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MG Midget and Sprite Technical - Changing a 1965 AHS Positive to Negative Earth

Hopefully a simple question....
I'm considering changing my AHS from positive to negative earth, partly because I have a non working positive earth tacho and a working negative earth tacho.
I need a definite answer on this - does the electric fuel pump need changing ? - I have a replacement SU fuel pump bought from Ron Hopkinson's in 1992. From the literature on the MASC site, the fuel pump is not mentioned, but MGB literature does say later (diode) pumps are earth sensitive.
Clearly, I don't want to swap over and find out that I have saved money on the tacho and cost money on the pump.
Cheers,
Malc
M Gilliver

Have a look on ebay I got a working secondhand positive earth Tacho for approx £20 delivered

Shaun
Shaun

That was the plan, on the last tacho I went for the gauge shop man out bid me in the last second, I don't want a recon tacho because then I'd need to recon all the gauges and that isn't going to happen.
So does anybody know if the pump is earth polarity sensitive ?
Malc
M Gilliver

Some modern replacement fuel pumps have a diode or capacitor across the points for arc supression. I have heard that wiring these up the wrong way will maybe blow the diode or whatever. It is wise to check your fuel pump to see what has been fitted. If you are unsure of what a diode or capacitor looks like then remove the fuel pump and post a picture here.

If it does have a diode then it will require reversing in the head of the pump.

Robert (Bob) Midget Turbo

check the pump for a red wire or plus sign atatch accordingly an try to start

if it is not marked i'd just try it
Onno Könemann

Malc - See the article, Polarity Change in the Other Tech Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ If you change the polarity, See the article Negative Earth Stickers in the same section of my web site and get one to put on your car to avoid any confusion by maintenance people in the future. I have attached a picture of the diode/resistor assembly used in the later pumps. If the pump doesn't have one of these, or a printed circuit board under the end cover, then it is not polarity sensitive. If it does have the diode/resistor assembly, the leads on on the assembly just have to be reversed to make it compatible with negative ground. Cheers - Dave



David DuBois

Thanks All.
I have had a look at your data sheets, David, and thanks for them.

It looks like I have a AUF 214 or something very close, I'll remove it later - we have 30degC and Wimbledon on, so sitting in the garden with a beer in the shade is more important than swapping the polarity !

cheers,
Malc
M Gilliver

Beware though, the Sprite I have is 67 and has a negative earth sticker but the battery is clearly wired positive earth and the tacho has positive on it also.

So do not trust by stickers alone. Also try an auction snipe tool like bidmate works 80% of the time for me.

Shaun
Shaun

I appreciate the comment Shaun, I intend to check everything before I start to change.
Cheers.
Malc
M Gilliver

My tach, '67 midget, was originally a positive ground. When I got the car the tach didn't work. Turned out the seller had attached the battery as Pos and the Tach had been re-wired to be neg. Argh! Turned out to be an easy fix, though I'm little help now. That was 12 years ago. Someone should know or if needed I could crack open the Tach and see what had been done.
Also if using a Facet F.P. they are wired neg, so I had to swap the wire connectors so black was my neg and red my pos to the body.
cheers.
Kerry Cooper

This thread was discussed between 30/06/2009 and 01/07/2009

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.