I can only get heat in my Defrost Function in my 87 300D. The other functions will not provide heat. Furthermore, the round BLUE RED Climate Control dial does not modulate temperature or heat.
Is this due to my electric vacuum pump not being functional and not alllowing the climate controller dampers in the dash to operate properly? If this is due to my electric vacuum pump not being functional, can I hook my vacuum functions for this electric pump to my Engine Vacuum pump?
Or is the lack of heat and temperature modulation because of the standard dysfunctional Climate control circuit board that the W123 class suffered from?

Dysfunction of the vacuum activated vent pods in the dash default to the defrost mode. Use a Mityvac to check if pods are defective. I found a disconnected vacuum supply hose which gave me the same problem.

Climate control vac is suppled by the engine pump, but through the small line for accessories, not off the main line like the W123 chassis.

There is a restriction in that supply line below the five-way T at the front of the engine, so if you have a big leak somewhere, you won't have climate control airflow funtions, and the temp won't work right becuase the air only comes out the defrost vents.

I believe the climate control line is gray with a red stripe, but it may be green, I don't remember. It's the only one going to the dash area at the brake booster, easy to see on the diesel (the black line is a vent, never shows vac and is open under the dash). Hook up the mitivac and see if the climate control works better, if is so, check the recirc and EGR vacuum actuators (the two branches off the big T to the passenger side) for vac leaks. Mine were both blown, so I plugged the hoses.

My climate control works fine, now. If, however, you get appropriate air flow but no heat, the monovalve and/or aux water pump are shot, check and replace before assuming the pushbutton unit is bad -- the one in the W123 chassis is all electronic and doesn't suffer from the W123 problems near as much.

You also need to check to see if air is being drawn into the temp sensor grille beside the dome light. When climate control running, see if a 1 cm square bit of tissue will stay up there. If not, check the sampler motor (pull glovebox liner, motor is below the right side vent). The hose can fall off the motor if you try to attach it after clipping the motor into the retaining clip, put the hose of first.

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!

Thanks a lot...I get up at 4 am for work and it gets chilly already on the Interstate