Randall Young wrote:
> Tony :
>
> The problem is that, to engage the overdrive, the lever does not open a
> valve, but rather closes it. The valve is actually a bypass that bleeds
> pressure out of the O/D hydraulic system when the O/D is disengaged. If
> the valve does not close fully, then your O/D is operating on less than
> normal pressure, which can lead to slippage under load, especially when the
> oil is hot. Of course, slippage heats the oil, which increases the
> slippage ...
>
> Not a good idea, IMO.
>
> If you really want to reduce the operating pressure, there is an adjustment
> (at least on the early A-types) to do so. It is not dependent (much) on
> oil viscosity and so is temperature independent.
>
> Everything I've read on automatic transmissions (which bear at least some
> resemblance to our O/D boxes <g>) suggests that fast clutch engagement
> reduces wear. (as long as the other components can stand the strain <g>)
>
> Randall
>
Randall, not a flame, but I beg to differ on the function of the lever.
Quoting from the "Service Instruction Manual for the Laycock-de-Normanville
Overdrive
Unit with Electric Control" publication no. 502274, it states:
"The operating valve chamber is an enlargement at the top of a vertical hole in
the
overdrive casing. It is sealed with a screwed plug and contains a spring and
plunger,
which hold a steel ball downwards against a seating, to prevent oil from
circulating
to the operating cylinders. The valve is a hollow spindle sliding in the hole
with a
conical seating for the steel ball in the top end. When the valve is lifted by
the
cam lever it seats against the ball which is then lifted, admitting oil from the
pressure accumulator via the drilled passage to the passage and into the
operating
cylinders, moving the pistons forward to engage the cone clutch.
When the valve is lowered, the ball is allowed to come onto its seating in the
casing,
cutting off the supply of oil from the accumulator.
Further lowering of the valve brings it out of contact with the steel ball,
allowing
the oil from the operating cylinders to return along the passage and down
inside of
the valve, to discharge through the small restrictor jet into the sump of the
overdrive body. The jet is of such a size that the cone clutch takes about
half a
second to move back under the influence of the clutch springs."
The above publication applies to A-type units only.
Using the same logic that the restrictor jet smoothes out the disengagement, I
also
have my valve set to just crack open a few thou by setting it with a dial
indicator
directly on the ball. My OD never slips even lugging under heavy loads in the
lower
gears, and the engagement and disengagement is so smooth that if you didn't
hear the
change in engine RPM, you wouldn't know it even did anything.
I believe the reason for setting the lever as suggested with the alignment hole
is to
facilitate quicker assembly, and a more "idiot-proof" repair at the dealership
level.
On the later J-type units, this adjustment was elminated and the function of the
hydraulic circuits is drastically revised more in line with what you initially
described as the "bleed-through" circuit. The engagement was also significantly
smoother on the J's than on the A's
Again, not a flame, just presenting information from a rare factory publication
which
others may not have access to.
Brian Schlorff '61 TR-4 '64 TR-4 '72 TR-6 '79 Spit
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