Climate Control

Temperature Regulation Failure

>I just completed changing out the vacuum controlled heater valve. Now,
>the only place I get cool air is if the temp. control is on 65. Anything
>above that and it gets real hot real fast. Does that sound like a bad
>dashboard temperature sensor?

Here's my take on the binary temperature problem, based on a sample size of one:

There is a servomotor that controls your heat, located above the driver's right knee. You must remove the parcel shelf to get at it. It has a little arm which controls the temperature mixer flap. If the arm is all the way up, you get max heat; all the way down, no heat. Normal position with the inside temperature stable is roughly horizontal. So the first step is to find out what that little arm is doing. With the parcel shelf out, you can feel its position while sitting in the driver's seat. If it never stops anywhere but all the way up or all the way down, you have isolated the problem somewhat.

The key input to the servomotor is pin 4 of the upper plug (there are two). It connects to the outside temp sensor(in the air duct to the alternator), which connects to the inside temp sensor (behind the slotted fitting in the dash next to the glove box), which connects to a rheostat on the temperature control slider. According to the manual, the resistance between pins 4 and 12 should be about 3.7Kohms with the slider at 18, and about 4.7 Kohms with the slider at 30. I think that assumes approximately room temperature for the sensors.

Now, if there is any break in the chain from outside temp sensor to inside temp sensor to temperature control slider, you will get the binary temperature problem. So check that reading, if you're electrically inclined. If it's wildly different, you've found the problem. If not, maybe you'll be lucky and the connector just needed reseating.

-John White-
'84 928S