Fuel gage 73 chevy c10

[FrankSando]

I had a wire brake off of my sending unit so I traced the pink wire to my fuss block and then the tan wire out to the firewall ran a new wire from that to sending unit not when I turn my key of the gage goes way pass full .what I need to know Is it the sending unit or the gage before I buy a new fuel gage fuel gage is 49.00 the sending unit is 64.00. What do I do any help would be great.
Thank you
Frank Sando

[Palf70Step]

Normally that would mean there is either a bad ground on the sending unit at the tank or the sending unit itself is bad. Make sure you have a good solid ground from the sending unit to a clean spot on the frame, as more times than not that is the issue.

[FrankSando]

Thank you
For your time I guess I'll just replace the sending unit that why I'll only have to pay 1 time for the work done

[hatzie]

Loading the parts cannon is a way to spend too much money. The sender is usually way down on the list of failures in a non working fuel gauge.

You say you had a wire brake (break) off the sending unit and the gauge is pointing way past Full.
Does the broken wire run out of the wiring harness to the brass pin on top of the sender or from the frame rail to a tab on the sender locking ring?

Since the sender is between the frame and the sender terminal on the gauge... breaking either wire will interrupt the sender circuit creating an open sender circuit and driving the fuel gauge needle to around the 3:00 position.
The most common failure is a broken ground wire from the frame to the tank.
Test it by clamping a set of jumper cables from the frame to the pinch weld between the tank top and bottom. If the tank ground is bad the gauge will work with this temporary tank ground in place.

[FrankSando]

Ok here it is in a nut shell. There was a blue and white wire coming from the gas tank dont ask me why it was done that way when I bought the truck to me it looks like phone wire it came from the tank up the firewall tied into a 12 volt wire under the hood I brought it to a
Garage to have them drop the tank to put the wire back on they called me and told me if they hooked that wire up they would have blown up my truck so what I did I took my cluster out and traced the pink wire to the fuses block and from the other side of the gage I traced the tan wire coming out of the firewall of the truck .i cut the tan wire and put a but connected on the tan wire then I took it to some other Garage and asked them to hook it to the sending unit they did it now the fuel gage goes why passed full so now I feel like it can only be the sending unit because from back of the gage to the fuses block and from the other side of the gage is the tan wire going to the sending unit so I may bug e wrong but it can only be the sending unit I have a new fuel gage coming Friday I'm going to put in if it still does it then only one thing it can be. Or am I missing something here
Thank-you
Again for anything you can throw my way
Frank Sando

[hatzie]

Before you change the sender test that the wiring is actually hooked up the way it's supposed to be.

The wiring is run from a frame ground to the sender ring on the tank, through the sender, from the sender wire connection to the bulkhead plug on the firewall, from the bulkhead plug to the instrument cluster plug, through the cluster printed circuit to the gauge.

1- Check that the tank ground is not broken. Hook jumper cables from a clean spot on the frame to a clean area on the pinch weld of the fuel tank to provide a ground to the tank.

2- Buzz the sender wire from the sender connector to the sender terminal in the instrument cluster bucket.

[FrankSando]

Ok back to the ground on the sending unit I'll check the ground on the seeding unit it's self that could be the only place it's not grounded at why I say that is because all my dash lights work because a a ground is a ground when it comes to that circuit board it's all grounded am I right ?
Thank you Again

[Palf70Step]

Both the gauge and the sending unit need a ground. One does not share with the other in this case. Gauge should be grounded thru the harness.

[hatzie]

The gauge gets ignition power and chassis ground inside the cab. This powers one of the electromagnet coils in the gauge can that drags the tip of the needle toward the 3:00 position.

The sender grounds a stronger additional electromagnet coil that pulls the needle in the opposite direction, toward E, based on how much power is flowing through the 90Ω sender rheostat.
Dead short to ground on the sender wire is E, 90Ω is F, and ∞Ω (open circuit) is roughly 3:00 on the face of the fuel gauge.

An open or short can be anywhere in the sender circuit from the frame to the sender pin on the gauge.

This is a drawing of a single gauge setup with a rheostat sender element. IIRC The sender wire is attached to the wiper and one end of the rheostat is grounded.

[FrankSando]

It's a 73 chevy pick up as I posted at the beginning
Thank-you for all the help

[hatzie]

Originally Posted by FrankSando

It's a 73 chevy pick up as I posted at the beginning
Thank-you for all the help

The basic single tank sender circuit is the same from 73-91 (the 73-86 CK and the 87-91 RV trucks). I edited the picture to expand when you click on it and touched up the rheostat so it clearly shows that both ends are not connected.
Really it's pretty much unchanged since the 1940's with two minor changes. The older trucks used a 30Ω sender vs the newer 90Ω sender and 6v electrical system gave way to 12v on the newer rigs... I believe one changed before the other in the mid 1960's but I don't remember which one or which year(s). The broad strokes of how the gauge works are the same tho.

On the 1977ish and later rigs you'll notice the needle sweeps faster from 1/2 - E than it does from F - 1/2 and it drops really fast from around 1/4 - E. You are not imagining this.

GM changed the sender rheostats on most of their vehicles around 1977 to weight the needle sweep to point to near full longer. If I can find the Chevrolet Service News article about this I'll scan it and post it. Replacement senders rheostat windings are likely weighted the same as the newer units.