Greg writes
> When reverse is selected, it's important that the overdrive does
> not try to engage. Doing so would make the the annulus try to overrun
> the sprag clutch, but this time in the reverse direction. No can do.
> When the planetary gear set and the sprag clutch fight, the sprag clutch
> wins. So the cone clutch which controls the planetary gear set is forced
> to spin against the OD case, which is of course stationary. The faster
> you go in reverse, the harder the hydraulic pressure forces the spinning
> cone clutch against the stationary OD case.
Unfortunatley this is not typically the case. The instead of the cone
clutch sliping the sprag (unidirectional clutch) usually is destroyed.
I have found this to be the most common failure in the A type OD.
What I haven't figured out yet is what causes the OD not to disengage in
the first place. The oil pressure typically falls off fairly quickly
when the pump is not operating so as soon as you stop to shift into
reverse the OD should disengage. Assuming that the valve is stuck it
would start to reengage as the pressure built up again but unless you
are driving a backwards race or backing a long distance at higher than
normal speed I suspect that enough oil pressure can be generated.
In my case the cone clutch failed to disengage even though the pressure
had fallen off. The only explaination I have come up with is that due
to years of wear the break ring had small groves worn into it, and the
bearing for the cone clutch was also worn. I suspect that the cone
clutch was able to catch and twist enough to get stuck on these ridges,
thus never disengageing.
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Kevin Spooner
The Sports Works of Greeley
Specializing in Triumphs and British Sports Cars
britcars@hpfckjs.fc.hp.com
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