Fuel gauge issues

[Tyre Smoke]

I'm sure ,it's something simple, but my fuel gauge has decided to show full all the time.

I've taken the sender out of the tank, there's 12v at the plug, and that varies as the float moves up and down. I measured the voltage at empty and it is about 5v, raise the float to full and it drops to 0.6v. At the gauge, there's 12.25v.

Ignition off and gauge drops to empty as you would expect, disconnect the plug on the sender and it drops, but more slowly to empty. Turn the ignition on when the gauge is dropping and the sender disconnected and it will show where it stops and not drop any further. For example, disconnect sender, turn ignition off, gauge drops. When it gets to half a tank, switch on ignition and it will register half a tank and stay there. Connect the sender and it will show full.

Is it the sender unit, the gauge or an earth somewhere between the gauge and sender? I'm guessing it's simply a resistance on the +ve that registers full to empty and the float moves? It doesn't look complicated.

For what it's worth, the sender unit looks brand new.

[phillpot]

Have you had the car long?

New looking sender would suggest a previous owner also had a problem and the new sender hasn't cured it?

[Tyre Smoke]

October.

And it was doing this when I picked it up. A jiggle of the connection on the sender seemed to fix it. Until this weekend. Now no amount if jiggling is changing the situation.

The sender isn't new. I can see the track where the arm has moved up and down the resistor.

Edited by Tyre Smoke on Monday 7th February 20:06

[Tyre Smoke]

Update. Been out this morning with my multimeter.

The sender is faulty. The resistor has a broken wire, only the thickness of a hair. Also moving the earth wire on the sender started it 'working'. It now reads full to half accurately, but no lower than half because of the broken wire.

Finding it difficult to find a replacement, but I think this may work...

https://www.ebay.co.uk/itm/35cm-13-78-Fuel-Level-S...

The TVR tank is 370 deep and the float and sender combined on 'empty' seems to be 350mm

[Polly Grigora]

Very nice sender units, never used one though

You could have a problem with the sender at that link due to its resistance being 190 ohms

Poster ChimpOnGas comments - The TVR petrol gauge works on a 250 - 0 Ohm range (0 Ohms full & 250 Ohms empty)

if you go here https://www.ebay.co.uk/itm/194215820182?_trkparms=...

and select 350mm (240-33ohm) you're getting very close to the resistance of the original sender

Might be worth you measuring the resistance of your sender to verify before ordering

[Polly Grigora]

Adjustable sender here of similar resistance https://www.carbuilder.com/uk/fuel-tank-sender

[Tyre Smoke]

Thanks for that, I did measure 160 ohms empty, 6 ohms full on mine. But it might be completely fuggered.

[Zener]

Any good for Caerbont gauges ? https://www.racetechdirect.co.uk/car-parts/electri...

[Tyre Smoke]

Yes, that's exactly what I need, but it's out of stock.

From what I can tell, the sender unit is made by Smiths. It looks like a Landy 110 petrol sender except the TVR one has five unevenly spaced fixing holes and the Landy has six evenly spaced. It's even listed separately on Smiths website. Typical it being a TVR specific part!!

[phillpot]

In stock at Racing Green . . . . . . . and cheaper

https://partsfortvrs.com/tvr-parts/part-details/tv...

[Tyre Smoke]

Thanks for that. I've ordered the one from the bay now. But good to know RG have one if that is no good.

Although mine looked virtually new, the wiper had worn quite a track on the resistor windings and eventually gone through. Not bad for 100k miles and 23 years though.

[Zener]

Tyre Smoke said:

Thanks for that. I've ordered the one from the bay now. But good to know RG have one if that is no good.

Although mine looked virtually new, the wiper had worn quite a track on the resistor windings and eventually gone through. Not bad for 100k miles and 23 years though.

They always tend to look that good living in fuel I changed one on an 87 Morgan some time ago looked this good but same thing as yours worn track (common failing of swept tracks) it was an obsolete part but a new old stock Sherpa one did the trick found that out by trial and sitting there @ home in the eve (I seldom switch off) Phillpot nice find

[Tyre Smoke]

Right, I'm confused. New sender arrived today, but is still showing full all the time. I know the sender is good, checked and it's going between 0-190 ohms.

I can only assume there's a short between sender and gauge, would that make sense? How can I 'prove' that easily?

[Belle427]

Disconnect at the back of the gauge?
Maybe connect the sender up temporarily to the gauge see how it performs.

[Tyre Smoke]

Yep, I think it's a broken or partially broken sender wire. I'll do some diagnostic work tomorrow.

Found a useful site (for boats but the wiring is rhe same a a TVR) to crib from.

https://www.sailmagazine.com/diy/test-replace-fuel...

[ric355]

Be careful using that site. It states that an empty tank is near zero ohms. This is not the case on a Chimaera.

The original sender range is approximately 250ohms, with 0 ohms representing full and 250 ohms being empty.

So if there is a short circuit on the input the the gauge will go full deflection. If it is open circuit it'll be a zero reading.

If you have some resistors in the 0-250 ohm range lying around you can test the circuit by putting a resistor across the pins on the 2 pin plug at the fuel tank end. And you can do the same thing by putting it across the contacts on the back of the gauge (disconnect the input from the sender first).

[Tyre Smoke]

Thanks, I'm using it as an aide memoir for the wiring rather than the electrical stuff.

i.e. what should read what with various wires disconnected. Remove sender wire, gauge should read empty, etc. Except that was a bit obvious wasn't it?

[Polly Grigora]

Some gauges do work the opposite way to the norm - higher the resistance - higher the gauge reads - disconnect sender and gauge reads full. Doubtful that your car has this type of gauge

Are you sure the float hasn't jammed in the full position inside the tank?

What resistance is the new sender reading now as fitted?

Anyway, moving on

Test front to back Green/Black sender cable by disconnecting it from the sender

Switch ignition on

Measure voltage from battery negative earth terminal or known very good earth to disconnected sender Green/Black cable

Voltage measured should read close to 12 volts (same as battery/ignition supply voltage)

Above is due to battery voltage flowing through ignition switch and onwards through a gauge coil or coils and onwards along Green/Black disconnected sender cable at rear of vehicle

Shown below

[Tyre Smoke]

I've got 12v at the sender unit.

My tank is near empty, connect everything up and gauge reads full. Disconnect sender wire from gauge and gauge reads empty. Jump a wire from sender connector on the gauge to earth and gauge reads full. Disconnect earth on the gauge and gauge drops to empty. I'm beginning to think it's the gauge.

It was showing full when I bought the car in October and didn't move. I assumed because of lack of use the float had stuck or dry/corroded connection on the sender unit or elsewhere. Gave the connector on the sender a wiggle and the gauge worked fine for a few weeks. Last weekend and it's back to full all the time.

[Gladers01]

Tyre Smoke said:

I've got 12v at the sender unit.

My tank is near empty, connect everything up and gauge reads full. Disconnect sender wire from gauge and gauge reads empty. Jump a wire from sender connector on the gauge to earth and gauge reads full. Disconnect earth on the gauge and gauge drops to empty. I'm beginning to think it's the gauge.

It was showing full when I bought the car in October and didn't move. I assumed because of lack of use the float had stuck or dry/corroded connection on the sender unit or elsewhere. Gave the connector on the sender a wiggle and the gauge worked fine for a few weeks. Last weekend and it's back to full all the time.

You could try disconnecting the sender wire at the gauge and the sender and do a resistance check down to earth and hopefully not get continuity or a resistance reading ie open circuit ideally, other than that it sounds like the sender could be getting stuck in the full position