GMC Repair: 99 gmc safari heating/ac controls, gmc safari, vacuum storage

Question
My 99 gmc safari awd temperature control seems to be malfunctioning.
Whenever I go uphill or put a load on the engine, even if I have it set to cold air
conditioning, will begin blowing hot air instead of cold. This also occurs if I am
not running the a/c. Also, there is very little control of the temperature settings
between the extreme cold and the extreme hot condition. Any suggestions?

Answer
Hi
I would get the Vacuum system checked.
The system uses vacuum to control actuators that control airflow and temp control.
The vacuum would come off the intake manifold and go to a vacuum storage tank. (Usually mounted on the hood or in a fender well) From there it goes to the inside of the vehicle and feeds the vacuum to the knobs (HVAC controller) where it is distributed .
On this particular vehicle I would check the vacuum line that goes into the van on the DRIVERS side around the battery.
They have a way of either corroding due to battery acid, or breaking just because of the location to the battery.
You SHOULD possibally be able to hear the vacuum leak in the form of a hissing or whistle sound.
I really hope this helps.
Please follow post back, if needed.
Pawl