First U have 2 make sure U have Vaccume @ the belt driven Vac.Pump,then U have 2 repair the Vac.Leak. Some Dodge truck have Vac.lines near the battery & Bat.Acid will make the Vac. line Brake.Some of the Vac.Hoses are in a plastic loom that keeps the hoses & wires 2gether.Loss Of Vac.2 the control is the problem.
I have a 2001 - my problem solution was the vacuum hose that is located right under the weatherstriping for the hood. Just about the center of the block, there is a rubber elbow that faces down and bends to the rear of the truck. The is a small vacuum line that is in a plastic wire loom that should be connected to this elbow. My hose fell out of the elbow and was against the firewall. I re-connected this hose and put a small zip-tie on it(to avoid the problem in the furture) and it now works perfectly! P.S. The 4 wheel drive will not engage the front tires with this hose off either. I found this out the hard way as I got my truck stuck on a slimmy boat ramp trying to pull out my boat!
you have a vacuum leak. defrost is the default when a vacuum leak is present.
Because a diesel engine does not create vacuum and the vacuum pump is necessary to operate things like the vacuum assist brakes and cruise control.
Vacuum pump the same vacuum pump that works automatic locking hubs on the 4wd
You have a leak in the vacuum supply to the ac control.
More than likely the vacuum feed line from the engine has been disconnected or cracked. When there is no vacuum supplied to the HVAC it reverts back to the defualt position, the defroster.
Check for a vacuum leak either under the hood or under the dashCheck the vacuum actuators under the dashCheck the selector control Defrost is the default for this system
Check behind the heat control panel to be sure that the vacuum hoses are all securely plugged into the control switch. As a safety measure on all cars, if you lose vacuum altogether, the default position for air direction is on defrost, so you can always see out of the windshield. Jake McCahn
try your local junkyard
You have a vacuum leak in the fan control system somewhere... not easy to locate the leak
Selector control Vacuum under hood and under dash Vacuum actuated motors under dash
most likely there is a vacuum leak. check the vacuum lines leading from the intake the the interior heat and a/c control. look for cracks around the vacuum lines. check the vacuum canister and check valve.
Something about the vacuum linr being unattached, maybe from the firewall. im trying to figure out the same. The main vacuum supply line is disconnected or cracked, probably near the engine. All the HVAC doors operate on vacuum. When there is no vacuum the HVAC system goes into defrost mode.