How Ford Gauge Unit Work

[Flat Ernie]

Originally Posted by rotorwrench

I've heard both sides of the coin on the rheostat variable resistance types. Most resistance based units use a specific Ohm resistance range and they vary between manufactures of the different units. Many claim that the repos work OK and then many claim they don't indicate accurate quantities. I think if you play with the adjustments you can gain some accuracy within a narrow band of meter movement but you can never get them to be accurate through out the range of movement. The resistance levels are more minute for working a current based indicator and the rheostats just can't attain the accuracy in those small levels of resistance change but they will give you some idea whether you are getting close to empty which is likely the most important use of a quantity indicating system. You may not know how much is left but that won't matter as much as the system causing you to run out of gas on the side of the road somewhere.

I can't really argue with anything you've said here, except to add that virtually all gauges are really just ammeters calibrated to read whatever we're measuring. Therefore, there is a range of resistance that will calibrate correctly - whether or not the commonly available aftermarket sending units are precise enough is another debate.


And all I can add is if you've got a 1940 Car/Truck with a BATT gauge instead of an Ammeter, all it does is replace the sending unit with a 60ohm resistance built-in. Therefore, when converting to 12V, adding another 60ohm resistor in series with the gauge will produce a pretty good 12V BATT gauge.