jump up pretty high. if it was charged with the compressor off, im guessing that it is low on freon or air is trapped in the system.

just sounds like the pressure switch or whatever isnt getting to the pcm or the pcm output to the compressor is bad(wires,connectors) i dont have a diagram for that car though

I had the same problem with mine when I ad the compressor replaced. I had to replace the AC programmer to fix the problem.