i think you are right about the temp. sensor. i noticed on my 79 it has the sensor you are explaining and is not on my 81.
you may just have a bulb burnt out in the cluster.
i can look at the sensor for the 79 and get you the numbers off of it..

make sure that you can remove the plug that is there now,it was NOT easy to remove the one for my block heater..

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Gretchen 1981 240D 4-spd 213,000 mi
1979 240D 4-spd 298,000 mi parts car

1994 BMW 325is
1991 Dodge turbo diesel
1997 ford expedition

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