Ford 641 Fuel Gauge and Sending Unit

WildcatFord

New User
I have a 6 V POS GND Ford 641 with a recently replaced fuel gauge and sending unit. When it hasn't been run for a while, the fuel gauge will indicate appropriate level when the switch is turned ON. Once started and running, the needle tends to bounce around and eventually the gauge stops indicating the fuel level completely. This happened yesterday, and I just confirmed that the gauge will not work with the switch on tonight.

I read a reasonable resistance (about 35 ohms) between the sending unit terminal and GND when the switch is off. I used a jumper wire to make sure the fuel tank was grounded, and no change was noted.

Anyone have any ideas on what would cause this behavior with the fuel gauge and sending unit?
 
Anyone have any ideas on what would cause this behavior with the fuel gauge and sending unit?

If your gauge looks like pixs, wiring indicated below.

mvphoto25683.jpg

"T" - Tank Sending Unit
"B" - Battery Voltage
"G" - Instrument Lamp Ground (Fuel level indicator circuit receives ground from the case)

Simple explanation of any tank level indicator - Tank unit is a variable resistor that gives different resistance readings depending on the float level in the tank. The higher the float is in the tank, the more resistance the sender makes and the higher the gauge goes. The first troubleshooting step is to make sure your tank is fully grounded and all of the gauge wiring connectors are clean and tight. If that doesn't fix your gauge, remove the wire from the sender and see what the gauge does. If the gauge goes to empty with the wire off the sender, the sender is shorted out. If you touch the wire to a ground it should peg the gauge. If the gauge still pegs with the wire off, the sender or wire may be shorted somewhere between the sender and the gauge.
 
"T" - Tank Sending Unit
"B" - Battery Voltage
"G" - Instrument Lamp Ground (Fuel level indicator circuit receives ground from the case)

For clarification, the fuel level gauge circuit is grounded by the sending unit at the tank, and the varying resistance between the gauge and ground as the float rises and falls is what causes the gauge to change.
 

I should have been more specific. The fuel gauge indicates Empty, which is not correct.

Earlier today, I removed the connection from the sending unit terminal and touched it to the grounded portion of the sending unit which is in contact with the tank. The fuel gauge pegged Full, as expected. I confirmed ground by checking continuity between this surface and POS battery terminal.

Next, I reconnected the wire from the fuel gauge to the sending unit terminal. After playing with the wire some at the sending unit terminal, I got the gauge to indicate the actual fuel level in the tank. However, upon tightening the connection, the needle dropped back down to Empty and I was unable to get it to read correctly again today.

Does this point toward a faulty sending unit?
 
I might have solved it. I cleaned up the parts a good bit so I'd get a good ground and it seems to work. I need to get more gas for the tank to see if it will go higher that 1/4, which is where is it sitting now with the key on.
 

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