Question about 1991 GMC Sonoma
I recently solved this same problem, look under the dash and there will be a rod that connects to the black exchanger box and most likely that rod has come undone and is not switching like it should
Posted on Oct 15, 2009
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Posted on Jan 02, 2017
common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.
Posted on Feb 23, 2009
Not sure about your exact model but a lot of these are vacuum actuated. If one of the vac lines is broken, leaking or disconnected this will happen. Maybe take look under the dash to see if something is obvious while you try to switch it - look and listen, good luck.
Posted on Mar 29, 2009
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