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MG MGB Technical - Earthing conversion on 66 roadster

I want to change the earth (ground) of my car from positive to negative. Rather than try to change the existing positive Tachometer, I have obtained a negative one. Can anyone advise me of the changes that have to be made to the car's wiring. The postive Tacho. has three white wires, the negative Tacho. has one white wire with a black trace, a green wire and a black wire. Both have wires for the illumination of the Tacho. as well as the ingnition warning light.
Thanks
G G Lewis

G.G. Lewis,

Took on a similar task with my 66 roadster, although the prior owner had converted from positive earth to negative, but didn't deal with the tach. I changed this internally as described in a Moss Motors catalog. I assume you are aware of the reversing of the polarity to the generator field winding via repolarazation. Also the reversing of the coil connections.

Since you have a negatve ground tach, I would suggest a look at both electrical schematic diagrams. The 66 and the 67or68 when they switched to positive earth or ground. These diagrams and a lot of good tech data on the subject can be found at www.mgbexperience.com

When the web page is loaded the upper left of the screen has a menu. Click on technical. A whole list of data on this is available on this activity. Good luck

By the way , it works well for radiosand modern electronics.

Jim
James Hanks

G.G. Lewis,

Took on a similar task with my 66 roadster, although the prior owner had converted from positive earth to negative, but didn't deal with the tach. I changed this internally as described in a Moss Motors catalog. I assume you are aware of the reversing of the polarity to the generator field winding via repolarazation. Also the reversing of the coil connections.

Since you have a negatve ground tach, I would suggest a look at both electrical schematic diagrams. The 66 and the 67or68 when they switched to positive earth or ground. These diagrams and a lot of good tech data on the subject can be found at www.mgbexperience.com

When the web page is loaded the upper left of the screen has a menu. Click on technical. A whole list of data on this is available on this activity. Good luck

By the way , it works well for radios and modern electronics.

Jim
James Hanks

Let's see--according to the Lucas wiring code. a solid green wire goes to the accessory voltage source. A white wire means ignition, and the black stripe indicates it also goes to ground--sounds like the point's connection to me. Black wires are ground. Double check this on the schematic for the year that the tack came from.
R. L Carleen

Hi:

I think the post 1967 British home market MGB's use a negative earth tachometer, steel dash with alternator in lieu of the generator.

Rich Boris, 67 positive earth "B"
Rich Boris

When you make the conversion, are there any other electrical items that need to be changed?

My 67GT was changed to negative ground and there are parts that are not working (windshield wiper for one).
Bruce Cunha

Bruce,
The windshield wiper motor is not sensitive to polarity. It should work eaither way. Must be another problem. Either a worn out motor, bad connection or wiring.

James Hanks

RL is correct, you have the later negative ground tach which is voltage triggered, and not the earlier current triggered type. This picks up a fused ignition 12v source from any green wire behind the dash, like wiper switch or heater switch. Black for ground as before. The white/black must be run to the points side of the coil, which originally would have been CB on the coil but will now be SW. One of the whites was the 12v supply and should be taped back and insulated, the other two must be joind together and insulated to provide 12v to the coil. You will have to check by testing with a voltmeter or test-lamp which of the three wires does *not* have 12v with the ignition on, that is from the coil, it can be connected to either of the other two. Be careful, whites are unfused and will cause wiring and switch damage if you short them to ground.

The wiper motor actually *is* polarity sensitive as it has a permanant magnet stator, but if you think about it with the blades stopped at one extremity of their travel it doesn't matter which way the motor runs the blades are going to travel in the same direction :o)

It is the starter motor that is not polarity sensitive as it has a wound stator.

If you have the later heater motor with different coloured wires (permanant magnet stator) you usually have to reverse the connection to the wiring. Try them both ways and see which gives more blow.
Paul Hunt

This thread was discussed between 18/01/2005 and 20/01/2005

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