Temp Sensor backwards

[SeanDo40]

Does anyone know how to calibrate the temp sensor?
This is from a 1949 Jeepster. At rest it pins at the 212 degree and when I drop 6v across the posts it drops to 100 degress.
I should add that it doesnt move when the engine is running and stays at 212. In my limited research it appears these King Seeley are supposed to operate at 212 with no current, go to 100 with power on and then rest in the middle when running at proper temp
Maybe I have a bad sensor?
Photos incl 5494
5495

[LarrBeard]

Does anyone know how to calibrate the temp sensor?

The original temperature sensor for the King-Seeley gauge was a sensor that worked a lot like a furnace thermostat, not the modern thermistor sensors.

To look at how the gauge/sensor pair operated, let’s start out with the engine cold – like on a 10-degree Indiana morning. The element of the sensor is a bi-metal strip that forms a set of contacts. Those contacts are firmly closed when you turn on the ignition on that cold morning. Current flows through the temperature gauge and the sensor, and the bi-metal strip in the gauge starts to heat up. As it heats, it takes the gauge over to the 100-degree mark. (If you hook 6-volts to the gauge that will take it over to 100, with nothing attached, it goes over to 212. Your gauge is doing this - it's working OK.)

The engine coolant is cold, the sensor contacts stay closed because the self heating of the strip won’t warm the sensor enough to open them. Once you start the engine, two factors cause the sensor’s bi-metal strip to warm up; the self heating the bi-metal strip in the sensor and the engine coolant. As the engine warms up, the strip gets warm enough to cause the contacts to open. This stops current from flowing in the gauge and it tends to drift back toward the 212 side.

But, once the contacts open, the bimetal strip no longer self heats and the sensor cools off – closing the contacts again and making the gauge go the other way. The sensor goes into an on-off cycle. The longer it stays closed, the cooler the gauge reads. As things get hot, it stays closed a shorter period of time. If things get really hot, the contacts never close and the gauge reads 212!

I don’t know what sensor you may be using. If you have an original sensor, you can check it with a meter. At room temperature, the sensor should measure a few ohms (closed contacts and the resistance of the strip). If it shows a high resistance, the sensor is open and will have to be replaced. There is no “calibration” of the sensor

In a ’49 Jeepster, you probably have an L-Head engine. The L-134 has room for a long sensor. There are sensors for flathead Ford V8’s that may substitute for the original. Be aware that there are two sensors in those V8’s, one an actual sensor and one is just an over temperature switch.

If you are using a modern sensor, you probably have a thermistor sensor. Instead of the switch contacts, it is a resistor that varies in response to temperature of the coolant. The problem is that a modern sensor goes the wrong way – it puts the gauge at 212 cold and 100 at really hot.

Here is a good description of the old gauges and sensors:

https://www.kaiserwillys.com/temperature-gauge-adapter-fits-46-51-truck-station-wagon-with-4-134-motor

Did this help?

How about picture of the Jeepster??

[SeanDo40]

That is an amazing description of these sensors. Better than the shop manual. I have taken apart both the temp and fuel gauges. It amazes me how the wires for the bi-metal strips are hair thin and so delicate.
I am going to assume this is still the original sensor since everything else appears original, including the wiring. I will get back on it Monday.
Here it is when I was replacing the floor pans. Whoever had this before me patched the rockers with rebar pasted in with bondo. Ugh5496

[bmorgil]

LarrBeard is the "King" of those gauge clusters!

[LarrBeard]

Yeah, somebody did it dirty, but it has the makings of a very-few-of a kind vehicle.

Here is more than you may want to know about sensors. The diagram is one of the best I've found, it shows all the coils, bi-metal strips and pointers involved..

I can't speak to how well the Ford/Mercury sensors would work in a L-134 or similar engine/gauge setup, but at least they have the right technology innards..

https://www.fordbarn.com/forum/showthread.php?p=625104

https://www.macsautoparts.com/ford_mercury_late/water-temperature-sending-unit-6-volt-all-mercury-engines.html

https://www.macsautoparts.com/ford_mercury_late/water-temperature-sending-unit-6-volt-single-contact-ford-6-cylinder-and-v8.html

[SeanDo40]

Showing my true amatuer skills but I just discovered that my temp sensor doesnt even have a wire nutted to it. It was obscured by the plug wires so I didnt see it at first. Doh!5498

[LarrBeard]

If that is the worst thing you overlook on this job - you are going to be a LOT better off than most of us.

Hook that sucker up and let's see where the needle goes!

How about overlooking a missing head stud for a month?

It wouldn't run more than five minutes and we tore it down three times - DUH!

[SeanDo40]

Quick update. I am going to declare success. At some point someone wired the temp guage to the (-) post on the coil. Once I connected that wire to temp sensor and fired her up the guage went back to the left and crept towards center as the car warmed up.
Thank you @LarrBeard for the amazing explanation of the KS guages. 5502

[bmorgil]

At some point someone wired the temp guage to the (-) post on the coil


Oh man! A new one every day. They must have thought the temp gauge was a tach.