a friend called the other day, his 05 2500 dodge with gas engine would only cool at highway speeds(above 35-40 mph).
when he stopped for a red light it would start to blow hot, then start cooling again once driving.
i had him check the fan clutch and it is bad.
but he said it was also cycling the ac clutch.
this am i went over with my manifold set to check pressures to see if it needed a bit of charge.
engine off static charge was 110 psi both high and low.
once fired up the low side dropped to 45 psi, and high side started climbing. at 400PSI i told him to shut it off, it was at 450 and still climbing when he shut it down.
since i know absolutely nothing about dodges besides the fact i will never own one, where should we start looking?

 

The first thing to do is get the fan working properly, the AC will never work right below 35 MPH until you do. Once fixed you very well could find the high PSI issue is gone. In the meantime if you wanted to try something, then spray the condenser with water to cool it down and see if the PSI readings drop.

 

it hit 450 psi within 30 seconds of startup on a cold engine.
i could see it getting high with a hot engine and condenser, but it was only 70 degrees when i tested it.

 

Was the fan running? I fixed an Edge recently that acted exactly as you describe. The electric fan was running, but not up to speed. A new fan fixed the AC problem.

 

I'm not there to see it, but if you feel it's rising too high too fast then the only other possibility is a restriction in the high pressure line or condenser.

 

that was what i was thinking too.
and i am an idiot.
went back over there this afternoon, and while looking at it realized the fan clutch has nothing to do with the AC. the condenser is on the driver side of the radiator all by itself, with it's own dedicated electric fan..............that is not getting power .
so we now have to figure out why it is not sending power to the fan.
i have a bottle of nitrogen coming in to pressurize the system one circuit at a time to try and figure out where the blockage is.

 

well, finally got back to the dodge yesterday. it WAS sending power to the fan, but the fan motor is bad. so waiting for a new fan motor to be delivered on tuesday.
i put a large shop fan in front of the condenser and misted it with well water and high side dropped to 260 at 84 degrees ambient temp, with 42 duct temps.