Question about 1999 GMC Safari

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Vent selection not working After the transmission was rebuilt

After having the transmission rebuilt on my 99 safari the AC vent selection control no longer diverts the airflow to your feet or dash vents it stays stuck on the defrost vents. is it possible qa vacuum line has been dislodged when the transmission was removed for repair?

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They are controlled by vacuum ... a line must have been disconnected or broken when the trans was replaced. Remove the dog house and look for a small vacuum line broken or pulled apart.

Posted on Jul 28, 2010

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NOPE- Replace the 3 way check valve in your accessory vacuum line. Easy $5 fix!

Posted on Oct 10, 2011


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The controls for your heater/vent selection is inside the dash, behind the control switches you use to select the vents the heat/AC will come out.
When you move the selection levers, there's a few things that happen. Vacuum is applied to the doors (which are inside) and they open or they work using cables to move the doors or both. The different doors open, directing air flow to the vents selected.
Remove the controls from the dash (which should just be removing some screws, maybe a few small pieces of dash and pulling it out). The controls will be connected inside so they will not come out very far.
Just check to see if anything is disconnected and try operateing the controls with the heat on to see how they work.
Sometimes they just stick or the doors stick. This system is the way the 1991 models worked but since they are the exact same, it should be this way also.
Look it up in some sort of repair manual for more info. I find with these trucks, they don't heat very well using the front vents anyway. I find using the bottom vents work best for heat unless you need defrost.
The "heater" selection will atomatically use the bottom vents, "bi-level" AC will use the front and bottom vents if I remember right and regular AC will automatically use the front vents. For the trouble in removing these controls (the whole dash would need to be removed for repair) I probably would just live with the way they work if they do work atleast properly (enough heat and AC). If you find that you don't get enough heat or AC when in use, check the cooling system for proper operation and blockages/mineral buildup and check the AC system incase it needs recharging or you have leaks.

Apr 12, 2009 | 1992 GMC Sonoma

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