Bjbpe

I understand that the HVAC has two sensors. The obvious one is in the dash. Where is the other one? Are we dealing with hot and cold? A photograph would be greatly appreciated. Thanks everyone

davek9

Ones is on/in the Dash (small round grill) near center console, the other is for outside temp, if missing you won't be able to control the temp of the Heat (full hot only).
I's located just under the Left side Head Light in the air intake tube for the alt, most are missing that intake tube.
edit: it is bolted to the inner fender panel, just in front of the front tire.

Your year would help, as newer ones (don't knw when and what for) also have a temp sensor in the left side door review M

dr bob

The control loop has a "thermistor" temp sensor in the dash as mentioned above, and another thermistor in the left front wheelhouse area, under and to the rear of the headlight. In that area, there's a bit of cooling air ducting for the alternator, with a "rain" loop held up on the forward-facing side of the front wheelhouse liner itself. In that ducting there's a metal connecting sleeve, and the sleeve holds the ambient-air temp sensor. Those two sensors and the rheostat behind the temp slider on the control head determine whether the damper door should move, then indirectly whether the fresh air door should move more open or closed, and whether the heater valve should open or close.

Bjbpe

I should have mentioned that it is an 87 S4. The temperature regulation doesn't work right. If the slide is all the way to the right (hot) there is plenty of heat but pull it off the far right just a little bit and there is no heat. Several years back I had the freon 12 topped off but I got no benefit; not sure if this is tied into the sensors or something else. Going to have the AC compressor and other parts checked out but I think I'll have to address these temp sensors also. Thanks for any other comments you folks may have.

dr bob

Wally Plumley put together a rather comprehensive guide to 928 HVAC, and it's available in on the 928 Specialists website at 928GT.com Worth a read before you plunge in headfirst.

928 at last

Mirror mounted sensor is just for the digital dash readout, doesn't affect the HVAC system IIRC. Most common failure point is the sensor in the front fenderwell.
(Accessed mine by removing the wheel and liner).