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Confused with my Overdrive wiring.. Help please


Luke Skywalker
Hi all.
I can't seem to get a direct answer for this and I am getting confused with differant schematics I look at. I am planning on dropping the engine and newly added OD transmission into the '71 tomorrow. The OD is from a '77 and I am going to use the switch on the gearshift, not the stalk switch. The harness that came with the unit is what has me confused. The PO of the unit just snipped off the wiring going to the transmission and to the on/off switch on the shifter but the rest is there. What I have is, the two green that I know go to the reverse switch.. that isn't an issue, but on the OD switch on the unit, there is one white/brown wire and on the other tab, a yellow wire joined by a yellow/purple wire on the other tab. Two wires go to the on/off switch, yellow and yellow/purple, and a yellow/purple goes to the solinoid. My question is, what is the power source from the cluster near the fusebox? I have both an open yellow and an open white/brown wire there. Do I plug in the yellow wire AND the white/brown wire there, or just one of them? If I make my own harness, what is the simplest route to go so I can make this work? I am at work so I don't know if I can attach the pictures .





Great Pumpkin
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Re: Confused with my Overdrive wiring.. Help plea

I don;t remember a white wire going to the transmission, but I could be wrong. I don't know if you have this chart, but it can be very helpful identifying MGB wiring and what it's for. Here's the link. Hope this helps. PJ

The MG & MGB Experience: Library: Lucas Wiring Colour Guide


Great Pumpkin
Country flag
Re: Confused with my Overdrive wiring.. Help plea

IIRC, the '71 should have a white and a yellow to the box solenoid IN the harness. The schematics for the various years here is very good, too.


Luke Skywalker
Re: Confused with my Overdrive wiring.. Help plea

Looking at schematic #23 and #32 from there then, it looks exact to what I have harness wise. The OD did come from a '77 as I mentioned, so the yellow wire that I thought came from the engine bay as well as the two greens and the white/brown really goes to a spark control system and I don't require it. So I plug the white/brown into the open wire by the fuse box and get rid of the extra yellow? Then.. it appears the power goes to the interlock switch FIRST then to the On/off switch in the gearshift which completes the cycle to the solenoid?

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