The next questions from me are:

- what's a 1980 300D turbodiesel? From other posts I gather it's a conversion.

- what's the ambient air temp sensor for? I don't think W123s ever came with an LCD temp display in the dash. If they did, I'm sure the sensor would be in the in the area of the front plate as it is in the early applications of the LCD temp gauge. You say it's in the firewall? This is the first time I hear of an ambient air temp sensor in the area of the firewall in any MB. Are you sure it's a factory installed part? I think some glow plug controllers have a temp sensor. Is that what you're talking about? Maybe it's for the ACC?

Sixto
95 S420
91 300SE
87 300SDL
83 300SD