[Mgs] MGB O/D

[Paul Hunt]

It's a bit like a 2-speed auto gearbox and is a bolt-on addition to the 
manual gearbox.  The electrical solenoid closes a valve, which allows 
hydraulic pressure, derived from a pump on the overdrive unit input shaft, 
to operate some pistons, which moves a friction clutch from one position to 
another.  In the released position the clutch locks the input shaft to the 
output shaft so you get direct drive.  In the operated position the clutch 
locks against the casing which allows the input shaft to drive sun and 
planet gears (a bit like a diff which with one wheel locked turns the other 
at twice the speed of the prop-shaft) which 'gear up' the input shaft to 
give a faster rotation of the output shaft by about 25% (22.3 mph per 1000 
rpm in overdrive top gear as opposed to 17.9 mph per 1000 rpm in straight 
top gear).  In the released position i.e. OD not engaged forward drive is 
actually via a one-way clutch which positively locks the input and output 
shafts together, so there is no chance of the friction clutch slipping in 
low gears which is when the torque on the OD is at its greatest.  With OD 
engaged the one-way clutch is free-wheeling and forward motion relies solely 
on the friction clutch to give forward motion.  This is why OD is only 
available in higher gears - 3rd and 4th on most 4-cylinder cars and 4th gear 
only on the V8 (the restriction to 4th gear only on late North American 
models was for another reason altogether), i.e. when the torque in the OD is 
reduced and hydraulic pressure alone is sufficient to prevent the clutch 
slipping.  With OD disengaged and reverse selected the one-way clutch is 
also free-wheeling, but the mechanical forces are such that the friction 
clutch surfaces tend to be pressed harder together which resists slipping, 
even though you are in a low gear.  If the overdrive should happen to be 
engaged while you are reversing, then the action of the friction clutch and 
sun-gears are trying to turn the output shaft faster than the input shaft, 
but the one-way clutch is trying to resist it, and you get the 'irresistible 
force and immovable object' scenario.  Except that the friction clutch is 
stronger than the one-way clutch and it is the latter that gets damaged, 
which is serious.  For this reason there is a lock-out switch on the manual 
gearbox linkage which prevents OD being engaged in anything other than 3rd 
or 4th.  This switch should *never* be disabled or bypassed by the user, as 
is done by some people to get OD in 1st and 2nd as well for some reason. 
*You* may remember to always switch OD off with the manual switch when 
reversing, but someone else may not, and they may destroy your OD.  Also 
using OD in lower gear causes glazing of the clutch linings, which causes 
slippage.

PaulH.

----- Original Message ----- 
> My question though may be simple but how does it actually work?