Fuel gauge will never read full

[barronj]

The best I can get the fuel gauge to read is 5/8 full on a full tank. It's a '68, and the 2nd tank has been out for a while, don't think it had a fuel level sending unit in it, as there is no switch on the dash for fuel tanks.

What do I need to clean or replace?

 

[Viperwolf1]

Clean the ground wire at the tank. Trace the sender wire along the frame to make sure it isn't damaged.

 

[broncnaz]

Might be the sending unit. I would start with it. You could do a ohms check on it when its full and empty that would tell you if the sending unit is working properly. Of course all your wiring connections need to be checked and cleaned as dirty connection add resistance which I believe would cause lower gauge readings if the sending unit is working correctly.

 

[barronj]

Thanks. When it's fixed, I'll update.

 

[bronconut73]

73 empty and 10 ohms full.

 

[barronj]

I take ohm readings by bridging the ground or the sender wire? Sender, I presume? I penetrate it w/ the red probe and ground the black, or bridge it?

I've never used the ohm settings on my multimeters.

 

[gearida]

It has been a long time since I have worked on a fuel gauge, but from my memory, by grounding the sending unit wire the gauge should go to full. Then again if I am correct the gauge may have an adjustment screw on the back to set the needle where it should be if the readings are truly off. Please confirm, but I may be on the right track.

 

[chuck]

disconnect the sender. put the meter leads from sender stud to grd. If your tank is empty the meter should read around 73 ohms, if the tank is full the meter should read around 10 ohms. If your sending unit is bad you will read around 40 ohms. While you have the sender wire disconnected grd. it with the key on and the gauge should go past full.
PS I am a slow typer, gearida is correct there is a adjustment screw but only mess with it as a last resort. To adjust it you need a 73 ohm and a 10 ohm resister.

 

[barronj]

Thanks all!

 

[Viperwolf1]

disconnect the sender. put the meter leads from sender stud to grd. If your tank is empty the meter should read around 73 ohms, if the tank is full the meter should read around 10 ohms. If your sending unit is bad you will read around 40 ohms. While you have the sender wire disconnected grd. it with the key on and the gauge should go past full.
PS I am a slow typer, gearida is correct there is a adjustment screw but only mess with it as a last resort. To adjust it you need a 73 ohm and a 10 ohm resister.

5/8 full would be about 20 ohms on a good sender. It's a log scale.

 

[chuck]

As the senders are not linear you might be right. The is how I came up with 40. 73-10 is the range so the total diff is 63 ohms from E to F so 5/8 of that is 63 divided by 8 times 5 = 39.375. I am sure my number is off because the scale is nonlinear. I have a chart around someplace that I did when I was scaling the sender a couple of years ago but not sure where it is right now.
Dah you are right, I was looking on the wrong side of the scale. 63 minus 40 = 23. Thanks for the correction.

 

[wegngis]

Clean the ground wire at the tank. Trace the sender wire along the frame to make sure it isn't damaged.

My gauge is simply not working, I'm going to try this soon.