Two temp sensor leads, one temp sensor
 

My 1980 300D wiring harness has two leads/connectors
for two temp sensors in the engine head.

My 1981 donor engine only used one sensor.
I tried hard to get the plug out of the 1981 engine head so I could
add the second temp sensor, but this never happened.

So, I have one sending unit and two sensor leads.

Over the years I have tried:
1. Leaving the unused sensor lead unconnected.
2. Connecting a sending unit to the unconnected sensor and left it hanging there.
3. Running a wire from both sensor leads to the one sending unit.

I have mentioned before that my engine temp is a dead on 80C as checked
with a mercury thermometer in the radiator and using a infrared thermometer.

My dash gauge has started reading higher than 80 recently.
Looking for clues.

I swapped the sensor from the 300SD into the 300D and am seeing a
high reading dash gauge still.

Someone here mentioned adding a ground wire to the instrument panel
and I am working that out now.

Any suggestions on what to to with my unused temp sensor lead ?

One of those sensors controls how long the glowplug light stays on dependent upon temperature. Other than that it plays no role so isn't really necessary.