Fuel gauge

[nigel jarrett]

On my 1949 Javelin std with the metal dash and square gauges the fuel gauge does not work .I have just spent the evening looking into this ,after a lot of head scratching and testing out 2 gauges one being a nos I have come to the conclusion the gauge work thus . One wire is connected to the live ,the gauge body Is earthed this sends the needle to full ,the other wire then goes to the tank this gives a vairiable earth which will then cause the needle to move accordingly .
Does anyone know if I have come to the correct conclusion your thought would be appreciated

[Keith Clements]

The tank sender varies from 3 ohms when empty to 93 ohms when full. The moving iron fuel gauge has a coil in parallel with a resistor to control the armature which deflects to read full if the sender wire is disconnected as current flows through the deflecting coil. When the sender is connected it drains the current from the deflecting coil by an appropriate amount to indicate how full the tank is.

If you have the gauge connections the wrong way around it will always read full and may burn out the deflecting coil as there is no resistor in series.

[Srenner]

Yep, that's pretty much how they work. One can find loads of info here; http://mgaguru.com/mgtech/electric/fg110.htm. Actually, the MGA electrical uses many of the same bits and all of the same principles, so the site is quite handy for trouble shooting. It includes photos and tips and often has reviews of the modern replacement items for the old Lucas stuff.

[nigel jarrett]

Thank you Keith that makes sense .I know what direction to go now

[nigel jarrett]

After an evening fettling I now have a fuel gauge that reads full when the tank unit is disconnected and still reads full when connected I now have to check the wire to the tank but I feel that it's the tank unit fault
Has anyone any ideas where I may be able to get a replacement

[Keith Clements]

JCS, Internet...but take it out and give it a clean. Check with resistance meter. They can be rewired with a bit of care as it is just a slider on resistance wire.

[Srenner]

I just dealt with this on a Jupiter. The gauge always read full when power was switched on. This is a grounding problem. The gauge needs grounded and the fuel tank needs grounding so the circuit is closed. Open circuit gives a full tank reading. Check these first, then test the wire to the tank. Remove and touch to the chassis. It should go from full to empty. Then move to the sending unit.

The sending unit is one continuous wire. This wire is coated, so be careful not to abrade it. One end of the wire should be grounded underneath the fiber washer that helps isolate the threaded stud (the washer has to be removed to find it). The other end of the same wire is visible stretching from the threaded stud to the far end of the bobbin and is wrapped around the bobbin. Visually check the wire and if good, check the continuity of this wire with a tester by putting the tester on the stud and a CLEANED area of the body of the unit. If the wire looks good, but the reading indicates a break, test in segments until you locate the problem. It could be corrosion under the washer not properly grounding the wire. Try finding the end under the fiber washer, slide it aside, remove a little material from the surface of the body and move the wire back. Using a cotton swab and alcohol, gently clean the copper wiper arms and the sides of the bobbin.

Good luck!

[Keith Clements]

All components on my Jup have separate earths and do not rely on the body or chassis for grounding. This includes the sender which has an earth wire attached from the rear loom.
There is a possibility that the only earth return is via the petrol pipe as the tank is protected with fibre padding under the straps holding it in.

[nigel jarrett]

Now that's interesting as as mine has a flexible fuel pipe at the rear by the tank.i will look into fitting another earth to the tank unit

[nigel jarrett]

Finally got around to further inspection of the tank unit,removed the cover in the boot and removed the cover on the tank unit,all looked ok checked for resistance that seemed ok so removed the tank unit to found the float swivel seized.
So I filled the unit with WD40 and penetrating oil and left it soaking for 4 days then came back to it to find it starting to move,with a little persuasion full travel and finally all freed off,so I reassembled the tank unit and fitted it back into the tank .At long last I have a working fuel gauge.

[SteveWood]

Thanks this is very helpful. I need to know the connections on the back of the gauge - ie which stud is connected to live and which is from sender. They dont have the identification letters given in the wiring diagram. T B