The AC Compressor on the truck won't run..... I've shorted the terminals on the low pressure switch, will not work; replaced the relay, will not work, checked the fuse....good..... BUT if when I attach an external power source to the compressor, it works......... could this be a dashboard switch problem?

Did you check the circuits to the low pressure switch(does this use a pressure cycling switch on the accumulator?) and relay?

Yes, I checked the circuits, it does have a switch on the accumulator, I replaced the relay...and then I even crossed the terminals on the low pressure switch with no luck....the compressor will not run UNLESS I put an external power source to the compressor and it works fine.

Are you even getting voltage on the circuit?

I just checked and no I am not....I have checked the fuse, and replaced the relay, so I'm getting ready to check out the switch on the dash this afternoon (It has a separate AC Compressor Switch on the dash).

Mike asked you the same question I asked you.......

You need power at term A of the low pressure switch....No, ck A/C fuse

You need power at A4 of the relay......if no power, not getting a signal from the control head.....

You need a good ground at B6 of the relay.....if no ground, relay will not work....

You need power at B4 of the relay.....this checks the circuit from the low pressure cutout switch.....

If you jumper B4 to A6, the compressor should come on......if it doesn't....disconnect the high pressure switch and jumper the harness.....should now run.....if it doesn't there is an open in the circuit.....

I found the problem! There was an obscure short in the the low pressure pig tail. I remedied that, and wa-la...back in business, topped off the system with a little refrigerant and we're good to go! Thanks.

I've always wondered how people 'top' off an a/c system. Done correctly, the existing system should be completely recovered, vacuumed down, and the mfr'* correct amount should be put back in. It is possible that you overcharged your system. Undercharged and overcharged are just as bad.

I have an evacuator, I have a refrigerant scale, I did everything to specs.... I made sure the system was empty, then I top it off to the correct amount to make sure it was cooling properly..... I just used, "topped off" as a figure of speech. Yes, sir, it was done correctly.