I've been trying to figure out the ac on a 2000 peterbilt 379 lately and have hit a wall... I replaced a line, the orifice tube, compressor, accumulator and still can't get it to work. I had an ac guy buddy of mine charge it. But now the compressor won't turn on. I found one bad pressure switch... When I jump the connector on that switch and hit the ac button, still nothing happens. The only time I hear the ac compressor clutch, and ac relay click is when I check for power with a test light on circuit a495. It goes to one of the switches, and the main ac switch. Can anyone shed some light onto what's going on here. I have a wiring diagram, but I can't even find the location of the ac on/of switch on it.. Please help! :Banghead
All the pressure switches are in series on the same ground circuit which controls the a/c relay in the fuse panel. Easiest way to check them is to jump across the plug for each one with a paper clip with the a/c turned on. Why do you think the pressure switch is bad if it didn't make a difference when you jumped across it? Here is a link with some wiring schematics. http://www.supermiller.com/ They can be found on the right side scroll down.
I ohmed out the pressure switch and it showed open... I compared it to the other switches, as well as another 379 we have. Someone before me had the switch by passed, and who knows what else. I did jump the one known bad switch, and I didnt' have the comp clutch turn on. Do you think even with the readings of the pressure switches right around a couple ohms, they could possibly be bad causing the clutch to not turn on... Furthermore... I have power at the a/c switch, and when turned on, passing through it... I have power on either side of the connectors going to the pressure switch, and power in the right places at the relay. I tried a different relay. Could i just be looking at a ground problem somewhere?
Not sure which pressure switch you're referring to....Nothing under the hood on the a/c system should have power except the wire going to the compressor clutch. Everything else is a ground. I think you will find it's a bad pressure switch or possibly a bad connection where the wire plugs up the the switch, but that's just my best guess.
I'm probably just over thinking this whole thing. based off the wiring diagram i'm having a hard time visualizing the way this system works. My assumption would be that there's fused power going to the a/c switch on the dash. When closed, power is sent to the relay. I see power at the relay on pins 86 and 30 in the fuse panel all the time, even with the a/c switch off. When I put my test light on pin 85, it lights up, the blower comes on and the a/c clutch engages. When i press the a/c on/off switch, i should see power given and taken away from the relay... correct? Not constant power all the time on pin 86. 30 is constant power, 86 is switched power, and 85 is coil ground
Yes that's correct.