My 1987 300D Turbo has a Evaporator Temperature Sensor. With the climate control unit working properly, this device keeps the evaporator from freezing.

I know because the one in my car was defective...the symptoms were when the engine was cold, the air conditioner worked fine. One the engine warmed, the temperature of the air also warmed.

It is a small sensor directly above the accelerator pedal on the left front side of the heater...once replaced, the problem was solved...about $15.

The way I found out about it was the ACC Mono Valve. It wouldn't close. That's why the AC worked well when the engine was cold, but once the engine warmed, the air from the vents got warmer. The sensor opens the valve to let coolant into the heater core to keep the evaporator from freezing...the defective sensor was keeping the valve open all of the time...leave it the Germans to over-engineer...

The Tenor Man

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Currently Driving.....
1987 Mercedes Benz 300D Turbo - Daily Driver.
2004 Buick Rendezvous...Smaller than it looks!
2005 Mitsubishi Galant - Not bad for a tin box...Lousey Seats
Past Vehicles.....
1994 Dodge B-250 Van- GAS HOG..Succumbed to rust!
1989 Mercedes Benz 260E - Totaled but no one injured.
1977 Mercedes Benz 240D - Loved it, but SLOWWW! - Succumbed to RUST!
1976 Mercedes Benz 240D - Same as above! -
1975 Mercedes Benz 240D - Same as above!