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MG MGB Technical - Overdrive Wiring

I have just put the engine and trans back in my 73 B/GT after being out since Nov. and went to hook up the wiring for the overdrive and reverse light. and found it to be not what I had read in the manuals.
I have a group of three wires which come from the main harness in the engine compartment (2) green/white striped and (1) solid green. I had labled the to two green/white striped (Front and Back) meaning they came off the front and back spade connectors on the right hand side reverse switch. Then I have a single Red wire which comes from my stalk lever and ends below shift lever it has a lead which then comes off the spade connector and drops to where it should be connecting to the overdrive solinoid (but was broken off during motor extraction). I'm thinking that the Solid green goes to the limiter spade as does the red.
Any thoughts on this are appreciated ....Tom
Thomas Koch

Hi Tom.

There is a common fault on cars with the overdrive switch on the gearknob.
The wire chafes in the area of the stick and, since the circuit is unfused, this causes the wiring to the OD switch and the reverse light wires to turn into a melted mess.
I suspect that this is what has happened to your car, and the wiring has been replaced with non-standard colours.
If there isn't a fuse in the overdrive feed, I suggest you fit one.

HTH.. Don
Don

Thomas,

It so happens I just dealt with this yesterday afternoon. Your '73 B does not have the switch on the gear knob, but fusing it is a very good idea anyway.

The color codes you mention are not correct. There is a yellow lead that comes from the stalk which comes out in the maze of wires in the back right corner of the engine bay. Through a single bullet connector, it should connect to another yellow lead which goes to the transmission in a sub-harness along with the reverse light switch leads.
Some P.O. may have replaced it with a red lead. As long as it connects to the yellow from the stalk, you know it's the right one. It goes to the 3rd-4th gear limiter switch on the left side of the transmission. The other spade connector on the limiter switch should have a yellow/purple wire which connects to the solonoid pig-tail. That's it. No green wires.

There are two green leads to one spade connector which goes on the reverse light switch. the other spade on the reverse light switch connects to a green/brown wire. The two light-green/white wires you mention are for the seatbelt interlock system which I've never seen in functional condition on any of these cars. But then, your color codes maybe incorrect in this regard also. There does need to be a solid green to one side of the reverse switch. If there is no green/brown, which ever lead you use should connect in that aforementioned maze through a bullet connector to a green/brown wire. You'll notice that one of the wires on your reverse lamp plugs (in the boot) is green/brown.

' Hope this helps. The Haynes wiring diagram ('73-74 North American models) is correct regarding overdrive and reverse light switch wiring. These diagrams are hellish to read; I can only suggest xeroxing them enlarged as much as you can. (Or perhaps buy one of those color-coded wiring diagrams advertised on Ebay.)
Then with your enlarged diagrahm from Haynes, you can trace the wires you want with a colored pencil.

I got mine working yesterday. That's the good news. Good luck with yours.

Allen
Allen Bachelder

Thanks Allen....I spent some time tracking the wires tonight and found that the wires I labled when pulling the motor..going to the reverse switch are green to one spade and green brown to second spade. Then there is a green/yellow striped which starts with a bullet connection off of a yellow in mass of wires next to fuse panel. The wire (red) which comes from the dash..I think the stalk... comes down under consol to the 3/4 limiter switch spade...it also has a pigtail which is sodidered to it which went to Solonoid pigtail. I think the above mentioned green/ yellow should now attach to he enpty spade on the 3/4 limiter.
Does this sound reasonable and are the two spades on the limiter interchangable....?....Thanks for any input....Tom
Thomas Koch

"These diagrams are hellish to read; I can only suggest xeroxing them enlarged as much as you can. (Or perhaps buy one of those color-coded wiring diagrams advertised on Ebay.)"

For an easier to read set of 11x17 color coded diagrams for the MGB, check http://www.advanceautowire.com/mgb.pdf

These are not just color coded, but also redrawn to make them much easier to read than the originals. And they are free! No need to buy anything.
Dan Masters

Thomas
I rewired my O/D by putting in a fused relay. The white/brown (77bgt wire colors) wire goes to one side of the relay coil. The yellow wire connects to the other side of the relay coil. The yellow wire runs down to the O/D switch and the 3/4 gear switch. The yellow/purple wire from the 3/4 gear switch is connected to earth. This means that the switches activate the relay. The worst that can happen with chafing is that the O/D will be engaged anytime 3/4 is selected.
The white/brown wire is also connected to the fuse on the relay. The fuse is in line with the switched contact in the relay. A separate wire is run from the switched contact to the O/D solenoid. This wire is much less likely to chafe because it is not routed to the O/D switch. It would just blow the fuse if it did chafe so it is safe.
This solved 5 years of erratic O/D operation.

Send me an email if you would like a wring diagram drawn up.

Cheers
Ray Taylor 77BGT
RM Taylor

Thanks ......for your comments and the great color diagrams. I solved my problem ...after tracing the Red wire I had on my limiter switch I found that it did'nt go to the stalk as I thought..it goes to a little light on the consol intalled by the PO which lights up and says OVERDRIVE when it is engaged...its wired to the Solinoid pigtail and covered with a thick length of clear surgical tubing as is all of the rest of my overdrive wiring to prevent chaffing. It really threw me off. I will now install an inline fuse as many of you have suggested. Thanks for your help....Tom
Thomas Koch

A big thanks to Dan for the wiring diagrams!

My car also has a PO-added Overdrive warning light. But my PO knew something, Thomas, that yours didn't. If you want an indicator light, all you do is splice into the yellow wire from the stalk under the dash, and run the additional wire from the yellow to the light and from there to earth. If you don't want to cut into the yellow lead (and I don't) then replace the bullet connector in the "maze" (between the stalk yellow and the overdrive yellow (and that's a good place for your fuse!) with a double bullet connector, giving you a place to plug in an additional wire to run to the indicator and on to earth. That extra red wire all the way to the transmision is like running an extension cord from the second-floor bedroom to the basement when there's already an outlet behind the bed.

Allen
Allen

Forgot to add, Ray's fused relay is really the best way to go. I have done it and will do it to this car when time allows. One of the advantages of relays is the load on the switches is reduced to almost nil, making them last a LOT longer. And if you've replaced one of the stalks recently, you know how much money that saves! But if you don't do a relay, at least put the fuse on the yellow.

Allen
Allen

This thread was discussed between 17/06/2005 and 23/06/2005

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