The patient: e99 F250 Super Duty with 7.3 "Million Mile" motor, 350k miles.

Symptoms: AC won't engage, low pressure PSI reading is over 85, past my needle's capability.

Story:

Bought the truck and found a big fat rock hole in the condenser. Swapped condenser and dryer, vacuumed, oiled, and recharged the system. Worked great for 1 month. Now seems any time I turn the truck off or turn off the AC it sends the low pressure soaring above 85 PSI. If I "De pressurize" the system to normal, Turn the truck on, then the clutch engages, and things go back to normal, then below normal because I just depressurized 2 cans worth of the system. Add two cans and it's good to go.

Does that sound like voodoo to you guys? Gremlins? Sign of the end times?

 

Go read the HVAC FAQs (link in signature). Your answer is in there.

 

Symptom: No cooling. No engagement of compressor clutch
Problem: Compressor clutch does not engage. (Ambient above 45F)
1) System discharged (low refrigerant level). Measure system static pressure. If below 45 psi, repair leak, and properly recharge.
2) Faulty low-side cycling switch on the accumulator can. To test, unplug the connector from the switch and insert a straightened paperclip into the harness connector. Test only for very short duration. If the compressor now engages, the switch is bad (assuming you met the minimum pressure requirement in #1).
3) A/C clutch gap excessive. Measure the gap. If too great, the clutch simply cannot engage. Remove a shim, if possible.
4) Other electrical circuit fault. Includes open clutch coil (should be around 3-4 ohms), failed clutch relay on those vehicles that have one (some control the clutch directly via the cycling switch), open high-side cutoff switch, open fuse that supplies the circuit, a faulty selector switch, or some other circuit fault that will require in-depth troubleshooting.

Problem: That el-cheapo gauge from Wal-Mart says I’m supposed to have 30-40 psi in the system but it shows I have between 80 and 120 psi.
Q. Should I let out some of the refrigerant?
A. NO! That is your system static pressure when the compressor is not running. You need to diagnose why the compressor is not running. See above.

 

Well I did the exact wrong thing, doh!

I forgot to mention that I removed a compressor shim also. It was cycling great before the overpressure problem.



2) Faulty low-side cycling switch on the accumulator can. To test, unplug the connector from the switch and insert a straightened paperclip into the harness connector. Test only for very short duration. If the compressor now engages, the switch is bad (assuming you met the minimum pressure requirement in #1).


Is this the same as the dryer? If so, I'll try that next.

 

Yes, "accumulator" is another name for the "dryer" since the can also holds the bag of desiccant.

The 99 Super Duty has a very simple compressor clutch control circuit. It's just a switched fuse F24 under dash > mode switch > low pressure cycling switch > high pressure cutout switch > clutch coil. Pretty easy to troubleshoot with a meter or test lamp.