>> I finally got busy and dug out an old copy of the "MGB Driver's Handbook"
>> dated 1967. I knew that I had read somewhere that the clutch is not
>> necessary to allow an O/D shift to take place. I will quote from the
>> "Bible", p. 30
>>
>> "Overdrive (optional)
>>
>> The overdrive can be engaged at any speed within the third or fourth gear
>> range at any throttle opening between light and full throttle. DO NOT
>> depress the clutch pedal to select 'overdrive' or 'normal'."
>>
>> Note that the words DO NOT are printed in bold face type.
>>
>> There is no other explanation as to the reason for forbidding the use of
>> the clutch.
I am not sure if this is really 100% correct, but it seems to me (someone
with a 67 MGB with overdrive and the silly vacuum switch/relay system) that
there is a simple explaination. If you look at a detailed description of
how the overdrive unit works, it is clearly laid out that not only the
solenoid, but oil pressure engage the overdrive system. I will quote from
The Complete Official MGB 1962-1974 (forgive my typing - I am not a trained
professional in that area):
" The overdrive unit comprises a hydraulically controlled epicyclic gear
housed in a casing which is directly attached to an estension at the rear of
the gearbox.
The gearbox third motion shaft is extended and carries at its end the
inner member of a uni-directional clutch. The outer ring of the clutch is
carried in the combined annulus and output shaft.
Also mounted on the third motion shaft are the planet carrier and a
freely rotatble sun wheel, and sliding thereon is a cone clutch member the
inner lining of which engages the outside of the annulus while the outer
lining engages a cast-iron brake ring sandwiched between the main and tail
casings.
The cone clutch is held in contact with the annulus by compression
springs, thus locking the sun wheel to the annulus so that the entire gear
train rotates as a solid unit, giving direct drive. In this condition the
drive is taken through the uni-directional clutch. Reverse torque or
overrun is taken by the planet wheels, which being locked by the sun wheel,
transmit the drive via the planet carrier and third motion shaft.
Hydraulic pressure produced when overdrive is selected forces two
pistons in the unit housing against the cone clutch and overcomes the spring
pressure causing the cone clutch to engage with the stationary brake ring
and bring the sun wheel to rest, allowing the anulus to overrun the
uni-directional clutch and give an increased speed to the output shaft, i.e.
'overdrive'
When changing from overdrive to direct gear, if the accelerator pedal is
released the vacuum switch holds 'overdrive' until the engine takes up the
drive. If the accelerator pedal is not released, when contact between the
cone clutch and brake is broken, the unit still operates momentarily in its
overdrive ratio as engine speed and road speed remain unchanged. When the
load on the engine is released it begins to accelerate, speeding up the
sunwheel from rest until, just at the instant when its speed synchronizes
with the speed of the annulus, the whole unit revolves solidly and the
uni-directional clutch takes up the drive once more. The movement of the
cone clutch is deliberately slowed down so that the uni-directional clutch
is driving before the cone clutch contacts, ensuring a perfectly
self-synchronized change."
Later, in the service diagnosis section the first thing to check if
overdrive does not engage is the oil level in the unit.
It makes sense to me that if you depress the clutch, you could let the gears
freewheel, therefore not making the proper oil pressure to engage overdrive.
I have never seen this happen in real life (I used to depress the clutch
until I read the owner's manual), but it makes sence that the pump must
deliver pressure to engage overdrive, therefore the pump must be turning,
therfore do not depress the clutch. Note that the oil pump is on the third
motion shaft - the one connected to the overdrive, and on to the prop shaft,
so even with the clutch disengaged, it should turn, but maybe not if the
overdrive was in the process of engaging.
Look at that manual; what a bunch of run-on sentences, huh?
Phil Bates
58 MGA
67 MGB
75 Jaguar XJ12C
66 Land Rover
52 MG TD replicar (VW)
86 Peugeot 505 Turbo Gle
86 Honda Accord LX-i
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