85 300d. Ac works when the temp wheel is all the way to min. Once rolled off the min temp of the wheel, the temp in the car goes very hot and does not mix with cool outside air at all. Monovalve??? I either have very hot air comming in or very cold , no medium.

i have the same thing happening as you darylorr. i think that the sensor which is underneath a small opening on top of the dashboard needs replacing. there is also a foam tube that runs underneath the dash that gets deteriorated. both can be accessed by removing the glove compartment.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Try wriggling the temp wheel up and down slightly. If this makes the blasted thing control temp, even if it isn't the temp set, the pushbuton control unit has a bad connection for the temp wheel, Very common, the TE is doing this right now.

If the air blows hot, then cold, then hot, with large temp fluctuations, check the suction for the cabin temp sensor -- there is a small grille by the dome light on the W124 (don't remember where it is on the W123, same place?). With the ACC on, there should be enough suction at the grille to hold a small peice of tissue. If not, check the foam hose behind the glove box, replace if fallen apart (you can use heater hose rather than the foam if you like), and for operation of the small fan down there, too.

If that all works, most likely it is the PBCU. Check the current draw of the aux. water pump, too, before replacing the PBCU, you don't want to fry another one.

The monovalve should have 12V while cooling, going to zero and then switching on and off when heat is called for. If you get 12V only in max cool, the temp wheel connections are bad.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!