TR2/3/3A TR3 Temperature Gauge Face Markings

[MGTF1250Dave]

Aloha All,

I have a Jaeger Temperature Gauge (non electric) with five marks on the gauge face. The lowest is 90 degrees F, the center is 185 and the highest is 235. There are two other marks, one between 90 and 185 and another between 185 and 235. Does anyone know what the corresponding temperature is for the unlabeled marks? It doesn't seem to me that they are linear since the lower half is a 95 degree range and the upper half is only 50 degrees.

I am prompted to ask because my car is running much cooler than usual. After my twenty minute drive to work, the gauge indicates about halfway between the lower unlabeled mark and 185. I suspect that my thermostat is stuck in the open position, but plan to wait until the weekend to tackle the problem. If the car remains stationary at idle revs for several minutes, it will warm up to 185 and the reading is confirmed with a non contact laser thermometer.

 

[TR3driver]

Easier test for a stuck thermostat is to start the engine from cold and let it idle until the thermostat housing is hot to the touch. Then feel the upper radiator tank. If it's still cold, the thermostat is doing it's job.

I don't believe the original gauge was all that accurate even when new, and 50 years will not have improved it any. If you just assume that it is linear between the marked points (making the lower unmarked point 138F), I feel the result will be about as accurate as the gauge is capable of. However, I just did a quick polynomial curve fit to the 3 marked points and it shows maybe 143F or so for the lower unmarked.

Switching to a logarithmic fit brings it up to about 145F.

 

[Moseso]

I don't know about the lower mark, but when I tested my gauge with the old "pan of water on the hot plate and a thermometer trick," the mark between 185 and 235 corresponded nicely to the boiling point of water: 212F or 100C. And the 185 mark was pretty darn accurate too.

 

[MGTF1250Dave]

Thanks guys.

Randall thanks for the suggestion, I'll try that test before I drive home tonight. I can't recall, but I think I have one of those fail in he open position thermostats installed.

Moseso, it makes sense to me that the higher unlabeled mark is the boiling point, 212 degrees F. As the engine cooled this morning I was checking the thermostat housing (where the gauge bulb screws in) temperature with my non contact thermometer and the gauge reading. My observations are the lower unlabeled mark is 150 degrees and the midway between there and 185 mark is about 168. Also the midpoint on the gauge between 90 and the lower unlabeled mark is about 120. So based on these readings it seem the lower portion of the gauge is linear.