I have a 97 F150 4 x 4 with a 4.6L V8. When I first start the truck and turn the AC on it works fine for approximately 5 minutes at which time it shuts off. I notice a humidity smell immediately in the cabin and the windshield starts to fog slighlty.

I have done all the maintanence and repairs on the vehichle and would characterize myself as a more than average mechanic but I have very limited knowledge about AC and my Hayes manual only barely touches on AC.

During my diagnosis I preformed the following tests.
1. Replaced the control unit (***** switches for adjusting temp, etc) with the one from my similar model Expedition. Problem still persisted. So this rules out the control unit.

2. Replaced the cylcing switch. Problem still persisted so this rules out the cycling switch.

3. Checked the coolant level when the AC was not working (i.e. after 5 minutes when the compressor cut out) with an aftermarket pressure gauge that attaches the low side located on the side of the dryer. The gauge showed in the red zone which designated high pressure.


4. Watched the action of the compressor. During the first 5 minutes when the system is working the compressor is operating normally. Does not make any noise and appears to work normal. When the AC stops working the compressor is no longer engaged.

5. Felt the lines. The fatter upper was warmer than the lower thinner line when the AC was operating normal. When the AC was not operating both lines were fairly warm if not slightly hot.

What should my next step be? Is there a few more simple tests I can do before I give up my hard earned money for someone else to diagnose the problem.

Any help would be much appreciated!!

Thanks,

Jeff

 

Hi Jeff and welcome to FTE
I think that without putting actual gauges on it you might not figure out for sure what it is.
One thing to check though is the Pressure Cutoff Switch which is on the line that runs from the condensor to the compressor. It is sticking up near the windshield washer reservoir.

 

Thanks Racerguy for your quick reply ... I much appreciate it!! How do you check the pressure cutoff switch and can it be replaced without evacuating the system??

Jeff

 

The switch is similar to the pressure cycling switch and can be replaced without evacuating the system.
To test it you could probably just jump across it but it would be nice to know that your pressures aren't too high before doing that.