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MG MGA - Fuel Pump Polarity

Electricity always defeats me, so you'll have to assume I know nothing.

My car is positive earth (the pos. battery cable goes to earth), and today I'm replacing the fuel pump. The new pump is virtually identical to the old one - the tag screwed to the metal body of the pump is on the other side, but that's the only difference I can see.

But on the old pump, the shorter wire that earths to the car was connected to the pump body tag, and the wire out of the loom was connected to the terminal marked 12V at the back end of the pump.

How can this be correct?
Nick

Beause it knew your car was positive earth and it was internally wired correctly to suit Nick. If your new pump is also for a positive earth car then it should be wired the same.

You need to regard electric as a difference in pressure between one point and another. The earth, chassis is simply a datum that the other battery terminal is live to, whether that is negative or positive, that just determines which way the electrons will flow.
Bob (robert) Midget Turbo

Thanks Bob. The new pump came with a printed note saying that all the Burlen pumps are now dual polarity.

Does this mean I should extend the earth wire to the 12V terminal, do you think?

The second part of your reply was just beautiful metaphysical poetry to me.
Nick

No Nick simply connect the normal wire to the terminal and the earth as normal to the chassis. By dual polarity it means that internally it has no diode or such that can detect polarity, and magnetism is magnetism so it will work quite happily either way.
So just to be clear connect the loom wire to the 12 volt terminal and the one connected to the body of the pump need to be connected to earth/chassis
Perhaps that is what you suggested :-)
Bob (robert) Midget Turbo

No, it isn't. I would have done it the other way round.

Thanks Bob. It's such a comfort having you around. It will have to be tomorrow morning now, because I've been nabbed by SWMBO for clearing up the house.
Nick

A diode used to be wired across the points to reduce sparking on interruption of the coil current: however a diode is polarity sensitive. Burlen no longer use a diode but instead have replaced it with a voltage dependant resistor which is not polarity sensitive. mike
m.j. moore

This thread was discussed between 09/02/2008 and 10/02/2008

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This thread is from the archive. The Live MG MGA BBS is active now.