96 Jimmy A/C Problem

I have a 1996 GMC Jimmy and the A/C works great except when i go to accelerate it will stop blowing out the vents that is pointing towards the driver and
passenger and either go to the floor or to the defrost usually the defrost vents and then once i stop accelerating it will return to the normal vents. This happens anytime the truck is put under load even if its just going up a hill. Any ideas y?
Thanks matt
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mdavis84
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snipped-for-privacy@no-mx.forums.yourdomain.com.au says...

The blend doors are controlled with vacuum motors. Under acceleration the engine has a "no vacuum" situation so all the blend doors go to their default position. There is suppose to be a reserve vacuum canister along with a check valve that is suppose to prevent this from happening. I would check under the hood for a black plastic canister with one or two vacuum lines connected to it. Thats most likely the problem.
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Dennis Smith
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