In the last couple days I have noticed air coming out A\C vents when all controls where off. Now the only place the air will come out is the defrost vents. Someone told me I may have a vacuum leak that is not allowing the door modulator to work. I have looked under hood for broken vacuum lines and under dash. Any ideas on where or what the problem is? Could it be the electrical switch the A\C compressor kicks on.

 

Sometimes the vacuum pump fails, so check your vacuum line and make sure you have a vacuum to the cab with the engine running. If that is good, then go to the air motor on the internal damper (below).

Remove the right side kick panel and you will find an air motor that regulates the heater A/C internal deflector door. Once you have it exposed, check to see if it is stuck by adjusting your HVAC controls while the truck is warmed up and running with the A/C switched on. If that motor is functioning, then look at the cable controlled arm that is located on top of the heater core. The cable on that will break loose from the dash cable control and then the system gets manually stuck in one position until it vibrates to another. If the motor is not functioning, then pop the vacuum line off and install a vacuum gauge on the line and switch the top control from defrost to recirculate, stopping at each selection for a moment, and read the vacuum. If you do not have vacuum on any setting, then the selector in the HVAC switch has failed.

If the vacuum is good, and the deflector panel works properly, then move to the manual cable on the regulator valve arm that is located right next to the coolant feed tubes to the heater core itself. Twist the selector switch and watch that arm to see if it moves the regulator damper itself. If not, then the problem is likely the cable end has failed or the HVAC switch has broken.

Should you trace your problem to the HVAC control unit itself and have to pop the dash apart to remove the HVAC control assembly be sure to disconnect your negative battery cables first, then flip on your headlights to kill the charge in the capacitors for the airbag system. Then you may pull the dash panels and remove the HVAC control without the risk of triggering a very expensive air bag and blowing out your dash.

If you work better with photos of the assemblies, pick up a copy of Haynes Manual #30041 and that will walk you through the entire process.

 

I had a 01 2500 and the dash control failed and when it failed the defroster was stuck on, it happened several different times before it finally stuck completely. I troubleshot it and found that the in dash switch assembly failed and defrost is the default position. I replaced it and had no problems. kinda pricy..

 

Thanks for the responses and I will start checking out truck after work tonight.

 

Checked out truck after work and found the vacuum line that runs along firewall had a hole rubbed through it. The line actually was laying across the trans. dipstick and from the vibration rubbed a hole in it. Hey thanks again for the technical help.