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MG MGB Technical - Overdrive Wiring?

Confused of Holmfirth here.

Which wires go where on a 4 syncro OD box.

I'm fitting it to the 66BGT I've got (or should it was being fitted by previous owner who got stalled with the project and I took it on instead).

So it all came as bags of bits and pieces so I have no reference as to how it was fitted initially.

I've got dyslexic reading the wiring diagram and can't fathom it out, so if anyone has got a photo of there set up or can simply describe what goes where then I'll be a happy chappy.

Oh and why have I got three wires in one section where there can only be two terminal that it attaches to.

The loom in the car is a 71 just to confuse things further.

Thanks

Andrew

Andrew McGee

Andrew,

From memory, on my 69 CKD local production. Happy for anyone to correct me if needed.

A green/yellow wire with end spade connects to the OD cutout switch on the top right hand side of the extension housing. Other end to the yellow from the dash mounted OD switch in that amalgamation of wire ends in the RHS rear corner of the engine bay.

A two ended extension piece (attached to that) has a spade which connects to the remaining terminal on the cutout switch and a bullet end that connects to the internal solenoid wire that emerges from under the OD itself.

A green wire spade end connects to the reverse light switch on the left hand side top of the extension housing and to a green feeding those lights in that amalgamation of ends in the engine bay.

With a 71 loom and a 66 box (assuming of course that nothing has been substituted) I suspect you may not have the reverse lights so not need the green.

The mounting bracket (holding your wire set together)I attached under one of the extension mounting bolts, RHS I think, but could be positioned where best fits on your unit.

Hope both that I am recalling correctly and that this helps.

Regards

Roger

R Taylor

Andrew, just had a 'thought'. There may be two spades on that green to the reverse light switch - one power in and the other power back? In which case there are two connections in the amalgamation of wires for that green wire set run. Darned if I can 'see it' at the moment.

There may be additional elements to the 71 loom I don't have on mine. I'm sure others will clarify for you.

Regards
Roger
R Taylor

Andrew. Not sure what the photo was as I had no reference. Basic wiring of the overdrive is a power wire to a switch, from the switch to the overdrive inhibitor switch on the transmission, thence to the overdrive solenoid. Most of us add an in-line fuse between the power source and the on-off switch (something the factory did not do) as a precaution. With my two 79s and my daughter's 77, I simply spliced into the white wire circuit and ran a wire to the in-line fuse, then to a dash switch, then to the overdrive inhibitor switch, then to the overdrive solenoid.

I do not know what was the switching mechanism for the Mark I MGBs. My 68 GT had the switch on one of the stalks and my original 79 had the switch on the gearshift knob. But, I prefer a dash switch myself. You might check out, or someone may post, what switch was used with the Mark I series. If you have such a switch, simply run power from the white wire circuit (activated when the ignition switch is on) to the switch, thence to the overdrive inhibitor switch, then to the overdrive solenoid.

Expanding the factory wiring diagram (most copy shops here offer such a service for a nominal fee), then laminating the enlarged diagram, allow you to use a dry erase marker to trace out circuits. For those of us who are "electrically challenged" that makes life much easier.

Les
Les Bengtson

Green is fused ignition, comes from the main harness by the fusebox, and goes to one side of the reverse light switch (right of gearbox). The other side of the reverse light switch has green/brown, the other end of which goes to green/brown in the rear harness, again by the fusebox.

Your overdrive is complicated by being a 66 car, which would have had a D-type overdrive with vacuum switch and relay. But if you are fitting OD for the first time that shouldn't be there.

In which case yellow comes from the manual switch, out of the main harness and to yellow/red in the gearbox harness by the fusebox. That goes to one side of the overdrive switch (left of gearbox). The other side of the switch goes to the solenoid, probably yellow/pink.
PaulH Solihull

Thanks all. I think I've got that bit sorted.

It's a fit of a frankenstein car having a Laycock 4 syncro OD box (with dipstick) and has a loom from a 71 car in it.

Once I'd determined which way the wires went it was just a matter of getting my youngest lad to put the spades over the terminals simply as his hands were much smaller than mine are.... they do come in useful occasionally.

A.
Andrew McGee

Ah, thank you Paul for clearing up my colour blindness. Memory is an interesting thing - as I read your post I could 'see' it again.

Well done Andrew.

Roger
R Taylor

This thread was discussed between 27/02/2013 and 01/03/2013

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