96 blazer a/c blows thru defroster going uphill.

Hello,

I have a 96 blazer that the a/c blows though the defroster vents when I go uphill. If I take my foot off the gas, it blows through the vents it's supposed to. I live in a hilly place and this is getting old.

Any ideas?

My first guess would be you have a vaccume leak somewhere for the door that controls your vents. For me it would be hard to say where it could be. I havent done a lot of work on Blazers.

Mine was a 2-way vac diaphragm in the dash. The mechanic had a heck of a time diagnosing it until I was careful to give him a ride and explain that even though vacuum is used to control all, when control is set in one spot it uses vacuum, but isn't supposed to "use it up"

Once He got the concept of using it up or wasting it He found the problem. Yours could just as easily be a leak anywhere in the system or even the supply hose from the back of the manifold.

<FONT face=verdana>Many vehicle manufacturers use vacuum motors to operate heating/AC controls. Most vehicles have at least two of these vacuum motors. One motor is used to move a door that switches airflow “modes,” like defrost, heat, vent, etc. The other motor controls a door that regulates air temperature by “blending” outside or re-circulated air with freshly heated or cooled air. Other vehicles have a third vacuum motor to control a door that allows fresh air to enter the car or restricts the airflow to re-circulate. In yet another variation, manufacturers require two vacuum motors and two door to regulate the “mode” settings. When you accelerate rapidly, the throttle plate opens wide and intake manifold vacuum decreases. That means the vacuum motors lose vacuum and the doors they control move to their “default” position. But engineers know that vacuum always decreases on acceleration, so they design the vacuum motors to work despite the vacuum drop. However, if the vacuum motor develops a small leak, or there is a leak in the vacuum hose or the dashboard switch, the drop in vacuum may be too much to overcome. That’s when you get a change in airflow.