i have a 1995 Dodge 3500 dually, V10 with autotrans. Everytime i tow my trailer (a 32' Halmark Edge race car), and i accelerate the air conditioner or heater mode changes. Such as, the air cond vents close and the cold air starts coming out the defroster vents. When the speed of the truck settles (not under load) the air switchs back to the vents, and I hear a duct door slam shut.

Anyone have a solution???

the heat and air controls (as well as the CAD system) are vacume actuated.  I'd start by checking all the vac lines for leeks. if it is leeking someplace under the hood, it should be pretty easy to get to, but if the leek is in the switch, then you have to get behind the dash to get to it.

hope this helps,

mudrunner 

More than likely it sounds like your truck is using all the vacuum it's creating to operate other systems in the engine bay and there isn't enough vacuum to keep the A/C vents open until you get out of the throttle.  If it's only happening under heavy acceleration, this is probably the case.  If it's all the time, Mudrunner's prob. right about the vacuum leak.  But if it was leaking all the time, it would affect other systems in the truck, not just the climate controls.

thanks for you help...it only happens when i am towinf the trailer and the engine is under load...i will check the tech web site to see if there is a cure..

thanks for your help...it only happens when the trailer is being pulled, under load....i will check the web site for a possible cure  

thanks for your help...i check all the line i could see,  the truck runs fine the problem happens when the engine is under load, when towing my trailer (it is a 32' Haulmark Edge Race Trailer - GVW10,000)...other than that the heater/air conditioner works perfect...i thought of putting an aditional vacuum canister on the truck, sort of the model you would use for a street car that has a vacuum problem created by using a radical type cam...maybe the addition of the canister would cure the problem???