I did a search but did not see a solution to my problem. I have a 1998 F150 standard shift with the 4.2L. My issue is the air works at idle and when driving as long as I keep the RPM's low when shifting. I can get to highway speed and it will keep working most of the time. If I stop and start again I have to shift at low RPM's and work up to highway speed slowly. If I let the RPM's get above probably 2000-2500 the a/c shuts off and blows hot air until I turn it off for a few seconds then back on. It will then work until the RPM's get high from shifting or passing and it cuts off until I turn it off for a few seconds and on again. I am looking for things that I can check, I live in a small rural town and I don't have a reliable auto a/c company that I trust to work on it until I have an idea of what it could be so they won't try to screw me over just swapping and replacing things to try to fix the issue.

 

Some vehicles shut off the AC compressor when accelerating to free up the hp. It could be that switch acting up. I'm just guessing in the dark.

 

This is the key, but the description is not accurate enough for a diagnosis.

The distinction must be made to determine if the compressor is simply shutting off or if the temperature blend door is going to the full hot position. Therefore it's a matter of degrees(!). If it's going to HOT, as if the temperature control had been turn to the the full HOT position, you have a broken blend door. If it's simply no longer cooling as if you'd switched to VENT, the compressor is dropping off line, probably due to a worn out clutch.

 

The 98 F150 doesn't have that type of control, it was first implemented in the F150 for the 2000 model year.

 

Thank you projectSHO89 that gives me something to check and narrow things down. It does feel like the air is hot air coming from the vent, I will double check tomorrow when I take it on a drive and check the compressor to see if it is still engaged when the a/c stops working.

 

sorry for the delay, I had to go out of town a couple of days. I think I have determined it is not the compressor clutch. I have found that I can it works fine until I hit highway speed for a short time then stop like at a light or intersection. After stopping as soon as I start to take off and the rpms get very high I have been listening and can hear the door down under the dash flipping. Soon as I hear that sound I start getting the hot air. So I am down to thinking it is a switch of some kind bad or a vacuum issue.

 

Check the little vacuum hard lines behind the battery - they crack and break when they get old.

 

That's the set of symptoms of a busted temperature blend door.

If it was a vacuum leak, the airflow would divert to the defroster from the selected setting. Temperature control is electrical, not vacuum actuated.

 

The air keeps coming from the dash vents. Is there a switch or sensor that could possibly be causing the blend door to swap positions like it is doing? I would think if the door was broken that it would stay stuck in one position or the other (but my thinking has been wrong before).

 

No.

If the door breaks like it typically does, it becomes disengaged from the driver coupling. The door then flops back and forth from its momentum when either accelerating or decelerating with a correspondingly abrupt change of temperature. If it were the clutch disengaging, the temp would rise from cooling to ambient (like in VENT). When the blend door is busted, it doesn't go to or from ambient, it goes to or from HOT.

 

Thank you projectSHO89, that makes all the sense in the world. I now know what needs to be replaced so I can work on finding someone that can do it for me.

 

I have a 98 and my ac is blowing hot air, even when ac is off and using the outside air, still blows hot air. I checked my compressor and it seems to be engaging, I went through the lines and found a small air leak . I tightened the connection and still blows hot, I ordered oring kit with drier,cannister, orfice tube, orfices. Any suggestions for why its blowing hot?

 

sounds like you possibly have a broken blend door as well. The bend door is what changes the air flow from across the heater core to across the evaporator. When it breaks it can get stuck where you get heat no matter what you do.

 

If you would have read the WHOLE thread, you would know what the problem is. Please read it.

 

Hey sorry for crossing threads, new to the forum. Ok I'll look into the blend door, which is located inside the cab passanger side. Thank you for your time.