Well, my saga continues on my intermittent AC problem on my 99 f350 v10. As before, the AC blows cold but mysteriously stops working on occasion when climbing a grade or stopping at a light in warm weather. I bought a gage kit and runs ~ 45/250psi on the low/high gages. Clutch is stopping and the metal tube from the accumulator tank runs warm when this occurs. Seems like the problem occurs when the underhood temp reaches a certian level. Thought it might have been one of the switches but the pressure seems to be OK when these problems are occurring. Any thoughts on what/how to check to find the problem? some of my thoughts are:
1. Wire a light in the clutch engagement circuit to see if the problem is in the compressor clutch coil not working when powered up or some heat related failure of the wiring continuity/
2. Check the Hi and low pressure switches for continuity when this is happening. to do this I need to know a few things- location of the high pressure switchlow press in on the accumulator) and whether the switch should be open or closed in normal operation. Also can the swithes be replaced without losing the charge in the system? Any thoughts or help??????

Ross

 

Two switches in the AC circuit either of which could be causing the problem. I have listed both of them with their descriptions. Hope this helps.

John

1) The A/C pressure cut-off switch is used to interrupt the A/C compressor operation in the event of high system discharge pressures.


The A/C pressure cut-off switch is mounted on a Schrader valve-type fitting on the high pressure side of the A/C manifold and tube.
A valve depressor, located inside the threaded end of the A/C pressure cut-off switch, presses on the Schrader valve stem.
This allows the A/C pressure cut-off switch to monitor the compressor discharge pressure.

When the compressor discharge pressure rises to approximately 3103 kPa (450 psi) , the switch contacts open, disengaging the A/C compressor.
When the pressure drops to approximately 1793 kPa (260 psi) , the contacts close to allow operation of the A/C compressor.
It is not necessary to discharge the refrigerant system to remove the A/C pressure cut-off switch.

2) The A/C cycling switch is mounted on a Schrader valve-type fitting on the side of the suction accumulator/drier.


A valve depressor, located inside the threaded end of the A/C cycling switch, presses in on the Schrader valve stem.
This allows the suction pressure inside the suction accumulator/drier to control the operation of the A/C cycling switch.
The electrical switch contacts open when the suction pressure drops to 152-166 kPa (21-24 psi) .
NOTE: A/C cycling pressure switch readings during clutch disengagement (clutch off) may read as high as 186 kPa (27 psi) for a properly operating switch with a normal calibration of 152-166 kPa (21-24 psi) during clutch disengagement. This may be caused by the pressure reading being taken from a low side service charge port that is not adjacent (within 177.8 mm/7.0 inches ) of the A/C cycling switch. A pressure drop in the A/C line results in high readings.



The contacts close when the suction pressure rises to 269-317 kPa (38-46 psi) .
When the A/C cycling switch contacts close, the A/C clutch field coil is energized.
When the A/C cycling switch contacts open, the A/C clutch field coil is deenergized and compressor operation stops.
The A/C cycling switch will control the A/C evaporator core pressure at a point where the plate/fin surface temperature will be maintained slightly above freezing.
This prevents icing of the A/C evaporator core and blockage of airflow.
It is not necessary to discharge the refrigerant system to remove the A/C cycling switch.

 

A a close to this old thread, the solution (and cause) was too large of airgap on the a/c compressor clutch. I was inspecting the surfaces which looked like they were slipping and reduced the shims to close the airgap and everything was good again. Evidently when eather got warmer and system pressure was higher, it was enough to cause the clutch to slip.

 

FWIW, If you have power to all your switches and the compressor (gray wire with white trace, runs in wire loom on passenger side front of engine) try pressing in the clutch with a broom handle or whatever you feel safe using while the engine/system is running. If the clutch grabs and starts working, your clutch face is worn. You can replace it, or just pull it, watch for the shims on the backside, they will fall, then either grind them down a bit, or replace them with something that is thinner. A magnetic fields strength drops quickly with distance, and once the back of the clutch face is worn too much, it will no longer pull in. Usually happens when the system has been working for a hour or so, then just seems to quit cooling. Goes back to working after the truck sits w while.

 

Considering they posted that 16 years ago, there's a good chance they don't own that vehicle any longer.

 

I often wonder how in hell newbies find necro-threads like these?

 

I imagine from using the search feature.

 

Blame it on Google....

along with obliviousness to dates.