Previous posts on these King Sealy gauges explain how they work, but they do not work as designed by using a modern replacement variable resistance sender unit. You could try using one and might get lucky if it works correctly but don't bank on it. If a car is running a 12 volt system conversion and a voltage reducer/regulator has not been fitted to the gauges it is possible that the gauge and tank sender bi-metal windings have been overheated and damaged or burnt out. These are 6 volt gauges and never designed to operate on double that voltage. Some will say theirs work OK on 12 volts without a reducer but I say they have been very lucky so far. Regards, Kevin.
The gauge itself doesn't "know" what voltage it's supposed to operate in, but it has an operating range for whatever it is measuring. Often, these are physical limitations on the gauge itself. Virtually ALL gauges are ammeters of some sort. Double the voltage and leave the resistance/impedance the same, and the current doubles. Now the gauge, without a reducer of some sort, is outside its operating range and may be damaged, that much is true.
But if you're trying to regulate the amount of current through a gauge to render a reading of some sort, there are a few ways to do this. The King-Sealey way was to introduce full system voltage (6-8VDC) in a chopped manner (on/off switching of the bi-metallic strip moving contacts). "Modern" guages regulate current by changing the resistance and introducing lower than full system voltage continuously. So, yes, modern variable resistance sending units can be made to work perfectly fine with KS gauges.
Besides the ammeters, the only early Ford gauge that I'm aware of that does not require a voltage reducer is the 1940 BATT gauge. The early Ford service bulletins tell us that entire circuit is 60ohms, so we can run full system voltage to it and add an additional 60ohms in series with the gauge and it should operate as intended. If you regulate voltage via a step-down or CVR to the BATT gauge, it won't read properly.
The rest of the gauges require a step down of some sort. I don't care for choppers or dropping resistors and greatly prefer solid state CVRs. The L7806 will provide a steady 6V to the gauges and they're pennies to buy.
The gas tank sending units of modern design can be made to work reasonably well. An original KS is probably preferable, but given that they're not reproduced and originals are often missing or broken, the aftermarket replacements are more than acceptable for the overwhelming majority of applications. The challenge, as previously stated, is that they're of a universal design and the float arm must be very carefully adjusted to obtain reasonably accurate readings at each end.
Adjusting the gauges via the star wheels should be a last resort. Disassembling a KS sending unit to clean the contacts is a worthy endeavor because if it's not working now, you're probably not going to hurt anything anyway. Just be extraordinarily careful you don't bend anything internally - use some very fine wet/dry sandpaper/polishing paper and clean well when finished.
