jtexcellon

Last week I bought an '87 300e with 158k miles on it. Nice ride, just a couple of problems. Not new to MB's, just a new problem I haven't seen before. I only get heat with the thumbwheel on "max". The wheel needs to click on the max detent to get heat. Otherwise, cold air on any setting of the temp. dial. All of the fan and vent settings seem to work fine. The servo and the monovalve must work as I get heat, just on the one setting. As soon as you click off max, you can feel the air start to cool down. Any ideas if there is a temp sensor out or if the control head is bad. Any suggestions for troublshooting are appreciated.