fuel gauge issues

[fgross2006]

The other day I couldn't get my Mustang to start after just moving it to edge of the driveway. I saw that the fuel gauge read 1/2 full. I assumed it was correct as I replaced the sending unit a few years back and I didn't think it would fail so quick.

After verifying spark at the coil and distributor, and fuel pump relay is energizing, it must bad the fuel pump, which is a Walbro I put in about the same time as the fuel tank sending float. Then I noticed my low fuel warning light was on. I had seen that a few days ago and tried swapping the sending switch behind the glow box but the light stayed on. Like a stunod I assumed the fuel gauge was correct and the dummy light was not. I was wrong.

Just to see, I added one gallon from a gas can and the car started. Now I'm curious how much gas is actually in the tank so I go to the gas station and fill it up. it took 14 gallons. That means it was bone dry. So after my fill-up I see the gas gauge goes far right and after a few minutes driving it the needle hides so far off the scale I had to hold my camera sideways to catch it.





This summer I had the cluster off and I ended up replacing the printed circuit board and the IVR. The IVR had failed a few months back and my fuel, oil pressure and temp gauges all peaked far right. Right now my oil pressure and temp gauges are reading normal so I don't know if the IVR can fail just one gauge.

Does anybody have any suggestions on how to troubleshoot this?

[fgross2006]

I noticed after turning the car off that the fuel gauge needle settles at 1/2 tank position powered off with no voltage.

At first I thought there was a voltage problem causing the needle to sweep far right, but the IVR if faulty, would cause fuel, temp and oil gauges to all float far right, not just the fuel gauge.

I'm thinking to pull the cluster yet again, remove the fuel gauge and manually position the needle to the empty position while it has no voltage. Put it back in and hopefully it corrects the position with car running.

[scenarioL113]

When gauges are pinning or experiencing unexpected fluctuations I tend to point toward bad ground somewhere.
I would experiment and remove voltage and or ground and see where the gauge goes down to. It should go to empty. You can play around with grounding it and see if it goes to Full but no pinned.
You would also have to see the voltage references from the IVR

You may have to try and check the gauge for proper function while taking the IVR out of the equation

I dont have a procedure for you to look at so it would just be troubleshooting

[fgross2006]

When gauges are pinning or experiencing unexpected fluctuations I tend to point toward bad ground somewhere.
I would experiment and remove voltage and or ground and see where the gauge goes down to. It should go to empty. You can play around with grounding it and see if it goes to Full but no pinned.
You would also have to see the voltage references from the IVR

You may have to try and check the gauge for proper function while taking the IVR out of the equation

I dont have a procedure for you to look at so it would just be troubleshooting

with the car off and the key not in the ignition the gauge stays at 1/2 full when it should drop to empty. I'm inclined for now to think the needle is off. If it was a voltage issue, no voltage should still cause the gauge to drop to empty.

[scenarioL113]

Maybe, but maybe you are getting "stray" voltage or something backfeeding.

Bad grounds can cause Gremlins that make no sense


Disconnect the battery and see if the needle changes position, if nothing changes then touch the 2 battery cables together...(obviously fully disconnected from the battery terminals) this will chase out any residual voltage that possibly might be in the wiring...like micro current.
The smallest amount can effect gauges since the gauges use different voltages to make the needles move

[fgross2006]

so I pulled the battery terminals today and the fuel gauge still stays at 1/2 or just under a half tank even with no voltage and no battery.

I have to assume the needle has to be adjusted. I don't know how or when it lost its calibration but I'm sure thats it.