In my 92 firebird i just replaced the head unit for my HVAC controls. On the old unit the vacum was leaking and the switch for the blower motor had to be jumped in order for the fan to work. well i solved the problem with the fan controls. but i still can't get my vents to change from the dash vents to the heater or defrosters. i have vacum and i can hear everything changing over but it still only coming out the dash vents. you can hear the diffrence from max ac to norm but the vents still won't change over any suggestions?

 

Is the vacuum line at the engine connected? Is the check valve under the hood working properly? Has your car been wrecked? Is the plastic vacuum resevoir intact? Any of these things can cause your problem.

 

As far as i know the car has never been wrecked and there is vacum up to the head unit which is working. I think the physical mechanism behind the dash is jammed i was wondering where to look or how involved it is to get back there to check it.

 

I know when you lose vacume, the air blows from the defroster, safety reason. so maybe the actual mechanism behind the dash is screwed up.