Fuel Gauge stuck on full?

[Foghorn]

Hey everyone my 1996 Thunderbird fuel gauge is always stuck at full. I unplugged the fuel pump and It still stayed at full. Does this mean that the problem is for sure from the gauge?

The bird's mileage indicator also stopped working at 225k miles.

Should I replace the instrument cluster to solve the problem? It's been fun guessing how many miles till empty but I think Im done with that.

Thanks, everyone!

 

[XR7-4.6]

They’re not related issues, the odometer gear failing is very common and if it were a general electrical issue the speedometer wouldn’t work either as the odometer motor drives off it’s signal . Fuel gauge reading full is unlikely the gauge itself but may be a bad slosh module on the side of the cluster, it’s replaceable without needing to replace the whole cluster.

 

[Grog6]

A grounded wire could do that. iirc, the fuel sender is 127 ohms full scale, and it goes toward zero for full. A grounded wire is a full pegged reading.

 

[Foghorn]

A grounded wire could do that. iirc, the fuel sender is 127 ohms full scale, and it goes toward zero for full. A grounded wire is a full pegged reading.

I don't have much knowledge on the subject but isn't unplugging the fuel pump also disconnecting ground? Or are there other cables/points that I'm unaware of?

They’re not related issues, the odometer gear failing is very common and if it were a general electrical issue the speedometer wouldn’t work either as the odometer motor drives off it’s signal . Fuel gauge reading full is unlikely the gauge itself but may be a bad slosh module on the side of the cluster, it’s replaceable without needing to replace the whole cluster.

Good to know I haven't heard of a slosh module but I'll be sure to check it out!

 

[Grog6]

The wire runs all the way to the gage; it could be grounded anywhere. If you unplug the cluster and it's still pegged, the gage itself is bad.

 

[Foghorn]

They’re not related issues, the odometer gear failing is very common and if it were a general electrical issue the speedometer wouldn’t work either as the odometer motor drives off it’s signal . Fuel gauge reading full is unlikely the gauge itself but may be a bad slosh module on the side of the cluster, it’s replaceable without needing to replace the whole cluster.

I tried taking out the module and doing a bypass trick online. The fuel gauge went from full to beyond full and the oil gauge pegged itself on high. Does this narrow it down to a bad ground? Possibly even a faulty fuel pump?

At 9:13 you can see the module that looks like mine. 4 pins on both ends as opposed to the 3 and 2-pin combo a lot of other people have on their module.

 

[Grog6]

Looking at those components, the ones most likely to fail is the mov, which fails shorted, and the capacitor, which dries out over time. I'd replace the capacitor, its rating and value is on the wrap on the outside. If nothing else, a 10uF 25V capacitor will work, most likely. The mov is going to be 16 to 24V; I think it probably is your problem. it being shorted, would make everything on that circuit peg.
Unsolder one leg, and reinstall the module. If it works, you found the problem.
If not, solder it back, that means I'm wrong about its function. Check it with an ohmmeter, while it's loose; if it's below 10k ohms, it's bad anyway.

 

[Foghorn]

Looking at those components, the ones most likely to fail is the mov, which fails shorted, and the capacitor, which dries out over time. I'd replace the capacitor, its rating and value is on the wrap on the outside. If nothing else, a 10uF 25V capacitor will work, most likely. The mov is going to be 16 to 24V; I think it probably is your problem. it being shorted, would make everything on that circuit peg.
Unsolder one leg, and reinstall the module. If it works, you found the problem.
If not, solder it back, that means I'm wrong about its function. Check it with an ohmmeter, while it's loose; if it's below 10k ohms, it's bad anyway.

Thanks for the reply, do you think installing an aftermarket fuel gauge would be an easier approach?

 

[Grog6]

You could. If you run it empty after that, you could add 5 gallons at a time, to 'calibrate' the gage.

 

[Grog6]

Look at other vids; there's a way to bypass the module, and read the gage directly. Just remember that when you hang a corner, nail the throttle or brakes, your gas gage may read funny, lol.

 

[XR7-4.6]

If the sender’s bad an aftermarket gauge would just be a waste of money and for the money you could just get a used cluster for less if the gauge is in fact bad

 

[Foghorn]

Look at other vids; there's a way to bypass the module, and read the gage directly. Just remember that when you hang a corner, nail the throttle or brakes, your gas gage may read funny, lol.

I tried this but still no luck

So I unplugged the gauge and took it out of the car, still reads full. Then for the hell of it, I took the plastic cover off and moved the Fuel gauge from F to E manually. I plugged it back and slowly but surely it returned to the exact position past full.

Does this mean that I have a faulty ground? Or is this some type of bad electronic within the gauge?
Or even a bad electronic within the fuel pump? The car drives just fine.

Also, I noticed the yellow and white wires go to the cluster. Could I perform a test with this information somehow?
It appears that grey/red leads to the ground.

 

[XR7-4.6]

IMO all signs are pointing towards a bad or stuck sending unit in the tank.

You did right moving the needle to empty to test as if it were a fault in the gauge itself it wouldn't have moved back to full, that also proves there is in fact a path to ground through the sending unit; That's how the basic circuit works, power goes through the gauge, through the slosh module, through the variable resistance sending unit to ground. The sending unit varies the resistance in that circuit depending on fuel level - high resistance(low voltage) = full, low or no resistance(near battery voltage) = empty.

If there were a bad ground the gauge wouldn't react at all after you manually moved it, it needs both the cluster ground and fuel sender ground to operate. There's a chance that the wire in between the gauge and sending unit is corroded and artificially simulating high resistance but I wouldn't bet money on it

 

[Foghorn]

IMO all signs are pointing towards a bad or stuck sending unit in the tank.

You did right moving the needle to empty to test as if it were a fault in the gauge itself it wouldn't have moved back to full, that also proves there is in fact a path to ground through the sending unit; That's how the basic circuit works, power goes through the gauge, through the slosh module, through the variable resistance sending unit to ground. The sending unit varies the resistance in that circuit depending on fuel level - high resistance(low voltage) = full, low or no resistance(near battery voltage) = empty.

If there were a bad ground the gauge wouldn't react at all after you manually moved it, it needs both the cluster ground and fuel sender ground to operate. There's a chance that the wire in between the gauge and sending unit is corroded and artificially simulating high resistance but I wouldn't bet money on it

Great thanks! I guess a new unit is next on the list. Quite a shame because the pump itself seems to be fine.

 

[Grog6]

Matt, all the other fords I've had, a grounded wire read full scale. That seems to be the opposite of what you said.

 

[XR7-4.6]

Matt, all the other fords I've had, a grounded wire read full scale. That seems to be the opposite of what you said.

145 ohms at full

 

[Grog6]

Sorry. 30 minutes; the next time I have a cluster out, i'll, uh, have you got s spare cluster laying around? Preferably dead, but with all the electronics on the back. More and more problems are coming from those, and unless there are chips with house numbers(made for ford supplier), I can make a schematic of it. I've started doing it for some of the other modules, but my notes got scattered a few years ago, and I haven't found them yet.
If you want to get be a dead one, I'll document it, and send it back. I can measure what all the gages are at various points too.
The blue airbag module has a circuit designed to permantly kill it, lol. Theres a 130 degree thermal fuse, with a resistor glued to it with thermal epoxy. Resistor kills fuse, disables bag.