I'm scratching my head here. im working on a 2002 f150, 4.6. Here's the deal: first start up of the day and ac blows cold and all is well. shut it off and no more ac after starting the truck again. All fuses are good, relay is good, system has good refrigerant charge, cycling and hi pressure switch have power and ground. Just no power at the compressor's connector (wires). I've jumped the cycling switch and compressor doesnt turn on. same with hi pressure switch - nothing. It's a manual ac system and the function selector has power in and power out of the purple wire that sends the ac request signal. It just seems that nothing is sending the signal to the relay in the junction box. I've jumped the terminals of the relay with it removed and the clutch will kick on. Put relay back in - nada. The best i can figure is maybe the function selector is screwy or i have a pcm problem. Also there is no cruise control available and the airbag light is on now. could they be related? why would it be intermittent? any and all help is greatly appreciated!

 

won't anyone give an educated response? we could really use the help

 

Verify that the clutch relay coil has power on the high side of the coil. Verify continuity of coil low side back to PCM.

Check AC_Request PID with scan tool to see if valid request signal is being received.

If your scan tool has output test mode capability, use it to exercise the clutch relay to see if the PCM is putting out the correct signal to energize the relay. Verify that the clutch coil has power on the high side of the coil. Verify continuity of coil low side back to PCM.

 

Thanks! i'll try the PID thing. when the 'human' request for ac is present, there is no power to the clutch because there is no power at the relay to energize it. this has me thinking that there may be a bruised wire or a pcm problem. But why would it work intermittently?? I'm about to check the hi and low side switches (for the 4th time). maybe i'll just replace them (cant be too much) and see if that changes things for the better. My scan tool does not have output test mode capabilities so i may have to call in a favor and try that. i'll check wire continuity further. wish i had a pinout diagram of the pcm wiring to clutch and relay. perhaps my buddy's shop will let me take a look at his Alldata

 

would excessive compressor clutch gap stop operating after engine gets hot? this system works great at cold start up. run it for a while, it's chillin. turn off ac and it wont come back on. truck does have 200,000 miles on it. perhaps clutch is worn out and its coil wont complete circuit. anyone agree? the gap is just at .030. anyone think a shim-ectomy would help?

 

Service manual specs .014-.030" for the gap. If you have the chance and can catch it disengaged when it should be engaged, tap the clutch plate with a stick or screwdriver handle and see if it pulls in. If so, it's the gap and I'd do the shimectomy.

If the problem persists, I'd then suspect the cycling switch of getting flaky when hot.

 

now there's an answer that gives me some confidence! I was gonna replace the cycling switch for good measure, but the clutch gap is more likely the culprit (i'm hoping). I'll post the results very soon and thanks. anyone else's input is welcome!

 

changed the cycling switch but ac still disappears intermittently. sometimes while the ac request is on (manual controls), there's nothing kicking the compressor on. tapped on the clutch, no change. wiring so far has tested ok. I'm starting to think it may need a control head. it doesnt look like i can change just the function selector switch - it looks like one big unit, not EATC but manual. i'm losing this battle and am tired of messing with it. help!

 

update: has new cycling and hi pressure switches. no help. I ohm-ed the clutch and got 3 ohms which is good. have continuity from the selector switch to the hi and low switches (good voltage too), function selector switch is good, wires traced to pcm good. The last thing is the relay circuit which grounds the compressor clutch - there must be a bad ground between the junction box (relay circuit) and the clutch. that's all i can figure. it would explain the intermittent availability of a/c. Anyone?

 

The PCM sends a ground signal to the low side of the relay's coil to energize it. The high side of the coil is fed via fuse F23 (underfood). When the relay energizes, battery power is routed from fuse F13 (underhood) to the relay coil.

 

thanks for your help SHO. i'm thinking about trying Alldata DIY so i can get better diagrams and diagnostic flow charts. this repair has hurt my pride now. It must be beaten!