Triumph Driver wrote:
> OK, I have always heard, and always repeated, that a J or D type overdrive
> should NEVER EVER be driven engaged in reverse....
>
> NowI aint so sure. I hope someone can show me were I am wrong here, but I
> see no reason for the OD to be damaged if the solenoid is somehow left
> engaged and the car driven in reverse.
In the very back of the overdrive case is a one-way-clutch that
is there to take up the drive when shifting.
It is called a Sprague clutch I beleive, and it is this unit
that doesn't like to be reversed.
> Now, pressure is the key. It takes a bit of pressure for the system to
> work. Remember that 25 MPH figure above? It turns out (with a 3.89:1 rear
> end) that to generate the required pressure (something over 350 lbs/sq in)
> the output shaft of the gearbox (and so the input of the OD) must be moving
> about 1500 RPM.
...if your unit needs a sustained RPM to maintain pressure, then
you likely have an internal pressure leak somewhere.
The pump in the unit is a constant-displacement type, which means it
displaces the same amount of oil with each turn, at any RPM.
If you have no leaks, the overdrive should build up shifting pressure
at almost any RPM, it just takes a lot longer. Once it has enough pressure
to be engaged, it should stay engaged for a long time, even if you
stop the car.
> Drive shaft RPM is solely
> dependent on the speed of the vehicle (for any given drive axle gear ratio),
> not on engine RPM or gear selection.
Actually all of those terms are related. Engine RPM / gear selection is
the drive shaft RPM which when divided by axle ratio is the wheel speed.
> So to develop enough pump pressure to engage the OD I
> would have to pull about 6000 RPM in reverse.
>
> So even if your OD is stuck engaged, how often would you pull the 6000
> engine RPM needed in reverse to make enough pressure to activate the cone
> clutch?
If your overdrive hydraulics are not leaking, the car will
sit in overdrive for a while even when not moving, as long as
the solenoid stays powered. So you have plenty of time to shift
into reverse and attempt to back away.
> Or have I got this all wrong?
It would appear you are making excellent observations but
on a faulty system. :<
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
Thirty Helens agree.
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