feets

I converted my 92 D250 to R134a a couple years ago. It got a new compressor, drier, condenser, and expansion valve to replace bad parts and fully upgrade.
Now, I can't seem to keep the high side pressures down.
Two weeks ago, the pressure would hit 400 and even 450 if I didn't shut it off in time. We suspected the fan clutch but changing it didn't help.

I performed a vacuum check on the system Saturday and it was fine. After shooting oil back in, I ran 2.4 lbs of 134 into the truck and the pressure stayed between 220 and 240 at idle.
The only changes were cleaning off the battery connections and unplugging/plugging the compressor relay (along with the other two, don't know which one it is).
Everything was fine afterwards.
I put about 75 miles on the truck and this evening, while sitting in a drive though, the compressor started growling like it was spiking the pressure. Turning off the A/C made it go away. A few minutes later, I turned it on again and everything was fine. I did not let it run when the truck was sitting still.

What could be causing the problem?
Anybody run into anything like this before?

rockfoot

A blockage at the expansion valve will make the hi side go really high... Seems like the compressor clutch cut-out switch back by the expansion valve, would disengage the clutch before the pressure gets too high... I have watched mine cycle and when that pressure gets about 300-350 the clutch disengages and everythings kinda settles out then the clutch re-engages and starts the cycle over. The switch that controls it is feeling the superheat from the evaporator.... if that switch ain't workin it ain't gonna cut-out the clutch on the compressor and the pressure is going to continue to climb... there is a relief valve on the compressor that is supposed to go at about 450psi...

feets

the FSM makes it look like you've got to do surgery to replace the switch. It says there's a tube that inserts into the high side line to sense the pressure. How difficult is it to replace that switch?

rockfoot

Piece of cake... the hardest part is getting to the two little screws that hold the switch in place...

rockfoot

I did not read what you said in that last reply... I believe that tube inserts into the low pressure side after it leaves the evaporator...