E350 Super Duty A/C Issue
 

Why does the A/C cut out from the dash vents and switch to the defrost vents when I accelerate to pass or go uphill?

A lot depends on the year of the vehicle.
Older vehicles controlled the air directing door with vacuum and when accelerating vacuum drops momentarily , and the directional door reverts back to the default position which is defrost.
A lot of that was reduced by having a vacuum reservoir that looked like a large tomato juice can or a black plastic ball shaped reservoir. Some systems also contained a check valve in the vacuum supply from the engine so vacuum drop was not as apparent.

I have a '97 E150 Conversion van with a 5.4 and mine SHUTS OFF the A/C momentarily under those same conditions. It comes back on after things have calmed down. I assumed since it was originally built as an ambulance chassis that it was a safety feature to ensure the engine wouldn't stall under heavy load. Also, about every 10 years, the diverter system that controls which vents the heat and A/C come out of FAILS, and if I want anything but the defrosters on I have to have Ford tear out the dash and fix it. I've fixed it twice and the last time it cost me $1600 and it only lasted 8 years. It stopped diverting again about 6 years ago but screw it, I'm not fixing it this time. The defroster/defogger still works, I just don't get air blowing into the cabin or to the floor anymore.

Those vacuum controlled diaphrams most times last the life of the vehicle.. Something else may be going on to cause them to leak.

If oil is getting into the vacuum system , it can destroy the diaphram. That is why just replacing a malfunctioning part is only part of the repair and we don't know if that is happening.

Might be a good idea to have someone with the time and experience check the system more carefully.

Thanks. The first time the cable coming from the vacuum diaphragm that controls which vents the air goes to broke. The 2nd time they never told me what the problem was, they just said they fixed it. This time I don't want to spend a few thousand to fix it on a 27 year-old van. I've been told by my local shop that they've seen this problem more often than they'd think, and they won't even touch it, "go to the dealer" is what I hear.

It is unfortunate that your shop does not want to handle this. It does tie up a man and does not produce a lot of revenue.

Can you consider this,
all you need is a phillips head screwdriver , remove the screws that hold the glove compartment in place. Place them so you know exactly where to reinstall them. Some have small hex head screws that hold the box in too.
Remove the glove box and you should be able to see the back of the dash with all the ductwork and controls.. The hardest part of this is emptying the glove box of all the junk, lol
.If you can see what is supposed to move and is not moving, you are half way there. Do you think you might want to try this. ?

Thanks for the suggestion but I have a '97 Explorer Conversion van (custom made). I dont have a glove box in the dash, it's just a flat panel, part of the whole dash. All the storage normally in a glove box is in a wooden center console attached to the doghouse. I would try it if I didn't have to remove so much just to get to it. I was told the original broken cable and diaphragm were in the upper area of the dash somewhere behind the passenger airbag and the radio. Thanks again for your concern, but I'm going to leave this one alone.

Someday, when the spirit moves you , you just might decide to tackle the job. If you have the tools, and access to the info to read up on, you never know , you just might decide you are going to fix this. Many times what looks like a terrible , rotten job, once started and without any outside pressures, when it is done , you just might look back at it and wonder why you never attempted to do it earlier.

Of course , I agree with your present decision, got enough to do now the way it is lol.

A/C temporary Fix
 

My 1985 and my 1996 f250's both have the A/C problem, and I don't have the time to track down the vacuum leak. So I took a piece of aluminum flashing and made two little scoops and wedged them between the dash and the windshield. NOW when the A/C reverts to default out the defrosters I have plenty of cold air blowing into the cab. Just the opinion of an 87 year old do it yourselfer. 🤠