1938 Plymouth fuel sending unit

I've replaced my fuel tank and the sending unit. Both are new. The gauge will properly indicate "full" when the float is fully up. The gauge will not go below 3/8 of a tank at it's lowest setting. I've verified wiring, it's correctly grounded. I considered perhaps the float was hitting something inside the tank so I pulled the unit out but the read is still the same. The sending unit is adjustable and I've moved the bar (hence the pivot) up and own. No difference. I have not put any fuel into the tank.

My 38 was converted to a 12 volt system before we bought it. The gauges are all original and they (the others) all seem to work accurately. Is there a problem with 12 vs 6 volt? Any advice is appreciated.

Not a problem with 12v per se, but I wouldn't want to run 12v through the gauge for very long as it may burn out. You should have a voltage reducer on the power lead to it.

Sounds like they might have used the same resistance range in 1938 that they appeared to have used in the early 1930s and that is not a current standard (no mainstream off the regular supplier sending unit). If so then you have only a couple of options if you want to use the original dash unit.

There are vendors that claim to have a sending unit that matches that the range needed by the original dash unit. I will leave you to search out those, you can start with the usual old Mopar vendors.

Or you can modify a modern universal sender to that it better matches your dash unit.

First, you might want to verify the resistance range needed by your dash unit. I made up a box with a variable resistor to simulate the sending unit and a switch and some wires. My setup was to turn the resistor to a point where the needle was on a known level (empty, 1/4, 1/2, 3/4, full) and then switch the resistor so that it was across my volt-ohm meter where I could read out the resistance needed to display the level. You can follow along at http://www.ply33.com/Repair/fuelsender2.html

Not perfect, but mine reads full when full and empty when empty. 1/2 tank reads a bit more like 1/3 tank though.