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MG MGB Technical - Voltage stabiliser

1965 Roadster: if the car is converted to negative earth keeping the original type petrol tank and sender unit connections, should the voltage stabiliser be replaced to ensure accurate readings on the petrol guage?
Thanks
Ron Stevenson, Ayrshire,Scotland.
R Stevenson

Ron - Unless you recently purchased a solid state voltage stabilizer, there is not need to change it, the original units are not polarity sensitive. Cheers - Dave
David DuBois

David
It still is the original unit (3 wire connections) but the guage is showing erratic readings which I thought might have been the stabiliser needing changed for negative earth. If this does not matter maybe I should fit a new unit anyway. Which one do you suggest? (solid state?)
Thanks
Ron Stevenson
R Stevenson

A 65 would have had the voltage stabiliser, together with the slow-acting fuel gauge. As Dave says the stabiliser isn't polarity sensitive and isn't touched as part of polarity conversion. It should only have two male spades which are electrically the same, and two female spades which are also electrically the same. The males should have three wires on them, probably using two separate wiring connectors one with one wire and the other with two wires. There should only be one wire on one of the female spades, the other is unused. The unit body must make a good electrical and mechanical connection to the car body for earthing, a bad connection here can make the reading move erratically between correct and high.

There can be many reasons for erratic readings ranging from the supply to the stabiliser through the connections at itself and the gauge, problems inside the gauge, main to rear harness connection, rear harness to tank sender connection, the tank sender itself, and the earth connections at the tank and rear number plate bolt, as well as with the stabiliser itself. You should diagnose what is actually wrong before splashing out what may be wasted money on a new stabiliser, of either type. Personally I would only go for an original sender, they cause the gauge to rise faster when switching on the ignition than the solid-state type.
PaulH Solihull

In my experience, the symptoms that you describe point to a worn fuel level sender. This can be checked by moving the sender from the full to the empty position and noting the readings on an ohm meter. RAY
rjm RAY

The unit also needs to be near vertical with the top label actually at the top!
Allan Reeling

I have checked by taking off the green live at the tank sender unit (2 wires green/black on the unit), the guage goes to empty and earthing this wire the reading stays at empty. Does this indicate a faulty sender unit? The unit is the early type which screws on to the side of the tank.
Thanks
R Stevenson
R Stevenson

Ron. If the wire is not connected to the tank sender then it is not a sender fault. If when the wire is shorted to earth the gauge does not move then you have a fault in the wiring, or the gauge or the stabiliser. You need to start checking the voltage from the fuse to the stabiliser, the stabiliser to the gauge and the gauge to the tank wire.

Tony
Tony Oliver

If the gauge was showing a reading *before* you removed the green/black from the sender (incidentally there should only be one wire on this terminal, two indicates a rewire where the old harness wasn't fully removed!) and dropped to nothing when removed, then stayed at nothing when connecting the black earth wire to the green/black wire, then it is the earth wire that is bad. What gauge movement you *were* getting before will have been from an earth via the tank mountings which isn't reliable. The black earth wire should go back to a number-plate bolt, or at least it did when reversing lights were added as they were earthed at the same point, not certain about before that.
PaulH Solihull

This thread was discussed between 19/08/2010 and 22/09/2010

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