A/C blows warm when driving...but...
 

...but blows cold when I'm parked and idling. I can't figure it out. I have a 94 f-150. It is a r-134 system. The mechanic who did the a/c in the first place can't figure it out. I've been back to him at least 5 times to no avail. I bought all new parts for it. He's absolutely clueless at this point. Has anyone had this problem before? Any thoughts would help.

At higher rpms the suction side pressure will go down and possibly in your case low enough that the low pressure switch is opening. What are the high and low side pressures at 2000 rpm and at idle?

John :)

jt,

thanks for your input. I'll try to get all that info later this week. What I'm doing is trying to help this darn mechanic to fix it so whatever info I get from here, I pass it on to him. After going to him 5 times, I'd just as soon help him at this point get it fixed because yesterday, with the humidity factor, it was up to about 97 deg here in s.florida. I'm never going back to this guy, but for now, I need my a/c fixed.

Fill me in some more on this low pressure switch thing. I don't know much about a/c. The fact that the suction side pressure goes down makes sense to me, but what does the switch do exactly and where is it located. When is it supposed to close and open under normal circumstances? thanks much.

The low pressure switch should be threaded into the accumulator tank or line going into the accumulator of the suction side coming out of the evporator. It is a two wire connection and the switch threads directly into the fitting and has an o-ring for sealing. It is a schrader valve type of switch, like a tire valve, which means you can remove the switch without losing the freon. The switch is designed to open when suction pressure drops below a preset point, usually about 30- 35 for an R-12 system. If the R-134 charge is a little low the suction pressure will drop below the preset and shut of the compressor, which is what I think is happening to you. Also with R-134 freon many times the low pressure switch is set lower, like around 20 psi. Some low pressure switches are adjustable so remove the connector and look down into the switch connector and see if there isn't a small adjustment screw. If not you can replace it with one from NAPA. If it does have an adjustment, turn the screw CCW for lowering the pressure. Make small turns like an 1/8 of a turn to adjust. You have to have guages attached and engine rpm needs to be at about 1200 rpm to set the adjustment properly and also to check high and low side pressures. What are your high and low side pressures at 1200 rpm and at what is the ambient air temp at the front of the condensor? If youcan get me these I can probably help you through this. I said 2000 rpm in my earlier post when I meant 1200.

John

are you in NORM AC or MAX AC, Max AC recirculates INSIDE air, Norm AC brings in Outside air.
Cools 97* air to 40* won't happen period.
Best ANY AC can do is cool it 30* that is why it needs to cool Recirculated air.
Is your recirculate on air door operating correctly?

Could be the recirculate door vacuum servo is leaking.

Have you noticed whether you get cold air on deceleration? (high engine vacuum condition)

sorry it's been a while....been extremely busy.....

hhot71,

the recirculate door is definitely working. I checked it myself and felt the difference when I turned the switch. I actually never knew if it worked because since I've owned the truck, the switch has never seen "norm ac". It's just to damn hot.

jt,

thanks for all your help. the mechanic informed me he replaced the switch and made adjustments and that fixed it. Ahhhhh, and life is good. thanks everyone.

I have a similar problem but just the opposet. My A/C will work fine them the cold air turns normal (not warm) If I rev the engine to 2500 rpms it begins to blow cold again.
Can someone help

AL

Check your charge level.

Review previous A/C system posts for ideas.

Steve

Throttle body
 

Had the same problem on my 05 f150. Truck would also die occasionally, especially when backing out of a parking spot. As soon as I braked & shifted into drive it would just die. Ford told me I had a bad throttle position sensor & since you can't buy just that part by itself, we changed the whole throttle body. As soon as we did, ac started working like brand new....

The computer intentionally disables the A/C when there are certain other faults that affect engine operation.