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MG MGB Technical - What is the switch on the OD trans?

The attached photo is of an LH black-plate overdrive transmission that I am planning to put into my 1966 GT. The only thing that still puzzles me is the switch on the left side,facing front, which is marked with a white arrow in the photo. What is it for? I haven't seen it referred to in wiring diagrams for OD.

Also, I am assuming that the similar-looking switch on the left side, above the solenoid, is the 3/4 switch. Is that right?

Thanks for your help!

Mike

M.E. Whalen

Reverse light. Left switch is indeed the isolator. Some also have a third, smaller, switch which IIRC is connected in with the seat belt interlock.
Rob Edwards

I would recomend a new inhibit switch while its so easy to fit, they can wear and cause o/d drop out and are a pain to change in situ.
Stan Best

Not so sure about changing the switch as a matter of course, the wear can be as much on the lever that operates the plunger of the switch as the plunger itself. Usually there are one or two fibre washers under the switch which sets the depth of engagement, removing one of these is usually all that is needed to correct any intermittent operation.

Seat-belt interlock switch was only used for part of 74 and 75, but there was another small switch used for the Transmission Controlled Spark Advance system for a short period in 76/77 before American cars changed to having OD in 4th gear only.

There were two types of LH gearbox and OD - a top-fill with dipstick and black Laycock OD label on chrome bumper cars, and a side-fill blue label used on rubber bumper. The former has a 1280tpm speedo drive and the latter a 1000tpm. A 66 would originally have a 1040 Smiths (possibly a 1060 Jaeger) speedo making the blue label OD a better match.
PaulH Solihull

This thread was discussed between 13/12/2010 and 14/12/2010

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