MG Car Repair: Overdrive, overdrive switch, wiring loom

Question
Hi Barrie,
just a quick reintroduction, I own a 1972 MGB model G23N.

I tracked my overdrive switch (on the steering column) to the yellow wire at the harness connections.  However, the wireing diagram says it should connect to either another yellow or yellow/red wire, neither of which I can find.  Should I assume that perhaps this car is not equipted with an overdrive unit, even though the switch is present, and live?

Thanks
Joe

Answer

MGB Overdrive unit   Hi Joe.  

Unfortunately, overdrive was an optional extra on 1972-3 MGBs.  MG only made one wiring loom for 1972-73 USA cars, so you could well have the switch and the wiring but no overdrive.  

The easiest way to tell if you have one is to look underneath the car.  

An MGB without overdrive has a long gearbox bolted directly to the engine.  If you have an overdrive unit then the gearbox is shorter and there is a second big lump of alloy (the overdrive) bolted to the back of it.  I attach a diagram of an early MGB overdrive.  Yours may differ in detail, but the overall shape will be the same.