temperature select malfunction

screwy: When I turn my selector to vent it works sometimes and then it stops. If I turn it to off , then back to vent it does not give a/c or vent. It blows through the defrost and floor when on vent. Sometimes it will kick back to vent if I leave it at this position. Is there a vacuum that controls the selector so as to allow the proper opening and closing of a duct door? This is nuts!!!

I had a simular issue a while back and it turned out being a cracked hose on the driver side of the firewall (in the engine bay). It is apparently the hose that gives the air flow control the pressure it needs to switch from vents to defrost and all of the other vent areas.

I had a simular issue a while back and it turned out being a cracked hose on the driver side of the firewall (in the engine bay). It is apparently the hose that gives the air flow control the pressure it needs to switch from vents to defrost and all of the other vent areas.
Marr-TA, thanks for the info. I will look for such hose and change it. I will let you know what happens!cavf150

I have a '93 mustang that has a vent / vacuum problem. When at idle, the vents work fine, but on acceleration, (engine vac drops), the vents go to defrost. I have looked for a hose leak but haven't found one yet. The other vac. operated stuff, i.e. brakes, cruise ctr. work fine.