My 94 940T. I was looking around at things to see if anything obviously needed attention, and saw that there is no heat control valve, at least in the standard place. In its place is a straight piece of tubing. No, this is not to bypass the heater core, that is still attached. It is only that the standard valve is not there.
Now, the car has heat. When I turn the knob the heat def comes on, and goes off when turned the other way. ???

I have noticed that the vent position switch has no effect; no change in what outlet, dash, floor etc get airflow. I also note that when I step on the gas, there is no airflow anywhere; then when normal pedal is achieved, airflow returns. I know that this is a symptom of a vacuum lack under the dash and is covered in the FAQ. I am wondering if anyone has a guess as to what may have been done in terms of the heat, and lack of change of ports.

I have not investigated under the dash yet.

Dear jerryc,

Hope you're well. When "airconditioning" is selected, what is the temperature of the air coming from the vents, i.e., is the air "cold", or is it "lukewarm"?

If the latter - if the air temperature changes from "hot" to "lukewarm", when "airconditioning" is selected - then the heater valve's removal means hot coolant constantly flows through the heater core. When "airconditioning" is chosen, cold air - produced by flow over the evaporator - is mixed with hot air from the heater, producing "lukewarm" air, i.e., air that is neither hot nor cold.

If the former - if the air temperature, when "airconditioning" is selected, is truly cold - then I cannot explain how heat coming from hot coolant passing through the heater core, does not affect the temperature of air entering the passenger cabin.

Hope this helps.

Yours faithfully,

Spook

Spook, the coolant definitely is going through the heater core all the time.
The belt is off the ac, so that is not in play.
Why then does the air temp out the vents change from ambient to hot when the knob is turned, and back to ambient when turned back? Will get under the dash soon.

#1 Heater control valve under hood should be between pipe coming from water pump to the top fitting on firewall which goes to heater core.
#2 Heater control valve function is to close off flow of hot coolant to heater core when AC is in use. That way AC isn't fighting against a hot heater core to cool off car and is more efficient.
#3 Right hand knob inside on your console heater control still will/does function to control how hot or not you want it.
#4 Middle control is just to position where you want air flow going(up, down, middle, combo). If this is not working then vacuum line to manifold is leaking or off so there is no suction to control air flow directions.
#5 I myself have decided to eliminate heater control valve. In Winter(no AC useage) will connect hoses to heater core so as get heat. Then in Summer(no heat useage) will bypass heater core. This way in Summer when AC is in use engine still gets full flow of coolant rather then no flow when AC is on.

Hope any of this this helps.

One of my 740s has the heater valve removed. Heater function works normally. As stated, the heater valve is only there to make the air conditioning more effective. On that car the AC has been non-functional for years so no issue. One less thing to go wrong...
--
67 144, 85 740T, 86 740T, 91 945SE, BMW R69S, R60/2

#3 Right hand knob inside on your console heater control still will/does function to control how hot or not you want it.

Can you say exactly how this happens? I had thought the heater valve varied the flow to the heater core, thus the amount of heat. If I get what you're saying, the knob does something else; open a door underneath the dash perhaps? And, does the heater valve open fully, or close fully, and that's it?


#4 Middle control is just to position where you want air flow going(up, down, middle, combo). If this is not working then vacuum line to manifold is leaking or off so there is no suction to control air flow directions
It's not working. I have checked what I believe the line is just in front of the firewall, which has a one way black and white valve in it. This was disconnected. Reconnecting it had no effect. I have not gotten under the dash yet.

Thanks for this, more would be great.

Heater control valve is operated by a vacuum line. No electricity there. Right hand knob on console is soley electric. I have had the console unit off and saw so. I think(but am not 100% certain) this knob probably controls an electric motorized valve or flap that is within or part of the heater core inside of car.

Middle temp. console control knob is definitely vacuum operated. Usually when not working it is the vacuum line you have addressed under hood. However you seem to have only addressed it under hood. It goes thru firewall into car into rear of the console temp. unit. Line could be leaking /bad within interior. Various directional vacuum lines all meet at and are encased in plastic plug that plugs into rear of temp. control unit. Maybe it came off or is loose.

Final thought and I hope I might be right for it would be simple fix : Vacuum line that normally would go to missing heater control valve might need to be plugged up. I did so after removing my heater control valve. Small nail jammed into line.

Again I hope any of this is helpful.

Oh it's all very helpful. I had never thought through some of these things even though I've had the controller unit out of several cars, but I was concentrating on the ac and the relay issue on the board, did not think about the rest of it.

Your thought about the vac line to the missing heater valve is good. I didn't see one, will compare to my other cars.


I think(but am not 100% certain) this knob probably controls an electric motorized valve or flap that is within or part of the heater core inside of car.

If anyone knows what exactly the knob does, I'd really like to know, Thanks!

Indeed it does! I was getting lukewarm air out of the vents and decided to see if I could locate the flapper and see if it wasn't closing all the way. If you remove the glove box, there is an electric motor almost directly behind the temperature selector that connects to the flapper valve. It just has a little plastic arm on it with a threaded rod going through it. Turning the termperature control moves the motor in small increments. It works very similar to a servo in remote control cars/airplanes. Like the others said, I believe the heater valve is only when the a/c is turned on. But probably bypassed because someone didn't want to fix your vacuum leak in the first place.