Erratic fuel gauge, gauge or sender?

[eddypeck]

The fuel gauge has gone mental, jumps about all over the place and is currently showing full when I know there's only about a tenners' worth in there.

Is it likely to be the gauge or the sender?

Also speedo occasionally jumps about, which is leading me to think the problem is in the cluster.

[Grrrmachine]

Read the Instruments page in the Wiki and you'll see it's all covered. I'd link you, but i'm on mobile.

[eddypeck]

I already have.... didn't help. Said could be either, just wondered if someone could shed some light on it with a bit more personal experience.

Is the speedo behavior anything related.

[Rtaylor2208]

The wiki does cover it:

"Fuel Gauge Not Working

Fuel gauge problems are very rarely related to the gauge itself, and are far more likely to be caused by faulty fuel senders. Learn more about the fuel level senders.

If testing the level senders works out fine, try tightening the brass nut on the rear of the fuel gauge itself. Be aware that this involves the removal of the dash cluster, so if you are doing this it is also worth assessing the condition of the SI batteries, the brake pad resistor, and the temp gauge at the same time.
"

Anyway, common issues are:

1. Brass nut on the back of the fuel gauge may be loose, this is pretty common and causes the needle to be erratic.

2. Showing full when clearly its not can be a sender issue, follow the guide in the wiki in the section I quoted.

[Brianmoooore]

You can temporarily replace the sender(s) with a fixed resistor of about 33 ohms for 55 litre tanks and 56 ohms for 63 litre tanks, which should result in the gauge giving a steady reading of about half full. Depending on the results, this will point you towards the gauge and wiring, or a sender.

[KW355]

My 55 litre tank sending unit reads about 60 ohms empty and about 0 ohms with the tank full.
I believe the 63 litre tank has 2 senders that read the same as above but are wired in series i.e. 120 ohms empty and 0 ohms full
Easy to check with a multimeter if you remove the tank sender (s)

[eddypeck]

Thanks, I have a multi meter I've borrowed but I'm not sure how to use it to test like that. Don't suppose you'd be kind enough to give an idiots guide?

[KW355]

Take out the rear seat base, remove the fuel gauge sender black cover (2 for the 63 litre tank ), unplug the fuel sender (s), undo the four 8mm nuts and pull the fuel sender out.
set the multimeter to resistance or ohms (ohms symbol is like a horse shoe ) put one lead on the metal body of the sender and try the other multimeter lead on to one of the three sender terminals and turn the sender upside down.

One terminal should give a reading of 0 ohms whether it is upside down or not, this is the earth terminal.

One terminal should give a reading of 0 ohms the correct way up and infinity when upside down, this is the fuel warning light.

One terminal should give a reading of 60 ohms the correct way up and 0 ohms when upside down, this is the fuel Gauge signal and the one which is probably your problem

Thanks, I have a multi meter I've borrowed but I'm not sure how to use it to test like that. Don't suppose you'd be kind enough to give an idiots guide?

[eddypeck]

Take out the rear seat base, remove the fuel gauge sender black cover (2 for the 63 litre tank ), unplug the fuel sender (s), undo the four 8mm nuts and pull the fuel sender out.
set the multimeter to resistance or ohms (ohms symbol is like a horse shoe ) put one lead on the metal body of the sender and try the other multimeter lead on to one of the three sender terminals and turn the sender upside down.

One terminal should give a reading of 0 ohms whether it is upside down or not, this is the earth terminal.

One terminal should give a reading of 0 ohms the correct way up and infinity when upside down, this is the fuel warning light.

One terminal should give a reading of 60 ohms the correct way up and 0 ohms when upside down, this is the fuel Gauge signal and the one which is probably your problem

Thank you so much, I think even I will be able to follow that, lol

[Brianmoooore]

Not a good idea to disturb suspect components until their guilt is proven. The act of removing a suspect often provides a temporary cure, which will leave you non the wiser, until the part fails again. There's also the chance that you will kill a perfectly serviceable component by disturbing it, giving you two faults instead of one.
Try fitting a resistor as a test (at the plug under the rear seat base) before you take anything apart.

[eddypeck]

Ok, so I'll be dumb again.... where do I get a resistor from and how do I use it to test?

I'm quite happy with a bag of spanners unbolting and replacing mechanical items but when it comes to electrics I really get lost.

A resistor isn't going to have the same plug fitting sender does, I guess it will have 2 wires? how do I attached it to replicate the sender?

It's a 63 tank by the way.

Cheers

[Brianmoooore]

Resistors cost me less than 1p each. Maplins sell them retail, and they might be a bit more expensive and come in packs of five or so, but it's still hardly going to break the bank. Buy a pack of 1/8W or 1/4W 33 ohm.
Take up the rear seat base and unscrew the two black inspection covers secured with self tapping screws.
Pull the plug off of the sender under the LH (round) cover, bend over the wires of one of your resistors and push them up the buckets of the plug, so that they make good contact.
Drive the car like this and see if the gauge is now inaccurate but stable. If it is, there's a good chance that this sender is faulty.
If the gauge is still unstable, pull the three pin plug off of the sender under the oval cover, and connect another resistor between the brown and brown/green wires. The fuel gauge should now read about half, independent of how much fuel is onboard. If the gauge is still unstable, the fault is probably with the gauge itself.

[eddypeck]

cheers