Fuel guage question

[19Fordy]

On my 40 I get a reading of about 6.3 V coming from the gauge wire that runs from the gauge to the gas fuel tank sending unit.
Carefully check your wiring with this 40 Ford wiring diagram.
https://www.fordbarn.com/forum/showt...wiring+diagram

Just for the heck of it try using only a 6V dry cell to supply Voltage to your gas tank sending unit. Make sure your gauge is not grounded against your dash etc. Look here:
https://www.fordbarn.com/forum/showt...ford+gas+gauge

I also ran a separate ground wire from the gas tank sending unit to the frame to make sure it has a good dependable ground. Normally, the gas tank sending unit is grounded by being attached to the metal tank which rests on the frame. A separate ground wire is good insurance.

[JayChicago]

Deleted. Not sure if what I was saying is correct, so better to not say it.

[LM44S]

Finally got a chance to work on my truck, grounded the sending unit to the frame with a separate wire and grounded the frame that holds the gauges to the dash with a separate wire also. The gauges read full and according to the manufacturer of the sending unit that indicates a break in the wire to the unit. Ran a jumper wire from the back of the gauge to the sending unit and stiil read full. Not sure what to try, any ideas? Thanks.

[JayChicago]

Originally Posted by LM44S

The gauges read full and according to the manufacturer of the sending unit that indicates a break in the wire to the unit.

You were given incorrect information. Some gauges work that way. My '58 Chevy works that way. But on all four old Ford gauges, if connection to the sensor is broken, needle moves to worse-case position. Fuel gauge needle moves to E.

When you ran a jumper wire to the sender, was the original sender wire disconnected at both ends? One thing that could explain needle always on F is a short to ground somewhere along the sender wire. That could also explain your low voltage reading on the gauge sender terminal, cuz voltage is dumping to ground.