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Q: The vents on the truck are stuck on defrost and floor vents. Need help. on 1991 GMC Sonoma

What needs to be replaced to make the vents get off the defrost and floor vents. The A/C works fine and also the blower motor. My only problem is I can not change the vent setting off of the defrost and floor vent setting. What can be the problem. Is it some vacuum hoses that are not attached are the control switch for the vent setting. Please help me out with this.
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look under the hood from the vaccum canister to the firewall by the pass side you may have a broken vac hose that controls the a/c and heater vents. if you can't find it and you have access to a smoke mach that will find it.
Is the vaccum canister to the firewall look like a round ball? And how many vacuum lines are supposed to be hooked up? Th motor was recently changed out and I don't know the original lay out under the hood.
Agree, most likely a cracked or loose vacuum line in the engine compartment that needs fixed or reattached.
Ok I just got into the dash and found out that under the dash that there are two round disk like things that open and close the vent doors for the defrost and floor vents. They do not want to operate rite so I just disconnected the vacuum lines that run to them and set the vent doors to close for the defrost and floor vents and rite when I did that the front dash vents started blowing cold air. I still have not found out how to fix the problem dealing with the vac lines though.
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ask the people that installed the motorif they could have crossed some of the line when installing the eng
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