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MG MGA - petrol gauge grounding

I'm preparing to rewire my dash switches and gauges. I'm studying the schematic and some of Barney's web pages. How is the petrol gauge grounded? The schematic shows three black wires at the gauge, and one of Barney's pages states "The only instrument which requires an electrical ground for operation is the fuel gauge". Maybe a look at the dash sub harness could answer this....

George G.

According to the wiring diagram I have, the gauge itself is supplied with a live which passes through the gauge to the level sensor in the tank. The other side of the sensor is earthed (grounded). Inadequate earthing at the sensor end can cause problems.
Graeme Williams

Originally the fuel tank sender unit was earthed via the metal fuel pipe to the engine.
A lot of cars today though (including mine) have flexible rubber/plastic fuel pipes or maybe an in line non-metalic fuel filter which prevents this direct connection.

So it pays to connect an earth to the fuel tank to be certain that your gauge will work correctly.

The fuel gauge is earthed through its fixings to the dashboard but as far as I know, I dont think it needs an earth at that end for it to work.

Colyn Firth

Graeme I think whilst what you say is correct, the gauge itself is also earthed.
Graham V

Yup, the fuel gauge must be grounded to work. There are a number of black ground wires in the the dash harness, all daisy chained together. The fuel gauge bracket was a convenient place to tie the ground wires together. The fuel gauge also serves as junction point for a few green power wires.

Each of the four dash instruments gets (at least) one black ground wire attached to the mounting bracket. The fuel gauge is electrical. All gauges have an illumination lamp that needs a ground return. If the black wires are properly connected to the instrument mounting nuts, the instruments do not need to be grounded on the dash panel. Turn signal indicator lamp and map lamp do ground on the dash panel.
Barney Gaylord

Barney has answered another question I had. I did have ground wires attached to the speedo and tach brackets, so I guess that is correct. Why doesn't the schematic show these instrument grounds I wonder....?

George G.

The wiring diagram does show all of the illumination lamps grounded (after the Panel "switch" dimmer rheostat). See here:

My diagram also shows the petrol gauge grounded here:
although the original Workshop manual did not show this ground. It also shows that the safety gauge (pressure/temperature) is another common grounding point for the black wire daisy chain, even though the safety gauge is not electrical.

These are logic diagrams which do not show physical configuration or physical location of the parts. For that I have physical diagrams for the 1500 and 1600 dash panels here:
Barney Gaylord

I don't see the earth as part of the fuel circuit. But as Barney says, it's for the instrument light. The holder of which just has a single live connection to the lamp and the circuit relies on the body of the gauge for it's earth.
The live on the gauge, again as pointed out, serves several other locations as a convenient power source.
The point Colyn makes is important though as I think the sensor is mounted on a gasket and could have issues with a satisfactory earth at that end.
Graeme Williams

Omission of the fuel gauge ground is an error in the wiring diagram in the Workshop Manual. The fuel gauge absolutely must be grounded to work.
Barney Gaylord

Its not just for the lighting circuit. Take a look at the detailed circuit of the gauge on Barneys site.
Graham V

This thread was discussed between 26/08/2016 and 28/08/2016

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