I bought a new switch and replaced it. But it only powers the clutch for a second before shutting off. It does this several times in a row rapidly, then it quits, the control panel in the cab starts blinking, and I have to unplug the battery to reset it.

That should mean it is still low on freon. Do you have any idea how much you have put in so far. There should be a decal on top of the accumulator to tell you how many lbs it holds. Also remember that most of the little cans of freon are only 12 ounces not 1lb (16 ounces). Is it getting cold at all?

Hard for me to tell. The can still seems to have a lot in it. I'm going to try to put more in again today and see what happens.

I'll bet the switch was doing its job. :)

I've got an update.

It turns out something wrong with the can of R134. After several attempts trying to add it and it wasn't taking, so I went to the store and got another can: different brand and this one didn't have the pressure gauge. This one worked. It sucked it all in right away and now I'm getting cold air in the cab. I've driven it two days now and it's still blowing cold.

I've decided to leave the low pressure switch disconnected and the wires jumped together.

I would strongly suggest not leaving it jumped like that. It will burn up the compressor. A compressor needs to cycle on and off or it will freeze up the system. If it does not come on with it plugged in you are either low on freon or the switch is bad. I think you said you replaced the switch so you are probably low on freon. Also remember it can take several minutes for a system to take in a single 12oz can of freon.

I didn't realize this. Thanks for telling me. I'll connect it back up to the switch.

Okay I reconnected the low pressure switch and have been running the A/C and it's working fine.

question: why would the compressor burn up if I leave the wires jumped together? It seems to me that even with the switch connected, the compressor is always running anyway.

Your compressor will cycle on and off according to fan speed and outside temperature. If it is not allowed to cycle it will freeze the system up - usually at the orfice tube. Also if you got a leak in the system the compressor would continue to run possible with no oil and that equals trashed compressor and the entire system would be contaminated with the compressor locking up. Just a safety item that is there for a reason.

So the low pressure switch also does the cycling the compressor based on the temp and fan speed? The switch monitors all these things?