What determines hot vs. cold out of my vents?



I have a '01 Ford Taurus. My heat isn’t consistently coming out hot. Sometimes it works fine, sometimes it doesn’t. I thought that was controlled by vacuum, but when I pulled the controls out, the vacuum lines only control where the air goes. What controls the hot/cold mix? That knob has electrical connections on the back.


The temperature is controlled by a blend door in the vent system. This blend door positions it self to allow air to either go past the heater core or past the evaporator or anywhere in between. This blend door can be controlled with either a vacuum motor or with an electrical stepping motor. Sometimes removing the glovebox gains access to this blend door motor.



Thanks, Tester. I’m slowing starting to figure it out.

I tested the reistance in the potentiometer in the knob. It seems like it works. As the resistance across one side goes up, the other side goes down. I have no idea what the numbers mean, though.

I put 12 volts on the gizmo that actuates the blend door. That seems to work. The thing turns as I turn the control knob. I have no idea how far it is supposed to turn, though.

Mechanically, it seems okay too. If I move it with my hand, it goes. I have no idea how easily it should move, though.

I’ll figure it out. I’m pretty certain my problem has something to do with that blend door just by the way my heat is acting.

My next test is to hook up all the electical connections and just keep it mechanically separate.


Sounds like you might have the infamous Ford broken blend door problem.

If this is the problem, the DIY fix is fairly simple & much better than spending major money at the dealer/mechanic.

See this link:


Your Taurus probably uses the same defective blend door.


Excellent suggestion… but the more I play with it, now I’m starting to think maybe the blend door isn’t the problem. I drove around a little bit today without touching that dumb knob and the heat still goes in and out! Now my best guess is that the computer is getting a bad reading and somehow randomly redirecting the coolant right past the heater core. I don’t know if that’s possible, but I know the EGR valve has a closed-loop default setting it uses under certain conditions, so I suppose it’s possible the coolant could do something similar? I dunno.

First thing I should do is check the level on the coolant! That would probably be smart… Actually I’m due to change out the anti-freeze anyhow.