Fuel Gauge Stuck Past Full

[LachlanMann]

Hello,

I went camping on the weekend and rudely discovered that the isolator for the dual battery in the car has failed. I managed to get the car home and after running the fridge again to see if I can reproduce the error, I took the batteries out (and put them on charge) to trace cables and see why it had failed.

I put the batteries back in and attempted to start the car. The starter battery didn’t quite have enough charge to start the car, but the gauges did move. While trying to start the car, the fuel gauge needle went from 1/4 tank (current position) all the way around to the right (past full) and has stayed there ever since (along with empty light) I put the battery on a trickle charger overnight (battery still in car) and got it fully charged. I started car this morning (hooray) But the fuel gauge was still stuck on far right. I’ve attached a photo of it (hopefully)

Anybody ever encountered this before? I’m thinking there might be a fuse in the circuit which I have blown? Only other thing I could think of is to disconnect the batteries and see if that resets it?

Please let me know if there’s something I can try. Hoping I haven’t cooked something important...

Thank you for the help

[erad]

The fuel gauge (like the temperature gauge) is merely a micro amp meter. The fuel level sender is a variable resistor - the resistance varies from high when low levels to low when full, allowing more current to flow in the fuel gauge circuit. if you have a short circuit in the fuel wire, this will emulate a full tank. Similarly, the temperature gauge resistance decreases as the temperature increases, again allowing more current to flow hence a higher reading.

Now, if your temperature gauge also reads high, the probable cause is the power supply for the gauges. In older cars they used to have a stabilised power supply for the gauges so that the levels did not vary as the voltage varied during normal operation. Nowadays, this is done via the ECU, but if both systems are showing high, you possibly have a problem there. Looking at your photo, I think that you may have spiked something in the ECU. Hope not for your sake.

[LachlanMann]

Hello Erad,

Thanks for the reply.

I am really hoping that I have not damaged the ecu; quite concerned actually. Any way I can test this? Or just see if I can reset the ecu and see if it comes back?

[Keithyv]

is to disconnect the batteries and see if that resets it?

Yes, do this then report back please !

[LachlanMann]

Hello mate,

Will give it a test when I get home from work.
Will let you know how I go

[Kingbrown]

Does the fuel gauge needle return to zero with the vehicle turned off ?

If not then your gauge may be mechanically stuck on full.
In this case give your dash some solid whacks near the instrument cluster to try to dislodge the jammed parts.

The presence of a low fuel light suggest a fault of electronic/electrical nature rather than the above mechanical scenario.

I once had a Nissan where the gauge needles had distorted due to cabin heat. Every so often one or more gauges would jam in a particular spot and required the above 'method' to release them.

[erad]

Kingbrown: That gauge needle is WAY past full - something has given it a real spike.

[LachlanMann]

Hello,

After work yesterday, I started the car and noticed the fuel gauge had moved to half way, with the light on empty. It had moved, so I figured it was still sensitivity to voltage.

I took the negative off the starter battery, and left it for ~1 hour while I sorted the wife and kids out for dinner. Afterwards, I reattached the negative terminal and started the car. The fuel gauge was back at empty with the empty light on - pretty much where it was meant to be. (See photo).

I took it for a drive and filled it up with fuel. The gauge went all the way back to full. Took it for a drive on the highway and the temp gauge behaves normally too.

Very thankful it’s behaving as it should - was concerned I had permanently cooked something important!