theshummer

Howdy folks,
Got an interesting issue that just developed. When I get up to high speed (above 60 mph or so) the air conditioning will change from the face vents to the defrost vents to the floor vents and back to the face vents all by itself. It only seems to do it in Max AC, not in Normal and it does not do it at slower speeds. What's going on here?
Thanks!
Bryan

chelios

Oh how I've seen this before. There's a leaking vacuum line under your hood. Forgot which one it is though. Might as well check em all.

wde3477

Typically the white hose going to the recirc valve located on top of the evaporator box on the firewall in the engine compartment.

ymeski56

ymeski56

Dup

Forgetful

Had the same problem. I posted some pics to make it easier. Mine's a '94.

https://www.f150forum.com/f10/ac-max...defrost-51243/

Good luck.

dr_bowtie

usually its the line going to the vacuum ball on the fender or can as ford uses...this is what hold the reserve vacuum for the doors when it changes in the engine as throttle position changes...

theshummer

Forgetful: Thanks for the pics.

Everyone else: Thanks for the pointers. I'll check it out and let you know what I find.

blade z51

Good post! Thanks for the information. I suspected some problems with that vacuum solenoid. I'll check the hoses before I look into that blower motor resistor...

blade z51

OK, here is what I found out today. The temps finally went down a little bit and this evening it was probably in the 80's for a change. I drove the truck for about 25 miles going to the farmers market and back and the A/C was actually blowing cool air . I then began to notice that the loss of air from the vents coincided with my foot pressing down on the loud pedal and the air flow came back each time when I took my foot off the pedal... Obviously it seems to be very much related to engine vacuum.
After looking at the other thread mentioned (https://www.f150forum.com/f10/ac-max...defrost-51243/). I looked at the vacuum valve over the fan enclosure. It's a bit rusty and all but the hose seems to be OK although it's quite discolored. I don't think it is leaking vacuum at the hose or the connection point. It seems that the engine vacuum is activating that valve in real time... I think from reading on the subject that there is some kind of vacuum accumulator and it might be that it is not doing its job... Perhaps the hoses to and from this device are leaking.
I would really appreciate some advise (preferably with good clear photos or drawings) about how to trouble shoot and repair or correct this problem. If any of you have dealt with this issue, I would really appreciate your advise and help.

Thanks!