2nd Gen Non-Engine/Transmission AC trouble shooting tips

[bnbrown]

I am trying to find the problem with the AC on my '99, it worked fine in Feb (not that you need it), but just quit when the weather got warm. The compressor does not even click on. I checked the r134 charge, seems to be good, checked the fuse, swapped relays, checked the low pressure switch (seems to operate properly)...
don't know where to go from here (other than to the shop), so any suggestions would be appreciated.

 

[vssman]

I had a similar problem except mine would work intermittently. It turned out to be a broken wire at the high pressure switch connector. If the compressor clutch is working correctly and has power to it, jumping both the low and high pressure switches should 'force' it to work when you have the blower switch 'on' in the cab. Don't run it too long if your not sure if you have a proper charge in the system.

 

[bnbrown]

would be a decent way to test the compressor and hopefully eliminate it or the clutch atleast...

 

[fest3er]

Mine died much the same way. Worked fine when I brought it to a shop. Didn't work after. But that was coincidence.



An AC shop 'rewired' the compressor, taking the PCM out of the circuit. Essentially, he ran a wire from the AC relay in the PDC to the 'firewall' side of the low pressure cutoff switch. The circuit's been working fine for seven years now.



There are eight things that can prevent the A/C from working (in circuit order):

• A/C switch in the cab does not send 12VDC to activate the A/C relay in the PDC to feed the compressor clutch circuit.
• The A/C relay in the PDC is bad and is not feeding 12VDC to the PCM.
• The PCM is bad and is not feeding 12VDC to the low pressure cutoff switch.
• The low pressure cutoff switch is bad and is not feeding 12VDC to the high pressure cutoff switch.
• The high pressure cutoff switch is bad and is not feeding 12VDC to the compressor clutch.
• The compressor is bad.
• The A/C system pressure is too low.
• The wiring between any of the first six items is bad.

That's it in a nutshell. A voltmeter and a suitable jumper wire are all you need to troubleshoot the circuit.



You can swap around same-size relays in the PDC. You can remove the pressure switches and bang them a bit to loosen them if they are not working. You can even clean them with a suitable solvent but you have to be sure the solvent has completely evaporated before reinstalling the switch; you don't want the solvent mixing with your R134. You can jumper 12VDC directly to any point on the circuit, but only momentarily (to listen for a suitable relay click) unless you know just what you are doing. For instance, if you hotwire the compressor too long, it will make a very loud noise and you might need to change your trousers. Don't ask how I know this.

 

[bnbrown]

I probed around some with a voltmeter in the relay socket, I only found a 12V+ at one pole... I assumed it was the constant or switched hot, I did not find a 12v+ (or lesser voltage) pole for the relay coil... will have to take a closer look at the relay schematic and see where I should find 12v for the coil, and which pole should be for outgoing 12volt feed to PCM...
it blew the fuse once, and I replaced it, but since then it has not worked (yes swapped fuses to make sure the new fuse wasn't defective) have considered just hot wiring the AC clutch and starting to work backwards as I did jumper the two pressure switches to no avail...
First will try to find the 12volt from the AC selector switch, since so far that is MIA... dang near be winter before I get this figured out