I'm trying to understand and fix the climate control on this '85 300D turbo that I picked up recently and am restoring to be a daily driver.

I've not yet found a user manual for this type of climate control. It is significantly different from my '83 240D.

The fan on/off switch works, but when I press "auto", the fan stops. When the fan is running, I get hot air at all times.

I've not noticed the push buttons doing anything at all except the right most one with the "0", which turns off the fan.

I'm guessing that the buttons, left to right are:
send air to defroster,
send air to floor and dash,
send air to floor only,
send floor to floor and EC?? - no idea
turn off

And, how is the "auto" button supposed to work?
And, is there a way to vary the fan speed?

So first, I need to understand how the controls are supposed to work, and then set about fixing.

Since I get heat always, I suspect that the heater valve is stuck on, and I've read in the tech article that these are failure prone. But I also understand that they are normally on, so even if no power to the valve you get heat- so I guess the valve could be stuck, or it could be electrical?

EC is Economy Mode. The compressor is shut off.

The fan speed is varied automatically via the Auto button. The system should adjust the fan speed based on the temperature setting and cabin temperature. The high and low buttons are single respective speeds.

My 85 w123 intermittently goes into full boil mode when I change the temperature so I've been studying this link.

http://www.peachparts.com/shopforum/diesel-discussion/304409-1985-w123-air-temperature-vents-boiling-hot-2.html

From Left to Right:

Defrost - defaults to heat and defrost nozzle only
Bi-Level - Runs defrost and the lower vents depending on the sensed temperature. A/C engaged and temperature adjusted with the monovalve duty cycle.
"normal" - Runs the A/C and regulates temperature and fan speed based on temp wheel. Temperature adjusted by monovalve duty cycle.
"Economy" - Same as "normal" but without the A/C engaged. Think of it as "vent".
"Off" - System is off entirely.

The fan will run at various speeds. The top button is "Max" fan speed, the bottom button is "Min" fan speed. Auto will do just that, automatically adjust the blower speed depending on what the climate control unit demands to get to the desired temperature setpoint on the temperature wheel.

Everything is automatic. Set the temp wheel and stick the fan on Auto. If everything is working normally, the climate control system will keep everything in check.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Remove the guts to the monovalve and make sure that the diaphragm isn't torn. You would normally have heat that would drop at speed if the diaphragm is torn but you need to make sure that it is in good shape.

see this PN on this site. 000-835-06-44-MBZ I had continuous heat and stopped it by changing the electronic windings that surround the monovalve. Remove the 4 screws for access.

see this PN. 000-830-37-84-MBZ I like yard prices better.

The other issue is vacuum pods leaking ant the air flaps not working right or the push button unit having cold solder joints.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.