[TR] Overdrive Trannies

[Randall]

There is a one-way roller clutch (aka sprag clutch, aka overrun clutch) in the back of the unit. Normally it handles transmission of power while the OD clutch is moving between direct and OD positions.

So when turning forward, it keeps the input shaft from turning farther than the output.
Unfortunately, if the OD is being turned the other direction (reverse), it keeps the output from turning farther than the input.

 So, if the overdrive clutch is engaged (or stuck), you wind up with the planetary gears fighting the roller clutch. Generally, the roller clutch loses, the force breaks the outer ring which in turn breaks the OD tail casting.

If the OD happens to be engaged when you are stopped and you begin to back up, the effect is pretty obvious IMO. Feels exactly like the rear wheels are up against a curb. Jumping the "curb" is what breaks the OD. I've been there a couple of times for various reasons and found it very clear that there was a problem. Never broke a clutch.

The danger zone IMO is when the OD has lost pressure for whatever reason (mine drops pressure in just a few minutes of no motion), but the solenoid is powered or stuck. Then you could be moving when the pump builds pressure and the OD would have to absorb the force of suddenly stopping the car.

Of course, only the A type stores pressure; so this is even more of a problem with a J or D type.

If you know you have a problem but don't want to wait for a flatbed, my suggestion is to only back up very slowly and without fully engaging the main clutch.  I drove that way for a couple of days when the J type in Stag#1 malfunctioned and managed to not break anything. Of course, if the OD does grab, you have to either wait for it to drop pressure again or call for that flatbed.

Hopefully this won't confuse the issue too much, but the problem can happen even just rolling backwards with the gearbox still in a forward gear. It makes sense, but I had not thought of that aspect until it happened to me a few years back. I had just come to a stop at the top of a fairly steep hill, with the lever still in 4th and the OD switch on. There was a big truck swinging wide to make the corner, so I deliberately let the car roll back a bit to give him more room. Rather surprised to have the OD stop me from rolling backwards, but no harm done. Again, it might have been different if the OD had lost pressure and I let the car build up speed going backwards.

Does that answer the question?
-- Randall
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