Another mystery in the ongoing truck restoration saga.
About 1974 or so I replaced the original 134 L-head engine with a 134 F-head engine. Since that time, my temperature gauge has read backwards - a cold engine starts out at 212 and as it warms up, it goes to mid-scale, about 160 degrees.
In the restoration of Ham's 48 pickup, we have had a sequence of events that has left me dazed and confused with the temperature gauge.
In August 2015 I had the instrument cluster overhauled; all gauges checked and calibrated. When we put the engine back together, we ordered KWAS temperature switch P/N 905926 for "6-volt King Seeley gauges". That's what I need for the cluster on my truck - the gauges are King Seeleys. But - the temperature gauge still reads backwards.
So, let's look at this problem a little closely. The sensor in the engine (allegedly the 6 -volt, 905926 King Seely sensor) measures about 350 ohms at room temperature. In a cup of really hot water- not boiling but close, it measures about 10 ohms. The hotter it gets, the lower the resistance. Both the sensor currently in the engine and the one that had been in there since 1974 agree closely. Technically, these are both negative temperature coefficient sensors - probably the same part.
So, today I went to the truck with my box of fixed and variable resistors to try to figure out just what it took to make the gauge run correctly.
Switch OFF. Gauge reads just a little past full scale to the right, a little more than 212 degrees. Then, I took the wire off the sensor and replaced the sensor with a 100 ohm variable resistor. Not wanting to burn out anything, I started with the full 100-ohms in series with the temperature gauge. The needle moved hardly any, maybe 1/3 a needle width.
I carefully decreased the value of the series resistor and as I did, the temperature gauge read lower and lower.
A rough resistance vs. temperature gauge readings is
Hmmm .... these readings indicate that the temperature gauge needs a positive temperature coefficient sensor to make it read right. I wondered; "Do I have a bad gauge - did I damage it at some time in the dim dark past?"
So, I called Bob's Speedometer Service, the folks who overhauled the gauge cluster. I spoke to their gauge guru and told him the vehicle I was working on.
His first question "Is it the temperature gauge?"
"Yep".
His answer, "Your gauge should rest at 212 or above with the ignition OFF, then come down to colder readings when you turn on the ignition". OK, I hadn't damaged the gauge back in '74 or so.
He continued "The old King Seeley gauges on the late '40 Fords and the Willys gauges used a different sensor. It was about 10-ohms or so when it was cold and it went up in resistance as the temperature rose". That matches what I measured on my test setup.
As stated earlier, KWAS calls out the 905926 sensor for 6-volt, 46-58 Jeep and Willys with King Seely gauges. The package the sensor in the engine came from has a P/N 905926 label on it, so I can only assume that this is a 905926 sensor.
Is there anyone out there who can:
A: Verify or deny that this is really the right part for the 6-volt gauges, or
B: Measure the resistance of your properly working sensor to let me know what the resistance of a cold, correct, sensor should be, or
C: Give me the part number what you know to be the correct sensor.
Here are a couple of pictures of the sensor I have - the one that doesn't work with the gauges. The thread that screws into the head is a 1/2"-NPT as far as I can tell. (I don't know what the "240" stamped on the sensor hex means).
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