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MG Midget and Sprite Technical - Wiring fuel gauge woes

I'm trying to get my head around a puzzle and I'm sure I'm missing something obvious

GAN4 275 midget Positive earth.

All the electrics n the car are working fine, except the fuel gauge, which flips to full as soon as the ignition key is turned, and it stays there regardless of tank contents. Dual green at the gauge shows 12V wth ignition on, but continuity to ground when off. Disconnecting all free wires at all points allowed tracing this to the fusebox. Checking the fusebox makes it clear that the green wires are not shorting at any point, but the white on the other side is too showing 12V with live ignition, but continuity to ground otherwise.

Haven't yet delved under the car yet to disconnect the sender and fuel pump and check the continuity here, that's a job scheduled for Sunday

Any suggestions for me to mull over, or other words from the wiser?
dominic clancy

I wouldn't be surprised to see continuity on the switched live circuits to ground with the ignition off, especially if you have contact breaker ignition and the engine has stopped with the points closed, as you will effectively have the HT coil bridging the live to ground and giving a resistance of around 5 ohms. Also other items such as the fuel pump will be creating a low resistance to ground, giving a false continuity reading.

I suspect it's more likely a fault on the sender or it's wiring.

J Smith

You got love positive ground

Prop and the Blackhole Midget

Maybe this link helps with finding the problem?

Good luck.

pjw Seezink

Thanks for the tips so far. I have tested the gauge on the bench with a spare MGA tank sender (it's exactly the same setup apart from the length and shape of the float arm) and it all works as it should. Disconnecting the white from the fuel pump removed the issue with the grounding, so nothing out of order there. I am now suspecting that the connection between the connector bundle at the RH end of the dashboard and the tank sender is the issue, as it's showing open circuit, which means it's either a broken connection or the (new last year) tank sender has given up the ghost. So it looks like I am going to have to drop the tank to have a look.

dominic clancy

I am sure you are aware but through the model range the way the gauges and senders work has switched around so that early gauges are not compatible with later senders and vice versa. It's to do with the full to empty going from high to low resistance and vice versa.
The manual has a set of procedures to check what has failed in the system.
Graeme W

Im certianly the last one to even offer an opinion as I know just enough to get a good looking fire going

But with the wiring being positive earth im wondering if its getting te open circuit thur the dash... on my neg ground car (71 usa spec) the dash is grounded and so the gauges and there gauge light bulbs are naturally grounded

So I wonder if you put some kind of foam or rubber electrical resistant material between the gauge and the dash if that would take the open circuit anomaly out of play

Please dont do this thinking I actaully know what im talking about... my comment is just something to say....

"""Hhmmmm thats interesting"""

Prop and the Blackhole Midget

positive earth is no big mystery, the current just flows the other direction
dominic clancy

This thread was discussed between 22/10/2015 and 25/10/2015

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