Quote:
Originally Posted by Flat Ernie
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.
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The problem is that with fords ohm range being very low its hard to use a resistive sender.
Since the sender is wirewound it will go in steps changing from one wire to another and with a low ohm range each step will make a bigger change in the reading.
And the sender canīt be linear in itīs winding or only empty and full reading will be accurate.
This is one the reason modern instruments went to higher ranges like 120/240 ohm.