So last week I was in town with the a/c going, I parked and went into the feed store to get some stuff. When I came out the vent was blowing warm air. I checked the fuse, relay and the pressure and all where good. I took off and pretty quick the a/c started blowing cold again. It did it one more time while I was parked and started working again when I started moving. The other day I went into town and the a/c did not work at all. Parked or driving, nothing but hot air out the vents. I have been told that the switch in the cab could be bad, but when I switch from the vents to the floor it changes just like it should. Just wondering if I am missing something?

Could you tell if the clutch on the compressor kicked in when you turned it on? Working going down the road and not when stopped could be a couple different things. One could be not enough air flow sitting still through the condensor causing the high pressure switch to kick the ac compressor clutch out. The other could be a slightly low charge, enough for it to work when the RPM is raised enough for the compressor to work at its highest efficiency. If the charge was a little low from a small leak it might have lost enough for the low pressure switch to cause it not to run. Have also seen the high pressure switch stick keep the compressor from running. As for the switch in the cab, I'm not sure exactly how these trucks are setup but there has to be something electrical to activate the ac compressor clutch so it is something worth looking into. I would check the pressure in the system and if the compressor is running first though.

The a/c clutch is not engaging, so it might be the high or low pressure switch? I will have to check the pressures again, it might be a slow leak and now it has leaked down enough not to work? Thanks for some places to look.

Most likely low on freon. When properly charged with freon the AC will not cycle at all. The compressor will run constantly. Hook up your manifold gauges and check you pressures before changing any switches. Make sure your seeing at least 35 PSI on the low side with the compressor running. If there is less that 40 PSI the compressor will most likely not start.

I will check the pressures. Hopefully that is all it is, thanks for the help and I will let you all know what the results are.

Mine was doing the same thing. I added a can of r134 and its working way better when sitting. I think I'm just gonna take it in and have it charged the right way with a machine since I wasn't very good at a/c. I was spoiled and had the machines that automatically did all the recovery and charging.

It was low on refridgerant, I have a leak in the low side valve where you add refrigderant to the system. Not sure if that valve is serviceable or if I will need to replace the line? For now the fix is to leave the hose connected to the valve and the hose away from any heat.

You can replace the valve, it's just a schrader valve like on tires only bigger.