I have a 85 300 TD that has an ill behaved climate control system. When I bought the car it didn't blow air from the center ducts. This was fixed by the dealer that I got the car from. They replaced a vacuum control valve. Air now from blows from all the right places in the different heating/cooling modes. The problem is that when providing heat It provides to much heat at first and then a few minutes later blows cold outside air, then switches back blowing hot air. (unit is in econ mode for the winter months )It acts like an underdamped control system that oscillates back and forth. Eventually it settles down. The same problem seems to happen when the system calls for cooling except in reverse. i.E. Real cold air and then hot air etc. Where does the system sense the inside temp. My 78 300D had a thermister behide a cover in the dash . This car has a small square fent in the middle of the dash. I am assuming this is where the temp sensor is. Tha car doesn't seem to have any vacuum leaks as all the door lock work even after the car has been off a couple of days. Any suggestions would be apprecated
Russ

The temp sensor has a hose that connects to a plastic fitting in the passenger side footwell. This is where it senses the temp.
these are famous for falling apart. Access to the hose is easy take out the glove box liner (held in with plastic rivets) I repaced mine with a 3/4" piece of heater hose. A 20 minute job and it worked for me.