I am sure this has been covered. I have searrched some of the archives, but could not find the exact thing, but nothing is exact when it comes the the automated heating system control.

Vehicle: 1987 300D

Problem: for first few hundred miles of driving, the automated heater/ac control system works. After that, things get crazy.

The temp control is set at 18 and heat all of a sudden starts blasting in. Then it corrects itself and cold starts blasting in. Went to manual control with same results. Then it works for a time, but fan always stays on about mid setting and never changes speed, except to do the blast thing. Outside temp is approx 25 F.

The present control unit is a rebuilt one that was on the car when purchased.

I cleaned the sensor in the roof as it had a lot of fuzz around it. Plus it "seemed" to work better when I would turn one of the dash vents toward the roof. Sampler fan appears to be working.

Question 1: could the sensor be bad?

Question 2: is it the control unit itself? Any way to test unit?

Question 3: can a new control unit be purchased anywhere? I have only found rebuilt ones - Programa appears to be the only company rebuilding. Any suggestions on other rebuilders.

Question 4: is it just a forgone conclusion that one must purchase a rebuilt unit every so many years?

Any thoughts appreciated - thank you Ron