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MG MGB Technical - Overdrive manual control switch wires

I will be swapping in an OD trannny in my '78 B roadster. I'm presently restoring the interior and have a wire harness that contains an extra lead and connector. This is on the subharness which connects to the driver seat belt warning switch.

The Bently manual wiring diagram shows this extra wire connected to the "overdrive manual control switch* (*if fitted)" The harness lead contains a ground (black) wire, and a brown/purple stripe wire. These are terminated in a double male pin connector. It appears that this has the correct length to route up into the gear selector area.

The other end of the brown/purple wire terminates as a dead-end in the engine compartment. This matches the Bently manual which shows it connected to nothing at that end. I discovered this end of the wire previously while re-wrapping the harness in the engine bay. It was burried inside the old wrapping, simply cut off. But I let it stick out after re-wrapping.

Is this brown/purple wire intended to be the +12v supply for the OD switch in the gear shift? If so, what would it normally be connected to in the engine bay? Should it be connected to a brown +12v lead through a fuse? I've searched the achives and haven't found a reference to this wire. Does it even ever get used, or is it redundant?

Thanks,
Gerald
Gerald O'

Go to http://www.advanceautowire.com and make an enlarged copy of the wiring diagram for yourself.
Make an enlarged copy of the section of the diagram that pertains to the overdrive.

The brown/purple wire is not used. It is leftover from the seatbelt interlock system.

Look at the wiring diagram, you will notice the wiring coming from the inertia switch that goes to the OD.
Kimberly

As Kimberley says the brown/purple and black were used in the various seat-belt warning systems. On a 78 North American power for the OD, fuel pump and TCSA solenoid comes on a white from the ignition switch, through the inertia switch, to a 4-way bullet connector. From here one white goes to the fuel pump and another (possibly a white/brown) goes to the gearbox 4th gear switch (posibly changing to a yellow/purple en route). From the other side of the gearbox switch comes a yellow to another 4-way bullet connector. That has one output going to the TCSA solenoid in the engine compartment on a yellow/black, and another yellow through the manual OD switch on the gear-lever to the OD solenoid on a yellow/purple.

The diagrams show a microswitch between the 4th gear switch and the solenoid but this seems to be a mistake, combining two different ways of operating the TCSA at different times into a single diagram.
Paul Hunt 2

It seems then that the brown/purple wire with its companion ground lead could make for a convenient power source for accessories in the console area without having to run new wires. Maybe a light fitted into the compartment under the armrest, or an accessory 12v power outlet inside there.
Thanks.
Gerald O'

This thread was discussed between 27/08/2006 and 28/08/2006

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