Dodge Ram AC and Heat switches between defog vents and dash vents when going up a hill

My Ram, '91 model, flips between the defog and the dash vents when you start up a hill. It don't have to be much of an incline either, just 2 or 3
percent, just enough to put a little load on the engine. I've had it to the part changers over at the dealership and after $300 it is still doing the same thing. Now they want to put a $400 part on it, a gas canister. With it only doing it on an uphill slope it seems to me it would be a vacuum thing.
Anyone have any ideas?
KY
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It sounds like a simple vacuum check valve has gone bad. As I get older and the memory deteriorates, posts like this help me remember why I don't want to buy a new truck again or have anything to do with dealers.
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Wow that would be cool. where is it located on the truck?
Thanks so much.
KY

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Yep... check for vacuum leaks - common areas are the cruise control diaphragm that sits under the battery tray (acid drips on the vaccum lines and eats them away), the vacuum line going to your your transfer case, and the lines from the case to the front axle disconnect (if 4x4). As BIR suggested, it could also be the check valve on the line that runs to your HVAC controls. Find the vacuum line off the back of your intake manifold, and look for the check valve. Note which side connects to the engine, and which goes towards the passenger compartment. Remove the valve, and try pulling a vacuum (use a hand vacuum pump, or, in a pinch, suck it like a straw) from the engine side. Air should flow freely. Do the same on the other side. There should be restriction (no air should flow). If it does, the check valve is bad.
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Wow Thanks so much Tom. I'll never go to the dealer again. LOL This truck has had problems for a while now and no one seems to be able to fix anything any more.
KY

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snipped-for-privacy@roadrunner.com says...

Get yourself a couple of rolls of vac hose and start changing them out. You're sure to find cracked hoses in a 16 year old truck. The cruise diaphragm is a good bet too.
BDK
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thanks. I found a really small hard plastic tube that was broken off. looked like it went to the cruize diaphram.
KY
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I had a '94 doing the same thing. They come from the factory that way. There is a fix that increases the size of the little vacuum canister next to the throttle body. The factory one is about the size of your little finger, the fix is about the size of a 35mm file canister. Someone posted the part number and connectors for the fix on line but I erased my copy when I installed it. Parts cost about $10 and 10 minutes to put it on. No more problems. When you hit the throttle the little cannister doesn't provide the vacuum buffer and the vents change from dash to windshield. Si
KYHighlander wrote:

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LOL cool stuff thanks. any idea where to find the page online?
KY

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Go to http://dodgeram.info/tsb / and look under 24-Heat/AC for TSB 24-01-96A. This is the Dodge response to the problem but this is not the fix that was posted on line. I can't find a copy. The fix I applied was to replace the existing canister with a larger canister from Dodge. If I remember correctly it is the same canister that was installed on the Jeep Cherokee. The other parts were miscellaneous rubber connectors from Advance Auto Parts. You might talk to the Dodge parts man. Si
KYHighlander wrote:

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Ok thanks so much.
KY

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Must be built into the vehicle. My 98 ram van does the same thing
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