I have been having similar problems with an 83 240D. I replaced the sensor and installed different, but used gauges. I am willing to go with new ones, but I am not confident that it will work any better, so I am still plugging away. My daughters have the car now while I am fixing the 80 300SD. When I get it back, I thought that I would track the circuit board to make sure nothing was a-miss. The ground does go through the glow plug relay, but that is true for all of the instrument cluster and everything else seems to be working. I am getting power to the sensor.

I am stumped on this one.

Let's see if we get any better advice on this one.