I'm having problems with my AC/heater in my 99 ram. To start i only have hot and cold, nothing in between. Next, when i have my AC on it will work for a little bit, then you can hear the air mixture flap slap over and it starts blowing hot air. I can turn it off and on again and will work fo a little bit then it happens again. Has anyone had this problem or know a solution? Does it run off vacuum and maybe have a leak. Any advice will help. Thanks

 

Yes the HVAC controls run off vacuum. So assuming that your vacuum pump is working OK, then you have a problem somewhere in the vacuum lines, the HVAC control switches, or the vacuum motors that control the flappers. It may be as simple as loose or disconnected vacuum line, but since the system works temporarily, then its probably a vacuum motor or switch which is loosing vacuum over time.

 

BZZZZZZ incorrect answer... 1999 would have an electronic actuator to move the blend air door. Sounds like the adapter between the actuator and door has cracked. There's a reason I used to keep one in my tool box.

...another possibility, and a rare one, is that the actuator is weak and the air from the blower is actually pushing the door overpowering the actuator. In this scenario the temp control will work fine on low fan, but screw up in the higher settings.