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MG MG Y Type - Fuel gauge difficulties

A friend in Adelaide, South Australia, has asked me if I can assist with a difficulty he has with the fuel gauge on his YA. When the ignition is switched on, the needle on the gauge immediately indicates "FULL" and that is where it stays.

Any suggestions on what he should look for to cure this problem would be appreciated. Alf Luckman
Alf Luckman

There must be a short circuit of some kind? The sender is best explained as a variable earth. If you earth the gauge it will read full.

I would try to make a fresh bypass wire and connect that to the sender (with the old wire off) and connect it to the gauge. If that works, the old wire is faulty. If that fails the sender must be stuck?
Willem van der Veer

Hi Alf

Have you looked at the Hints and Tips page ... and read the one on FUEL SENDER AND GAUGE TESTING?

Paul
Paul Barrow

Paul is right! It's even available in PDF:

http://www.mgcars.org.uk/mgccy/hints&tips/testing%20the%20ft%20unit.pdf
Willem van der Veer

Hopefully by now most stuff is available on either Hints and Tips, or Technical Advice ... but if it isnt, then you can always drop a line through to our technical section and ask them! We will document the question and answer on the site to increase the overall wealth of knowledge about these great cars.

happY and safe MotorinG

Paul
Paul Barrow

I thought it worth activating this older thread, because it helped me to get started in sorting an inoperative gauge, and a couple more issues arose which might interest others. There's also a safety issue worth emphasising.
The IMGYTR Hints and Tips section on Testing your Gauge (and/or sender) is invaluable - how to carefully test each coil in the gauge casing, and how the needle should behave. On opening up the casing (not necessary, but out of curiosity), I found the hair-like bridging wire (see pic) was shorting against the inner casing, which no doubt contributed to the gauge's failure to work. After moving the wire away from the inner casing, the gauge behaved perfectly on test, even though it had resolutely indicated full while running the newly restored car.
That led me to re-check both continuity and voltage from A4 (ignition-fed supply) to the gauge, through the gauge windings, and down to the sender. I was surprised to find that while continuity readings were excellent, the voltage changed from 12.5 volts both at A4 and at terminal B of the gauge, down to 7.5 volts at terminal T, and at the ring terminal to be attached to the sender. My thought was : is the gauge (windings) actually designed to reduce the voltage to the sender?
I also realised (confirmed on the Y Type facebook page) that it would be prudent to add a dedicated earth wire from sender unit to chassis.
Result: after fixing the shorting wire inside the gauge casing and improving the sender earthing, the gauge works well, although I think it's reading a bit lower than reality. Interestingly, after fitting the dedicated earth wire at the sender, the voltage from the Junction Box to the sender unit has dropped even further, down to 4.5 volts! Still seems to be enough to operate the fuel gauge - and here's the safety issue: is the reduction in voltage supply to the sender a deliberate ploy by the original designers to reduce risk of fiery incident? I have a feeling modern cars have a voltage step-down to the fuel sender for the same reason, but am happy to learn more.
I hope this is of interest - we never stop learning. John.


J P Hall

This thread was discussed between 10/11/2008 and 19/06/2023

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