Alan:

I’ve managed to learn quite a bit in the last little while. This style of gauge is a heater type which grounds current through a negative coefficient thermistor in the sender probe into the engine block. At high temperatures the resistance of the probe is low (~30 ohms) and allows higher current to flow which heats a bimetallic bar in the gauge and deflects the needle. At low temperatures the resistance is higher (~1000 ohms) and the lower current heats the bar less which deflects the gauge less.

So I checked the ground quality between the probe and the airframe. The resistance was zero. Sometimes the ground can be poor due to thread sealant or gasket material. So the ground was good – I generally check the ground first in electrical gremlin hunts.

I then ran a shunt wire from the sender connection to ground and had a helper momentarily ground it while watching the gauge. It swung promptly and smoothly to maximum indication. So the gauge is working freely. This also indicates that there is little to no resistance in the wiring from the gauge to the probe.

I tried to check resistance of the probe but received confusing and contradicting results that didn’t match the expectations. So I figure it is the probe. I am ordering a new probe from Air Parts of Lockhaven. I plan on doing a calibration sweep at various temperatures before I install it and I’ll summarize all of this soon.