MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Polarity changes

I have a 1967 MG B Roadster and, in order to fit a modern radio I need to change the car's polarity from a Positive to a Negative earth. In addition I would like to replace the dynamo with an alternator.
Can anyone help with guidance on theses two things?
Graham Lewis

The battery posts are usually different sizes for Pos and Neg so if you still have the cup-type connectors you will have to cut them off and get two new bolt-up connectors, which are much better anyway. Before starting work always disconnect the battery ground cable first and reconnect it last, no matter what the polarity is.

You need to open up the electric tach and reverse the polarity of the ground and 12v wires inside, you also have to reverse the direction of the wire loop inductive pick-up.

If keeping the dynamo you have to repolarise it, but this is no more involved than removing the wires from the dynamo after you have changed the battery and tach, briefly tapping 12v onto the dynamo F terminal, and replacing the wires.

If you have the heater fan motor with black and green/brown wires these may have to be reversed at the connectors by the motor. If in doubt try them both ways (you can't do any harm) and if one way blows more air than the other that is how to connect it.

The wiper motor may run backwatds, but you shouldn't notice any difference expect possibly a slightly different parking position.

If replacing the dynamo with an alt you have the physical mounting arrangements to solve, making sure the pulleys are in line, disabling or removing the dynamo control box, and making a couple of wiring connections to the alt. The simplest way of doing this is to disconnect the browns and the brown/green from the control box, link them all together, then carefully insulate them to prevent them coming into contact with anything else. Then remove and link together the brown/yellows at the control box and insulate. But if you are going to add high-power items to the car and want a biggish alternator you will need to run a heavy gauge brown (or two if planning very high loads) wire from the alternator output terminal(s) down to the bolt on the solenoid that carries the main cable from the battery. Use proper conenctors at both ends, don't just wrap the strands around the bolt. Tape back the brown/green at the alternator to prevent it coming into contact with anything.
Paul Hunt

Changing polarity is a worthy task in and of itself, but check your radio, it may convert to negative ground.
Most (all?) modern electronic units do not however. Most vintage radios do. I am going to install an early 70s Blaupunkt Frankfurt AM/FM/SW in my car. It can be easily changed to positive ground and even 6V. Also check out the Becker series. Ebay is loaded with them.
Some Blaupunkt and Becker radios even have a 6 pin DIN tape player jack in the back that will take an adaptor to run Walkmans and MP3 players. But then your probably back to the (+) ground problem again if you want to use those.
Andy Blackley

Doesn't the fuel pump have to be changed for a neg ground type ??
Moe

If the fuel pump is an early one, it uses a capacitor and is not polarity sensitive. If it is a later pump It will have a diode in it and is polarity sensitive. the only way to check is to remove the end cap to see what is installed. If it is a capacitor, it will usually be held in place with a wire bracket around one of the pedestal screws. If it is a diode it will not be supported by anything other than the two connecting wires which will be black and red and come out the same end of the of the diode. To change the polarity if the pump is using a diode, one only has to reverse the connections of the two leads. Good luck - Dave
David DuBois

Greetings David, In the post above you say:

"To change the polarity if the pump is using a diode, one only has to reverse the connections of the two leads."

I am going to double check, but I am pretty sure I have a diode type since I just replaced the original pump last fall.

On the diode type pump, are the connections you reference above reversed inside the pump, or just the wires hooked to the exterior connections?? Please explain a little more. I've been delaying convertion since I thought I would need a new pump, this sounds like good news.

Thanks IA

Moe
67B Rdstr/Pos earth (for now !!!)
Moe

I had my 1967 B converted by the dealer I bought it from for exactly those reasons. He has swapped the tacho for a neg earth one (looks the same anyway), the fuel pump works perfectly and I swapped over the leads on the heater as Paul describes because it made a funny noise and pushed out even less air than designed. I have thought about an alternator, but as the dynamo seems to be functioning perfectly well at present and even at -5 degrees C the car turns over and starts first time I can't presently see the need to fiddle with it (if it ain't broke...). The original EKCO radio had a polarity switch on it but I have changed it for a Panasonic so I can get FM and tapes, however it is simple enough to swap them back. What I DON'T want to do as the car is so original is make big holes in it for the speakers. I was wondering about some little box speakers Blaupunkt make. What other options are there?
A Mills

Moe - change the wires *inside* the pump, i.e. you disconnect the diode and connect it the other way round. David is the expert on this, and in the past he has mentioned that even if you fail to reverse the diode it will not do any immediate damage as such, just that the quenching of the points won't be as effective.

But this reminds me of the so-called 'pointless' electronic pumps. Whilst these can probably be converted it is more involved, and unless David knows better I would say that reversing the battery and NOT converting or replacing it will at best cause the pump not to work and possibly destroy the electronics.
Paul Hunt

Inverting power supply never destroys electronic but RAMs and ROMs or highly sensitive micros .But, sure your breakless points pump will rev the wrong side . The breaker one's just need to correctly re-connect the diode as mentionned above by David; if diode equipped .
Renou

Gentlemen - The diode leads themselves, inside the pump, should be reversed on pumps that have diodes when reversing polarity. The all electronic pumps would have to have a completely different circuit board installed to convert them and unless one is versed in work on electronic components it is not something I would recommend. Neither would I recommend swapping the connections (power and ground) to the external connections of the pump. Even though the pump is isolated from ground by the rubber mounting bushing around it and even if both of the fuel lines have a rubber hose between the metalic tubing and the pump connection, you are still left with this lump of conducting metal hanging under the car with 12 volts sitting on it. Anything conductive that gets between that lump of metal (previously termed fuel pump) and ground, such as something conductive falling from somewhere on the car or a wayward wrench will cause a unpleasant shower of sparks and release all the smoke from the wiring (and we all know what happens when the smoke is released from Lucas electrics). If you are reversing polarity of your car and have an all electronic pump on it, do yourself a favor and get a new pump. The price of a new pump pales next to the cost and bother of replacing a wiring harness. Good luck - Dave
David DuBois

Great info above. My "new" pump was purchased from Moss and was specifically ordered as a pos. ground pump. Is there a way of telling (numbers, colors, etc ?) if it has diodes (which means I will try to convert it) or if it is the more modern type (No diode) and therefore I will have to replace it.


Thanks

Moe
Moe

Moe - If you purchased the pump recently, it probably has a diode (the older pumps are the ones without the diode). Check you pump for the part number AZX 1318 or AZX 1319. Those pumps are positive ground and have the diode installed. Pumps AZX 1307 or AZX 1308 are negative ground and have the diodes. The different colored tape around the end cap to coil housing junction denotes positive or negative ground. If you have any question, or can't find the part number (usually on a sticker ont he side of the end cap), just remove the end cap and check for the presence of the diode. If you want to send me your e-mail address, I will send you a picture of the diode. Cheers - Dave
David DuBois

David, Thanks for that last bit of info. I'll check my pump today and find out exactly what I have..I'll post the results later today.

Moe
Moe

David: I crawled under the 67B today and confirmed I have a AZX 1318. I have added "Neg ground conversion" to my "to-do list". Thanks for the help/info..

Moe
Moe

This thread was discussed between 02/03/2004 and 05/03/2004

MG MGB Technical index

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