My temperature gauge is alternating between working correctly and completely low. Sometimes when the car starts the gauge reads correctly and other times there is no reading. The wire at the sending unit is nicely secured. The balance of the circuit is obviously in question. I have not seen this before and I am wondering if the gauge itself can fail.
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Maybe check the Ground at the gauge?
it is 99% the ground at the gauge. Ground it by an extra wire and try again or disconnect the "measure" wire use a variable resistor per OHM value according the electrical diagrams, turn variable resistor up and down as see what the instrument is doing. Obv. ignition on RUN.
Also check the ground from the trans to the frame. Might want to remove it and clean the contact areas.
The ground from trans to frame is new and in good shape.
While the gauge was failing I began inspecting the ground and other connections at the gauge itself. The gauge started working while I was messing with it and I cannot tell which connection was the issue. But the item in the photo below is something I have not seen before. At first I thought it was a fuse but maybe it is a resistor?
That is a resistor that was added to get the gauge to read lower. It was a running change and there was a tech bulletin for dealers to retrofit to earlier cars.
Yes, it is definitely a resistor. I removed it for the photo and then connected the wires directly as a test. When the resistor was bypassed the reading on the temperature gauge increased significantly. This is a late 1974 car and the resistor was likely installed at the factory to help match the sending unit to the gauge. On a 1971 car that I had they actually soldered a bare resistor into the circuit making this white unit look sophisticated.