I’m trying to figure out a gadget that takes the temperature sensors that are presently available and converts their output to a signal that will drive the temperature gauge in the early Jeep trucks. We are talking about the upside-down gauges (46 through mid '48 or so) where the needle points from the top down. In the second generation of trucks, with the round speedometers, the gauge was changed to a needle-up gauge – which in turn led to a change in the temperature sensor in the cylinder head.
The needle-down gauge used a bi-metal thermostat switch type of sensor. The needle-up gauge uses a temperature sensitive resistor as a sensing element. As many folks have found out, when you use a temperature sensitive resistive sensor with the upside-down gauge, the temperature gauge reads backwards; and swapping the leads doesn’t do anything.
There does not appear to be anyone making replacement bi-metal thermostat switch sensors for Jeeps. There are some available for early Ford V-8’s, but they are way too big to adapt to the little fitting at the back of the Jeep engines.
Kaiser-Willys offers two sensors; one for 6-volt trucks and one for 12-volt trucks. The 905926 is offered for 6-volt trucks with King-Seeley gauges; the 914847 sensor is for 12-volt trucks with King Seeley gauges. I was curious to see if the two sensors might have different resistance vs. temperature curves; one might be easier to adapt to the gadget than the other because of different resistance curves.
I ordered one of each. After a bit of looking, it appeared that there was a lot of similarity between the two sensors. The 905926 is a plug sensor with a 1/4” NPT thread to screw it into the head. The 914847 appeared to be the same sensor, but with an adapter to screw into a larger NPT opening. The basic sensors appeared to be the same in both part numbers, even down to the little “43” stamped on the end of the sensor plug.
The 6-volt, 12-volt/adapter and Ford sensor are in the attached photo.
I spent a couple of days running resistance vs. temperature curves on the two sensors. My wife got tired of the mess I had made on top of her stove with an old aluminum pressure cooker, a holding fixture for the sensor with her meat thermometer probe coupled to an aluminum plate and a Simpson 260 multimeter all piled on the end of the cabinet by the stove. It took about a dozen cycles of “boil the water, let it cool, boil the water , let it cool …” to get enough data on the 6-volt and 12-volt sensors.
In the end, it turned out that my suspicion about the 6-volt and 12-volt sensors was correct. The basic sensor is the same – the difference is the NPT adapter added to the 12-volt 914847 sensor for later trucks.
The question becomes; ”How did they use the same sensor on 12-volt and 6-volt trucks”. The answer is simple. Willys-Overland, like most manufacturers (GM and Ford), did not change gauges - gauges are expensive - they just added a mechanical voltage regulator for the instrument cluster. The rest of the vehicle may have been 12-volts, but the gauges were still happily running on 6-volts. Even late 6-volt systems had instrument regulators to stabilize gauge readings as electrical system voltages varied with engine speed and voltage regulator settings. On the '48, when I turn on the headlights - I lose about a quarter tank of gas because the system voltage drops. I'd forgotten that in the last 35 years.
The resistance vs. temperature curve is not linear. I don’t know what this will do the design of the gadget. I am most interested in getting 160 degrees and 180 degrees (the two most common thermostat temperatures for L/F-134 trucks) to fall at about the correct place on the gauge. Cold is all the way to the left and if we get to 212 on the right, we know there is a problem.
Here is a summary of interesting data points:
212 degrees 14 – 15 ohms
180 degrees 25 – 27 ohms
165 degrees 32 – 35 ohms
100 degrees 180 – 200 ohms
Now, if deer season doesn’t get in the way of the project …..
