My AC blows cold as ice, but while running, it switches back and forth from the floor vents,the defrost vents, to the dash vents.I don't know if this would have anything to do with what caused it, but it first occured while I was pulling a 20 ft. trailer loaded down w/ about $750 dollars worth of lumber up a hill. I don't know if all the stress could have messed up a vacuum hose or what.If you have any idea what may be the problem and a round about price, I'd appreciate the info.
Thanks,

 

just what you said "a vacuum line", try under the hood hinge left side. that's were the lines go thru the fire wall. look for a crack or a rough spot, normal lines are smoooth, maybe all you feel is just a rough spot. shrink wrap will fix it, use a piece large enough to overlap around it.

 

Yup. The doors that control where the air comes out are controlled by vacuum motors, and if/when you lose manifold vacuum (like when put your foot further in the gas) the doors [/I]MAY[I] change their position, depending on, well, many things. Pulling a trailer would certainly necessitate having your foot further into the throttle, thus lowering manifold vacuum, and I would bet you got lousy mileage while doing it as well.

Try this- get your truck on a long straight where you can basically floor it from a dead stop, like on a drag strip. Before you hit the gas turn on your air and adjust the flow to "high". Then mash the gas and keep her down for as long as you can. Pay attention to where the air is coming from, and see if it changed (pay attention to your driving, too!). If it didn't change I would bet you don't have a leak, but your engine may be a little tired. If it did change you may have a vacuum leak somewhere.

Of course this isn't foolproof, but it's just another tool you can use. I used to get the same thing while pulling my boat, and it was only after I installed a manifold vacuum gauge in my truck that I found I had a leak somewhere.

Good luck!

 

Wait- bad advice! I meant to say that you should try this both with a load (trailer) and without. Also, if you can manage it, try and temporarily rig a vacuum gauge so you can see what's happening. I think what you are experiencing is your truck is working so hard that it is losing all (most) of it's manifold vacuum while towing, and only by comparing loaded to unloaded vacuum readings can you confirm this.

 

I have had this same problem for the last 10 years on my 88. It only shows up when your under steady acc. up a hill. The only thing that helped was that I added a second vac res to the HVAC vac line. Now it only happens in a steady head wind and and uphill grade. I just gave up and learned to live with it.

 

Some of the older Fords used the coffee can style vacuum reservior and they are subject to rusting out underneath at the mounting tabs, go to the salvage yard and get a plastic style reservior from something other than a Ford and replace it, coachtap76, you didn't say which engine you have in your truck, 300 6 cylinder I have seen melted the piping from around the back of the engine, vacuum hosing goes from engine intake manifold to reserviour to interior HVAC controls. Broken Wire

 

I do have the coffee can style reservoir on my truck.It has a V8 in it.It usually starts switching back and forth while in acceleration.Gas mileage has also gotten pretty bad.

 

light throttle its ok, heavy throttle it switches and then u let up it switches back. its a vacuum leak. could be the can or a hose. check all the hoses, they may show signs of dry rott. you can spray some either(sp?) around the line and if the engine speeds up you have found it, but don't neglect to check them all.

 

Try looking at the thin white vacuum hose that is on the passenger's side of the engine block, and attaches near the blower motor. It's usually covered with some wire loom, but it's notorious for rotting through. I had the same problem, and that's where I found my leak. After applying some heat-shrink tubing, I've yet to have a problem...