Heater sensor location 129 SL

[Ray Waker]

My heater only works when the thumbwheels are clicked onto full. I have been told that there are sensors that control and balance the heat inside the car.
I know that most U.S. cars had climate control, however I would assume that they would have the same sensors. Can anybody explain their location, and how to test them? The car is a 1991 129 SL 300-24 valve.

Thanks

Ray
in the U.K

[bwheitman]

In my W140 and my R129 the interior temprature sensors were located right above the rear view mirror behind that little grill area. I usually blow mine out with compressed air once in awhile because it collects dust and then it will act goofy, the 140 was freeezing me in the summer, but once it was cleaned it behaved correctly. Good luck!
Brian

[elau]

So that's what the little grille is for!! I wonder what in the world those grille doing up there all these years