I used to be a 97 F-250 diesel guy, but I recently switched to a 2004 Heritage F150 4x4 that is an ex county truck. It has 83,000 on it, but is as tight as new. Before I start I searched other threads, but did not find an answer.

On to the problem. I can get in the truck and start driving in high temp weather and the AC will blow cool enough to make me turn it down. Then after about 30 miles, it stops cooling and blows air at ambient temp. I can park it for an hour or so, and it will go through the same cycle. I checked the AC clutch fuse and it is good. The gauges read about 45psi on the low side and about 225 on the high side (the chart calls for 50-55 low and 275 high at 95 degrees). The manual says there is also a AC clutch relay and an AC clutch diode. Tomorrow I will try jumping the clutch relay. My mind tells me it could be the low pressure cut off not functioning properly and once the truck sits the gas pressures normalizes and allows the system to work again. Or perhaps the relay is heating up.

One question is this: I only see one cutoff switch, so are there actually two cut off switches, and if so, where are they located? Also, is the AC cycling switch just another description of the cut off switch?

So, I'm wondering if anyone has had the same problem or has advice to give. I don't really have enough money to take it to a mechanic right now.
As you can tell, I'm not an AC educated guy.

Thanks for your time.

 

Read HVAC FAQs (link is in signature), particularly the part about the clutch gap.

 

yes.... after 30 minutes, when it gets HOT and not working, stop and look at the clutch to see if it is engaging.

 

Thanks to the link projectSHO89 posted, I was able to find the low pressure switch on the accumulator below the mega fuse. It is behind a metal plate holding a wire bundle, is held on with 2-8mm bolts, and must be removed to get to the low pressure switch.

Before I did that I turned on the AC with the truck off and checked the clutch - it was not engaged and I did not hear it try to engage. I pulled the AC clutch relay in the box under the hood, jumped it and the clutch came on and locked down tight.

So, I'm thinking it is the AC relay that is causing the problem. I am going to replace that and hope it works. After that I will get some feeler gauges and check out the clutch clearance to be safe.

Thanks,

 

Just an update for the curious.

I replaced the low pressure sensor and relay and the problem persists. I drive about an hour and the AC shuts off. I checked the clutch and it looks almost brand new and the air gap is fine. So I took it to an someone with AC training and after running some basic diagnostics, he said the ECM seems to be telling the ac to turn off. Since he doesn't have diagnostic equipment I will have to take it to a dealer or someone with a diagnostic machine to find out what is causing the shutdown signal in the ECM.

 

The series circuit of CJB F05, the function (Mode) selector switch, the HPCO switch, and the low pressure cycling switch provide an input to the PCM to enable the compressor operation.

The PCM, under its programming (assuming no inhibiting faults), will then energize the WOT relay (A/C Clutch relay) which supplies power from F05 to the compressor clutch coil via the energized relay's contacts.

If the check engine light is on, start there as certain faults will disable the A/C operation.

There are a few voltage checks that can/should be made as part of a diagnostic procedure but, after that, a scan tool is needed to dig any deeper.

 

My mechanic checked everything he could including pulling the switch in the dash to test it. He did say adding power to the Low Pressure Cycling switch would turn it on as would jumping the relay under the hood. The Low Pressure Cycling Switch and relay are all new. It will take a diagnostics machine to pin point the problem it seems.

 

Providing that information sooner would have been useful...
No, it would take a mechanic who uses the schematic, his head, and a multimeter or test lamp.

If injecting power at the LPCS (upstream or downstream wire?) caused the system to work, then there is an open circuit upstream of that injection point. See the description of the circuit above.