I have a 2001 4.7 Dakota. My A/C has the proper charge and will cool nicely when first starting. After driving for like 20 minutes it stops cooling completely. I check the pressure and it has climbed to nearly off the gauge. If all cools down for a couple hours it will do this all again. I am thinking expansion valve but I am not an A/C expert at all and have never seen this issue before, Any ideas?

 

Is the high pressure off the wall or are you talking about suction pressure? Without knowing what the pressures are when u start it up and it cools and what happens to them when it stops cooling makes it hard to troubleshoot so, u need to post this. If the HP is up and the LP is down a possible restriction in the system. Usually an oriface in the receiver. Common for 134a. Another possibility if indeed HP is high is the condensor is blocked or not clean. At any rate need to know both pressures.

 

Sounds like its charged too much dump some 134a see what happens other option is moisture in the system.

 

I am not sure about the high pressure side. I don't have a full set of gauges just what you get with the A/C pro charge. So I am only reading the suction side. The pressure on the suction side will start at around 45 and you can see it pull down as the clutch kicks in so it appears the compressor is working and it does cool good for a while. Then it jumps to like 80 or so and all cooling stops and the clutch doesn't engage anymore.

 

how did you fill the system...overcharging will cause high pressures on both low and high sides.
i'm for checking the condenser for air flow blockages, cooling fan ducting in place and operating cooling fan...if the system can't get rid of the heat, the high side pressure cut out will shut the system off.

 

Get a real manifold gauge set. It's the only way to tell the state of charge. You need high and low side pressures.