Welcome to our resource for MG Car Information.



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 - Positive Earth To Negitive Earth

Just purchased a 1966 mgb roadster witch the Positive side of the battery earthed . Is it possible to convert it to Negative earth system
If so what is involved and is it something I might easilly be able to do myself ? Will it have any impact on the cars other elecrtical items ,lights etc ?
Carl Suyhenwood
Carl Suthenwood

Carl - Here is a procedure for converting a positive ground vehicle to negative ground:

To change the polarity of any positive ground vehicle (T series MG, MGA, Magnette or early MGB) to negative ground, proceed as follows:

1. Reverse the leads to the battery such that the negative terminal is connected to ground.

2. Reverse the primary leads to the ignition coil (i.e. the leads to the terminals marked either + and - or SW and CB).

3. If the vehicle is equipped with an ammeter (T series and Magnette), reverse the leads to the ammeter.

4. In the case of the 65 - 67 MGB it is necessary to reverse the pickup leads to the tachometer and have the tachometer rewired for negative ground. To reverse the pickup leads, look for the white wire that loops through the pickup adapter on the back of the tachometer (the white wire comes out of the harness, loops through the adapter and goes back into the harness). Mark the incoming wire in two places, about an inch apart with some colored tape; mark the outgoing wire the same way with some different colored tape. Cut both wires between the points marked by the tape, strip and splice the wires to the wires with opposite colored tape. Insulate the splices with tape or shrink sleeving.

Note: the tachometer will have to be changed internally to operate in a negative ground environment as follows: To reverse the power wire and earth wire inside the unit, it is necessary to remove the chrome ring, the glass face and the glare shroud. The chrome ring is usually removed with great difficulty by prying the tabs with a small screwdriver, then rotating until the tabs can fit through the slots in the case. Remove the two screws on the back of the unit that hold the internals to the case (not the two whose heads fit in holes in the case), and allow those
internals to drop carefully into your hand. Don't bend the needle! The spade terminal is the power connection. Just next to this is the earth connection. A resistor is soldered to one of these connections, and a green wire to the other. Unsolder these ends of the green wire and the resistor from their current positions. Re-solder the green wire to where the resistor was connected, and the resistor to where the green wire was connected. Reassemble the unit after cleaning the glass.

5. If the fuel pump has been transistorized, if it is a late production pump with a diode installed, have it rewired for negative ground.

6. If there is any auxiliary equipment installed in the car (CB, Radio, Stereo, after market electronic tachometer, etc.), They will need to be replaced, or if they are a dual polarity unit, the power and ground leads will have to be switched.

7. Remove both leads from the generator. Connect a 12 gauge jumper wire to the field (small) terminal of the generator. Quickly touch the other end of the jumper wire to the terminal of the starter solenoid that has the battery cable coming to it two or three times (Wire should be touched to the battery terminal just long enough to produce a spark). Disconnect the jumper wire from the generator and reconnect the original wires to their respective terminals.

8. At this point, the polarity (ground) has been changed and the car can be started.

David DuBois

nr 1 must be : Remove the leads from the battery.

I thought I was picky, wim.

You may also have to alter the position of the wiper arms on the spindles slightly. The wiper motor is polarity concious and so will be running backwards, therefore it will approach the park switch from the other direction which can alter the stop position.

In my experience the batteries have different-sized posts for +ve and -ve and it may be necessary to swap over these on the cables if they are the bolt-on type. If they are the original helmet type they are probably best replaced anyway with the bolt-on.
Paul Hunt 2

I converted my '67 over to negative earth 30 years ago and the wiper motor ran as usual. Rewiring the tach and ignition coil were all that was required. Still running strong to this day. Ray

You wouldn't normally notice which way it ran. When it parks it is at one extremity of its movement, so whan it starts up again it moves across the screen the same regardless of whether it is going forwards or backwards. It may be that it *parks* in a slightly different place, enough for moving the arms on the splines one position to make a useful difference. But then again you may not notice that either.
Paul Hunt 2

This thread was discussed between 23/08/2007 and 29/08/2007

MG MGB Technical index

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