Free wheeling & reverse

[Guest]

Today, I swapped out the T-96OD tranny that came with my 63 Lark 6 with another one I have and both of them don't allow me to roll backwards in a forward gear with the engine off. There's no lockout switch involved here. Is this normal? My other cars have to be in reverse if I have the cable in, to not roll back. ??
I've not attained OD yet - still test-driving.
Thanks!

Western Washington, USA

[gordr]

Hi Tom.

I think it has everything to do with the amount of wear on the transmission bearings and the parts of the sprag (over-running clutch).
Think of the sprag as a toothless ratchet; it permits free movement one way, and locks up the other way.

The center hub of the sprag, with its inclined ramps, is part of the transmission's mainshaft. The outer drum, which the rollers bear against, is part of the transmission's output shaft. In a perfect world, with the rollers on the low point of each ramp, there should be JUST enough clearance that the outer drum could turn freely either way without dragging on the rollers and causing them to climb the ramps and lock up. But a little slack on the bearings, or a weak spring on the roller cage, and maybe ONE roller touches, and as soon as it does, it drags the cage with it, and hence all the other rollers, and it locks up.

The lockout switch has nothing to do with it. The sprag is entirely mechanical in its operation. When overdrive is locked out, a dog clutch effectively bridges across the sprag, and renders it inactive.

If you can make the overdrive unit shift properly from direct to OD and back again, I wouldn't worry too much about the ability/inability of the sprag to restrain the car from rolling back on a grade when parked. You should not rely on it for parking restraint; either pull the lockout cable, or set the parking brake.

Gord Richmond, within Weasel range of the Alberta Badlands

[Guest]

Gord, Something tells me you've had one or two of these things apart.
Thanks for the info, bro!