So i noticed today while driving on the highway to and from work, and even around town that when i get to the middle- top end of a gear mostly 3-4 the A/C air coming out of the vents will slow down and shut off untill i lay off the gas. Basically the faster i go the air shuts off. I dont think this is normal. Any help would be nice as the weather is getting to be hot and having the windows down doesnt cut it anymore. The truck is a 1996 f150 inline6 5speed 2wd.

 

sounds like a vacuum leak. does the air start to come out of the defrost when it stops coming out of the vents? pop your hood and look on the passenger side by the blower motor. there is probably a white vacuum line that got old and cracked and may be leaking.

 

I had the same problem and it was the white vac line leaking it like octaneforce said.

 

Ill go take alook now and see whats up under the hood. It doesnt come out of any other vents when it slows down. it just stops. I was thinking, and yes its kind of far out there, could it be the tension pully belt thing ( idk the exact name cause im a car noob) thats slipping on the compressor because when its on just vent and not a/c it stays on for the most part.

 

Im not sure on that part. Mine use to go up the defrost or just shut off at 45mph when it was on max ac and taping the small crack the white vac line solved my problem. I have jas88 to thank for that.

 

No cracked lines there. So back to square one of not knowing why. lol.

 

Some great info and pics in this thread from another forum: A/C and/or heater stuck on defrost - FSB Forums

 

Weird. Today it just stopped doing it.... It started to do it then just stopped. So I dont think ill worry about it until it happens again. But thanks for the help

 

Its normal, the EEC cuts A/C under full throttle condition.

Even at that under full throttle engine vac drops sharply, dash venting requires vacuum to hold any setting other then defrost.

See here Ford Fuel Injection » Strategies, have a gander at the section "Full-Throttle Acceleration"

An excerpt,

All EGR functions are stopped and the canister purge valve is closed to ensure the exact air to fuel ratios desired. Smog pump air is dumped to atmosphere, air conditioning, electric fan, and all other high demand accessories are shut off to lessen the accessory drive load from A/C and alternator.

 

Under full throttle condition I would agree with danr1, but I saw no mention in the original problem statement of the OP using full throttle.

 

Nope neither did I however I thought it was worth mentioning so the OP knew about it and could consider what had happen with how it should/does work.

And just maybe at the same time the OP had been into that skinny pedal a little more then originally might have thought? Just maybe?

Anyway with that info a quick test ride should prove or disprove easy enough its normal operation or some other condition as the cause?

 

I thank you sirs for the information, it has helped alot. And i do think i might have been into it a little more than i thought its very possible. (and highly likely) i had no idea about the full throttle adjustments and stuff. Ill read up on it. but it seems thats what it was.

 

FWIW my '92 F350 exhibited the same type issue when I first got it 2 years ago. After searching around on the 'net I narrowed it down to the little plastic reservoir on the side of the blower case. A previous owner had covered it in epoxy in an attempt to repair the cracks. I ended up using an unused portion of the dual chamber vacuum reservoir as a suitable replacement thanks again to the same PO who removed the TAB/TAD system.

 

Yep the Vacuum canister should have enough storage capacity for any full throttle blast, You have a leak somewhere, Get out there and look and listen for it.