I am not an A/C wizzard.... but here goes. I have a 1999 F150 with a 5 speed and a 4.6 v8. I have the manual AC controls. The AC on the truck has always been great - never re-charged and lots of cold air. A couple weeks ago, I was on a long trip, and I had no cold air, the compressor was not running. I hooked up a pressure gauge to the low side and it read around 75psi. I think I fiddled with the cut out switch up front also and some how the compressor started, then the low side came down to about 25psi and all was well ( air was cold and started and stoped fine). the next day it worked fine. Then the truck sat for a week and again, no cold air. I checked the pressure and it was at 100psi on the low side.... no amount of fiddeling with the cut out switch would get the compressor to start. So here are my questions:

Is it normal to have like 75 to 100psi on the low side before the compressor starts? If not any clues to what is wrong?

What does that cut out switch do ( the one up by the high side valve)? Does it turn off the compressor if it can't make high pressure.... or does it stop the compressor if the pressure is too high?

Any help is appreciated....thanks....

 

The switch by the high side service port turns the compressor off when high side pressures reach about 435 psi. Usually these switches don't fail, it's the low pressure switch that you need to check. Disconnect the low pressure switch at the accumulator and jump the two pins at the connector to see if the compressor comes on.

 

Yes, it is completely normal. When the compressor is not running, all points in the system have the same pressure. Depending on ambient and underhood temps, this is anywhere from 60-120 psi.

You have either an electrical problem OR your clutch has worn to the point that the air gap is too great for the electromagnet to pull in the clutch.

Try tappin the clutch with a mallet or broomstick to see if it pulls in. If so, it's a worn clutch.

If hitting it didn't make it work (but did make you feel better), you'll need to test the electrical circuits. The quick test that Quintin gave is a fast way of checking if the cycling switch is the problem.

Steve

 

Thanks for the leads! I'll check them both out. I don't recall seeing the "low side" switch, but it is possible I bumped it and that is what made it start working before. Anyway, I am sure I'll find it. If it is the switch, is that something I can change or is it best done as an AC place? Does it the system have to be bled to change the switch? If it is the clutch is that is bad, how big a job is that to replace? Thanks again for the great feedback!

 

There's a schrader valve under both pressure switches so you can just unscrew them without fear of losing any refrigerant. There'll be a little hissing, but the system won't dump everything on you. The low pressure switch is mounted on the accumulator in the passenger's side rear of the engine compartment under the bulkhead connectors/starter solenoid assembly.

If the clutch is screwed up, it's kind of a tricky job to do with the compressor on the truck, but it can be done...carefully.

 

So I finally got around to working this. I first tested the low pressure switch.... hoping that was the problem (cause the fix sounded easy). But orf course it was not. Turns out it must be the electromagnetic clutch that projectSHO89 described. A few mallett wacks and it started, slipping, then ran for a while and then quit. So I quess my next question is what is the remove replace proceedure. I suppose all work is done from the bottom. Obviously the fan belt is removed... then what? does the clutch pull off the compressor?... Are there special tools required. I am reasonably skilled.... but is this job better left for a garage.... or could I do it?.... Thanks again for all the help so far!