Petrol gauge sender

[jimj]

My petrol gauge began reading full all the time. I had a spare sender so disconnected the 2 wires to the tank and tested the spare which was fine. I removed the tank, fitted the spare sender and now it reads full all the time again. If I disconnect the wires and test my old one that works fine now, even if I connect the wires upside down. It can`t be shorting out as that makes the gauge read empty. Are dark forces at work or have I missed something obvious?
Jim

[PeterK]

I thought that earthing it out would make the gauge read full

If earthing does make it ready empty, then maybe a broken wire or dirty connection that doesn't make a circuit when the sender is installed, but does when you're waving it about ?

[TeeJay]

Hi Jim.
Just rewired my +2 and my Fuel Sender has 3 wires.
Black to ground (earth)
Green/Black to Fuel Gauge.
Black/Brown to warning light.
Is yours missing the ground?

Also with the gauge reading "Full" it suggest a weak/broken connection.
Is the Fuel Gauge correctly connected to the Voltage Stabiliser?

[AHM]

poitive or negative earth?

what does it read when there is no sender connected?

[jimj]

It`s negative earth and the 2 wires work OK when connected to the one I took out which is the mystery. If there was a fault at the gauge end or the sender needed earthing surely that wouldn`t happen. It reads full if everything is disconnected, empty if shorted out i.e. no resistance. Am I not right in thinking that the sender is merely a potentiometer breaking a loop in the wire to and from the gauge, with no resistance when empty and lots when full?
Jim

[Quart Meg Miles]

As you have described, when the sender is s/c it reads empty and when o/c it reads full. So you have a broken wire in the connectors to the sender, and when you drag them up to connect to the spare senders to test them the break temporarily mends itself. EDIT: My early Positive earth car reads beyond Full when I disconnect the sender so is your Negative earth car a dynamo system?

In the early cars the sender is a coarse variable resistor changing the current to a coil in the gauge. The gauge has another coil in it going to the battery and the angle between the coils and their different currents form a magnetic vector that the pointer aligns with. It's a nightmare to calibrate and I won't describe it!

[MickG]

Hi Jim,

I have just checked the resistence of a spare gauge and I get the following :-
20 Ohms Full
247 Ohms Empty.
These readings may vary slightly so any thing there abouts is probably OK

Check the voltage stabiliser and connections, connect the spare gauge to the cable that goes to the gauge and then use a new wire from the earth terminal to the battery neg. If the gauge works correctly then as Meg has pointed out you have a broken wire. It may also be that the existing earth/neg wire is badly corroded internally giving a high resistence.

Regards Mick G

[AHM]

I believe that the gauge is a voltage divider, consisting of 2 magnet coils and a resistive coil. (I state their purpose rather than accurate physics)

As the resistance of the sender changes the voltage bias between the magnet coils changes and the needle is drawn to one or the other.

Very simple, but very clever.

Must be wired-up correctly or the resistive coil becomes a heater element!

[AHM]

or the sender needed earthing

you have just jogged my memory - I had this issue last year. I connected the wires and it worked for a while. But I hadn't connected the earth to the Frustacone bolt in the chassis (S3) - when I did It worked ok.

As the others say poor earth connection somewhere.

[jimj]

Does anyone one know which wire is the earth? the upper wire is green, the lower black. I`m assuming I can just temporarily earth the right one to test it.
Jim

[john.p.clegg]

Earth one and the light will come on...
Earth the other and your needle will rise ( slowly)
Earth the Earth and nothing will happen...

John

[Quart Meg Miles]

Does anyone one know which wire is the earth? the upper wire is green, the lower black. I`m assuming I can just temporarily earth the right one to test it.
Jim

Black is ground according to the manual but I believe it isolated from the metalwork anyway.

[EDIT: THIS PARAGRAPH IS INCORRECT, I'VE GOT MY POLARITIES WRONG! 1st JUNE] Mick, Jimj has a negative earth car so won't be using a stabiliser and your sender is the wrong type. I know it doesn't work because I had to replace it with my old sender when I bought a new tank.

AHM, that's was what I was trying to describe. But it's not a voltage divider as such, just a resistor which changes the current in one of the vector coils. The clever bit is that it is insensitive to supply voltage provided all the connections are tight.

[jimj]

Well I`ve tried connecting each of the 2 terminals to earth, there`s no extra earth and no lights to come on ?? When I earth the black wire nothing happens, it remains on full, when I earth the green wire the gauge moves to empty. If the black wire is just to earth then it`s like shorting out the circuit with the same result. I can`t see how the green wire could be faulty as it wouldn`t cause a reaction at all unless it was making the circuit.
Unlikely but is it possibly that the tank is accidentally earthed and shouldn`t be? This would explain why the sender works when not in the tank.
Jim

[AHM]

It sound like the gauge is ok
The senders are fairly crude and rely on the bearing of the arm to make the circuit. If you take it to bits you can check that the coil is intact, that the wiper makes contact , and that the bearing is making the circuit.

[AHM]

Unlikely but is it possibly that the tank is accidentally earthed and shouldn`t be? This would explain why the sender works when not in the tank.
Jim

The sender is most likely earthed by the screws there should be no problem as the whole body of the sender is earthed.

When you have it in your hand it is completely isolated

When you connect it to the tank you are either getting a short
or have the terminals the wrong way round.