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Re: 1994 Buick Roadmaster won't divert heat to defroster...
There is a vacuum line that's plastic on the passenger side back near the firewall. Sometime they have a little black ball looking thing (vacuum canister) that the vacuum line hooks to. Look for vacuum leaks in that area. That will cause the problem you described.
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MODE DOOR NOT SWITCHING FROM AIR TO HEAT IT'S UNDER THE DASH THERE ARE TWO 1 THAT WORKS THE VENTS ONE THAT WORKS THE DOOR FOR THE HEAT AN AIR IF VENTS CHANGE FROM DEFROST TO DASH AN TO FLOOR IT IS THE MODE DOOR actuator
If you are referring to the air won't blow through the defrost vents or on the floor, you may have a stuck or broken blend door actuator. This part diverts air through the different settings: defrost, vent, floor etc. If you aren't getting warm or hot air regardless of the settings, you may either be low on coolant or have a bad thermostat.
Most common cause is a bad blend door actuator. This controls how the air is diverted, Sometimes the actuator will become stuck in one position. There are 2 types, a cable driven or electronic driven. Search for blockages underneath the dash that could prevent the actuator from moving. Also, it's possible (if it's cable driven) that the cable is either broken or loose to the point where it won't move the door any longer.
The most common cause is the blend door actuator. It's the door that diverts air between the floor, vents, and defroster. Sometimes the part will get stuck in 1 position. Some cars have cable driven blend doors, others are electronic.
this vehicle has electronic acuators that move the doors and do not use vacuum. the mode door actuator is bad causing this problem. there is a defrost acuator that works the defrost position and a mode door that works floor/vent operation. the defrost moves and diverts airflow up defrost vwents this is why you can still have defrost and floor.
does it blow through the defrost vent only when you switch to vent/floor or does it blow through defrost and vent/floor? is it a switch that is just electric or a manual lever that you slide? the manual type will have that problem if the diverter or the linkage behind the lever(in the dash behind the panel......may be able to access underneath) becomes broken or disconnected so that the air is diverted to defrost regardless where you move it.if you can get to it, you will see what is broken or maybe a fastenerr fell off.