fuel Gauge

With the key off.... remove the two wires &  5 screws and pull out the float assembly

You'll need to cover over the float hole in the tank so that no fumes escape, reconnect the 12v and ground back to the float ( keep the flat away from the tank opening) with the key in the on position move the float up and down checking to see if the gauge needle moves.........

I'm with Al - sounds like a sender heading south.

The sender provides a progressively stronger connection to ground as a signal to the gauge, relative to the fuel level in the tank.  If the sender is dead (i.e., no resistance or an open circuit) the gauge will read "no resistance" or "Full".  That's how it works.

You probably don't need a new gauge.  You need a new sender.

"when I turn on the ignition the gauge immediately goes past full."

That sounds like a good problem to have. Whenever I get in my wife's car, the gas gauge immediately goes to empty ..... 

I read his post as a guage test with the sender disconnected.....  If so, I would assume the guage is shot if there are no "under dash" wiring problems....

Just my .02....

Joel:  To test whether the sender is working at all or if the issue is in the gauge, disconnect the wire at the sender and then connect it to a known ground.  If the gauge is working properly, the needle should swing to the other end of the scale.  If it doesn't move, then check the wire between the sender and the gauge for continuity - Test with an ohm meter at the gauge end between the wire's terminal end and ground (disconnected from the gauge).  It should show a short when the sender end is grounded.  

You can also test the gauge this way - attach a short wire to the sender terminal on the gauge and short it to ground.  When you do that, the needle should swing to the other end of the scale.  If not, the gauge is dead.

As Gordon suggested, it is most likely a grounding issue.   Check the ground to all three of your gauges, they are frequently all linked together to one ground. 

Curious.   What did you find? 

@joelabraham

Inquiring minds want to know what came of this?

I ran the tests that Alan and Gordon suggested and they show that the fuel gauge is functioning correctly but the sender must be dying because it moves the gauge out of the tank but not in the tank. I've cleaned the ground and it has good continuity. I've checked the  float in a pan of water and it works correctly  moving the sender mechanism. There was no corrosion on the sender. I'm waiting for a new sender.

Thanks guys for your help and education-I'll let you know the outcome.

Joel

Checked the ground from where you where? 

Troy-The ground goes from the sending unit to the fuel gauge-i've added a metal contact to the top of sending unit in case the screw  holding the wire doesn't make conductivity.

joel

On my last car the gauge case had its own ground. It was bad and I had your exact symptoms.  Humor me and ground the case to a known good ground under the dash. "key on past full key off back to normal (empty)" 

"I've added a metal contact to the top of sending unit in case the screw  holding the wire doesn't make conductivity."

That's a good idea.  I've seen a few senders that lost their ground connection through the mounting screws - especially if they were installed with locktite on the threads.  Even if no locktite, the screw threads can corrode over time and loose their electrical connection.  I would spiff up the threads of the mounting screws with a wire brush before putting everything back together.

I have always been amazed.....That we're putting an electrical thing down inside a tank full of explosively flammable fluid and trust some designer somewhere else that it doesn't ignite.  I don't unnahstan why it don't, but it don't.

That's why it's the ground that's variable, not the power. The gauge is always hot with 12v when the key is on. The sender just creates variable continuity to ground.

There's still the remote chance of a spark, but it really never does. 

I replaced the fuel sending unit with an Empi 98-2021-B. It appears to be the same construction and quality as the VDO it replaced except it was a mirror image of the VDO. Therefore I had to position it 180 degrees to get the five screw holes to line up-they aren't equidistant. Anyhow it works -I'll have to use up the five gallons of fuel in the tank to see if it needs more calibration (bending the float arm)

Thanks guys for your help.

Joel