I have 2 1985 W123s (a 300D sedan and 300TD wagon). The 300TD has a fully working ACC system but the 300D ACC is only partially working. To figure out what was failing in the partially working one I pulled the temperature regulator (000 822 1003), fan relay (000 822 1103) and pushbutton ACC unit (don't remember the number but it's one of these complete with thermostat and fan switch) from the working system and connected them to the partially-working system. The behavior in the partially-working system remains unchanged, suggesting to me that all 3 of those units are good and that something else is wrong. I pulled the inside temperature sensor of the partially working system and measured the resistance: 7 or 8Kish. It dropped as I held it between my fingers, suggesting to me that the resistance is changing appropriately and this sensor works.

Partially working in this case means:
- EC setting fan will not turn on unless I choose another setting first, then it works ok switching back to EC, even on automatic fan speed.
- temperature cannot be controlled with thermostat. I have to click into min setting to keep the heat from turning on.
- center vents are sometimes open and sometimes closed when in vent or AC (no heat). This is determined when the ignition first turns on. Whether they are open or closed then won't change regardless of settings. I have confirmed this isn't a pod/vacuum issue.

I've tried shaking wires and banging things (including my head against the wall) and get no change in the behavior. One weird thing: I had the ignition on and ACC in EC mode. I unplugged 000 822 1003 (temp regulator) and suddenly the fan started to turn on. As soon as I plugged it in again, the fan stopped. I'm sure the temp regulator isn't the issue though because, as I explained above, it was pulled from a fully working ACC system.

Ideas?

My vote would be for the in car temp sensor (in top center area of dash under a 1" square grille) or the ambient air temp sensor in the airbox. If they one or both is failing you can get the symptoms you are experiencing I believe.

Double check the vacuum switches behind and slightly above your ACC pushbutton unit if it seems your vents are not acting properly. I found 4 out of 5 of mine were shot! (Only after many consternating hours of trying to figure out what was going on I celebrated with adult beverage and spent the 30. each to replace 'em)

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1985 300D 172,110mi and goin' strong

I *think* the in-car temp sensor is OK based on the resistance readings I mention in the original post, but...where is the ambient air temp sensor and airbox?

And I had a terrible thought that maybe there is something in the "bad" system that is blowing one of the 3 units I switched out. To rule that out, I'm going to reattach the units to the good system and confirm they still work correctly.

Is there a wiring diagram to this whole deal somewhere?

Have you replaced the non existent foam tube, that runs from the sensor to the air box? By now they have literally turned to dust...

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I'm aware of the foam tube and it is missing. I don't think that explains any of the symptoms, however, except maybe the thermostat not working somewhat.