I converted my 240D to a turbo with a 617.952 from a '82 300CD. The glowplug harness and relay came from a '85 300D, and the gauge sensor wire is apparently in that harness. The only sensors the haness would reach was a temperature sensor in the middle of the head and the far sensor on the intake manifold (don't know what that is used for, can't be too important
) Anyway, my gauge was reading 90C-100C, and generally not acting like I thought it should. I modified the harness to connect to a sensor in the thermostat housing, and now my temperature is rock solid at 85C. It confirms my cooling system is operating properly, but I don't know if that's where it should be connected or not.
The sensor in the thermostat housing originally was either broken or was a switch - the resistance measured with my ohm-meter read open. I replaced it with the sensor from my old 617 engine, again from the side of the head.
What's more important? The head temperature, or confimation of thermostat operation? Any thoughts or observations?