An overdrive is an entirely separate gearbox that bolts onto the back of
the main gearbox. It has two ratios, when it is off the ratio is direct
(speed out = speed in), when engaged the speed out is greater than the
speed in (can't remember the ratio of the top of my head!).
It contains an epicyclic gearbox
(http://en.wikipedia.org/wiki/Epicyclic_gearing), which works on the
same basic principle as the 3-gear systems that were used in bikes when
we were kids, before derailleurs took over.
When you flick the switch, electricity is supplied to a solonoid. This
solonoid operated a valve, re-directing oil pressure in the O/D (the O/D
includes a small oil pump that uses oil from the gearbox). The oil
pressure is used to engage a clutch, which changes the gear ratio.
That's it in a nutshell, but I'm sure there's someone here who's had
theirs apart and can tell you much more!
Richard & Sammy ('73 Black Tulip BGT)
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