[TR] Overdrive Trannies

[Randall]

In the rear of the overdrive, there is a roller clutch (aka sprag clutch,
aka overrun clutch).  By means of ramps & rollers, it keeps the OD input
shaft from turning farther CCW than the output shaft.  

Normally, it helps out the direct drive clutch when making hard launches in
1st gear (when the friction clutch might slip otherwise) and while the
friction clutch carrier is moving between OD and direct drive positions.

The problem arises when you try to back up with the OD engaged.  In that
case, the OD gear train & OD clutch fight with the sprag clutch; a classic
case of irresistible force meeting an immovable object.  Generally the sprag
clutch loses, breaking the outer race and the OD housing around it.

Actually, if you start out stopped with the OD engaged, the problem is
pretty obvious.  Feels just like you are trying to back over a curb.  As
long as you don't give it enough throttle to get over the 'curb', it won't
hurt anything.

The danger zone IMO is when the OD pressure has dropped, allowing the
springs to shift it back to direct drive; but the hydraulics, etc are still
poised for OD operation.  In that case, the car is already moving when the
OD clutch engages, and the inertia of the car will break the sprag clutch
before you can do anything about it.

Pretty unlikely unless something is wrong.  If you do notice that the
overdrive is engaging when it shouldn't; my suggestion is to either call for
a flatbed, or only back up very slowly (and without the main clutch fully
engaged).  When the dashpot hung up on my Stag (causing it to go into OD
regardless of solenoid power), I was able to limp home that way.  Of course
if the OD does engage while you are backing up, your only options are to
call that flatbed or wait for the pressure to drop so you can back up a few
more feet.  But I only had to back out of the parking place at work, which
wasn't enough for the OD to engage.

BTW, it is only the motion that matters, not the position of the gearshift.
If your OD is stuck in overdrive; be sure to tell the tow truck driver NOT
to try to drag the car backwards with the rear wheels on the ground!

-- Randall
 
> With the notes passing about overdrives, I was always warned that if the
> lockouts failed and you put it in reverse you could damage the system.
> 
> Just curious, what happens and I presume you can tell from trying to
> drive the vehicle.
>