Looking for some insight on the operation of the heater control valve. My AC was giving me trouble by discharging 70 deg air max. Went through some posts & FAQs and checked my control cable. Sure enough the door was not closing all the way. Adjusted the cable and now get 55 deg discharge air on a 90 deg day (not bad for 1990 740 gle 107k miles.

Now I'm wondering about the heater control valve. It is vacuum operated but where does it get the vacuum from and in what position, what switch. I would tend to think it should not even be opened unless the temp control lever is off the left stop. As far as I know it is the original valve.

Any info would be appreciated.

st

Owning a 700 with the original valve is risk embodied: these crack suddenly and you lose all your coolant. Get a new one.

The valve is closed unless the temperature selector is moved from "cold" to a warmer position, in which case the valve opens through vacuum supplied from the HVAC control module.

I seem to have vacuum on the valve at all times. If I disconnect the vacuum line on the valve it changes position. As soon as the car is started vacuum is again applied.

Where is the HVAC module (behind glove box?) and how can I adjust/check it.

Thanks.

The control module is in the heater control in the centre of the panel. The FAQ has instructions for removing it. Vacuum port 3 using the white line is the vacuum signal to the water valve switch; port 8 using the violet line is the vacuum supply to the switch. The switch supplies vacuum in the "cool" setting through port 7 and the gray line directly to the valve, thereby closing it. In other than cool, it should be off. This is for the manual climate control unit.