Trying to figure out why my fuel gauge doesn't read correctly

[HeavyHauler]

On my 1968 Chevy C10 the fuel sending unit had already been replaced with a new one by the previous owner but that did not fix the problem of the fuel gauge not reading correctly. The gauge reads empty with the key on however when you turn the key on and then remove the wire to the sending unit the needle for the fuel gauge instantly moves to the 3 oclock posiition and stays there but when you put the wire back on the needle then goes right back to the empty fuel location.

Anybody have any ideas on what could cause this?

[VetteVet]

Your fuel gauge sending unit wire is grounded to the frame and is not going through the sending unit resistor. Trace the wire from the sending unit back to the gauge and you will find where it is touching the cab or frame. If you can get to wire where it runs under the door sill you'll probably find it is worn through the insulation and is touching bare wires to the metal.

[vitamin_c95]

Do I have the same problem if my gauge stays at 3 o clock all the time? I've found your older post about the fuse, but my fuse box did not look like the picture

[Mbeef61]

Originally Posted by vitamin_c95

Do I have the same problem if my gauge stays at 3 o clock all the time? I've found your older post about the fuse, but my fuse box did not look like the picture

It’s very possible. There is no fuse. No power involved

[vitamin_c95]

I just pulled up that will trying to reach my selector valve I will be replacing soon. I will look for that wire

[rockyrivermark]

Just by pass the old wire with new wire rather than tracking down the area of the short.

[WorkinLonghorn]

If he has a short somewhere along that wire then why would disconnecting the sender change that? I don't get your thinking. I think more likely he has a short in the sender.

[HO455]

This link will help

http://67-72chevytrucks.com/vboard/s...d.php?t=640615

[VetteVet]

Originally Posted by WorkinLonghorn

If he has a short somewhere along that wire then why would disconnecting the sender change that? I don't get your thinking. I think more likely he has a short in the sender.

\

Reading his post I answered his question about the zero reading. I also assumed that he was disconnecting the sending wire at the gauge, but you are correct that if he disconnected the sending wire at the sender he would be removing the path to ground for the gauge.
If the sending unit is grounded, an ohms check between the terminal and the flange would show that, provided there is fuel in the tank.

[HeavyHauler]

Originally Posted by VetteVet

\

Reading his post I answered his question about the zero reading. I also assumed that he was disconnecting the sending wire at the gauge, but you are correct that if he disconnected the sending wire at the sender he would be removing the path to ground for the gauge.
If the sending unit is grounded, an ohms check between the terminal and the flange would show that, provided there is fuel in the tank.

I should have been more detailed. I was disconnecting the wire at the sending unit. I have a full tank of fuel.

[VetteVet]

Originally Posted by HeavyHauler

I should have been more detailed. I was disconnecting the wire at the sending unit. I have a full tank of fuel.

If you do the ohms check between the sender terminal and the tank or sender flange with the wire off you should get close to 90 ohms with the tank full. If you do then I suspect that the gauge body is not grounded in the dash. This would not allow the needle to read.

Here is the diagram or how it's wired.

[HeavyHauler]

Originally Posted by VetteVet

If you do the ohms check between the sender terminal and the tank or sender flange with the wire off you should get close to 90 ohms with the tank full. If you do then I suspect that the gauge body is not grounded in the dash. This would not allow the needle to read.

Here is the diagram or how it's wired.





Attachment 1915518

I completed ohms check with my multimeter set at the lowest setting which is 200. My meter read .9 when checked between the sender terminal and tank or the sender flange. This reading was checked with sender wire unconnected. I have a full tank.

[VetteVet]

Originally Posted by HeavyHauler

I completed ohms check with my multimeter set at the lowest setting which is 200. My meter read .9 when checked between the sender terminal and tank or the sender flange. This reading was checked with sender wire unconnected. I have a full tank.

What I'm seeing from this reading is that you don't have even 1 ohm of resistance when you should be seeing 90. Basically its a short to ground which is causing your gauge to read empty. So your sender is bad. You can check this if
you can wire some kind of resistor in series with the sending unit wire and the terminal on the tank sender. It should cause the gauge to read higher than empty.
Or you could just pull the sending unit and install a new one with a new filter sock since yours is probably deteriorated after all these years.

I got mine from Parts Geek .com and it's a beauty. Works great.

https://www.partsgeek.com/catalog/19...ding_unit.html

[Wrenchbender Ret]

The float on the end of the sending unit is probably full of gas.
George

[RustyPile]

Originally Posted by Wrenchbender Ret

The float on the end of the sending unit is probably full of gas.
George

That's a very good possibility.. I've had 4 float failures within the past year on my new sending unit.. Each failed at the solder seam joining the two float halves.. The float fills with gas, causing the float to sink to the bottom.. the sending unit goes low ohms resistance and the needle points to "E"..