Near finished restoring 1978 Dodge Power Wagon.
The fuel gage needle is barely noticeable in the top left portion of gage.
I switched the posts on the sending unit with no change.
I put 15 gallons of fuel in the tank.
The sender was working ok when I started the restoration 5 years ago.
The dash gage assembly was completely replaced, so I don't know the history of the fuel gage.
What is the best way to evaluate what the root cause of the issue.
Gage, Sender, wiring???

Thanks,

SLD

EZ to check..
Ground the lead back @ the tank..
What happens...
1. Guage moves goes to full, indicating a sender issue, everything is fine from the tank forward
2. Guage does nothing, indicating voltage is not reaching the tank problem is in the wiring, dash or guage circuit

Just my $0.02...

Just remember if you ground the sender wire don't let the gauge peg to full for more than it takes to be sure it's moving. The gauge will burn out if left to peg for long. Ground the wire then turn the key on and if the gauge starts to climb turn the key off because you now know it's working. Do you have the sending unit grounded? It takes a strap that jumps the rubber fuel line that joins the sending unit outlet to the metal fuel line going forward. example-- http://www.ebay.com/itm/Mopar-Fuel-Sendi...r-/230843842227

Good info Stumpy.
I am not aware that this part ever existed when the gage used to work before tearing the truck down.
My sender has 2 posts, one red, the other black.
I have the black post grounded to the frame.
The red connected to the wiring harness.
No metal fuel line being used.

Original problem was that the dash plate had a break in the copper circuit behind all the gages.
Once replaced, now the gage moves toward full when there is only 5 gal. in the tank.
Just as you described Stumpy.
I have replaced the fuel sender, but has been years ago and not used for the last 5 years.
So, is it time to replace the sender?
Or any other info I can check.

Thanks All,

SLD

Pull the sender,but leave the wiring hooked up, and see what happens when you move the float arm.

ok there are several checks you can do before you pull anything.

first get a check light and see if you are even getting voltage from the gauge.
I would probably find the wiring diagram and pull the side harness that goes back to the sending unit and start there.

if it blinks you have power going back to the sending unit.

If you ground that wire there and the gauge moves then everything to that point works.

then you have to check from the rear harness to the sender.

but lets say it doesn't blink first.
If it doesn't blink then you check the vr in the dash see if it is working but at this point you might need to get behind the cluster or pull it.

the vr might need to be replaced, or the circuit board died again. you can try the temp gauge to verify if it is the vr or the circuit board.

you can check for blinking at the temp sending unit under the hood and ground it to see if it moves the temp gauge.
if the temp gauge works then the vr isn't the problem.
If both don't work, that is the common source.
OR it could be dash grounds and you can verify that by just tying a ground wire to the dash or instrument cluster.
but you would know the grounds were bad if your instrument lights don't work as well.

Finally if you had to pull the cluster or if you can reach behind it, you can tie 2 AA batteries in series and wire directly to the posts on the gague. it should move to the half way point if the gauge works.