My 2003 u140f 4-speed transmission contains 2 one-way clutches, neither of which are supposed to be externally controllable, and yet by using my O/D button (which is supposed to simply either allow 4th gear or disallow it), I can engage/disengage engine braking,
and I am puzzled as to how this is possible.
This transmission DOES have a problem…4th gear (O/D gear) does not ever engage. Instead, when the O/D button is in the “enable 4th gear” state, the transmission stays in 3rd gear, but engine braking is completely disengaged. Pressing the O/D button into the “disable 4th gear” state completely restores engine braking.
I’m guessing that somehow the transmission’s inability to achieve 4th gear is somehow allowing the O/D button to achieve the seemingly unachievable ability to turn engine braking off or on at will, but I have no idea how it is doing that.
Automatic transmissions need overrunning one way clutches because at the moment the transmission shifts from one gear to another the output shaft would briefly be driven at two different speeds. That can only be done if the shaft is broken. You can’t drive one end of a shaft at one speed and the other end at another speed. The overrunning clutch allows the lower gear to be briefly overrun until it is disengaged.
The original hydramatic transmissions did not have overrunning clutches. That meant that the engine speed would necessarily flare when the transmission shifted by first disengaging from one gear then engaging the next one. That’s why people who learned to drive on hydramatics learned to lift their foot momentarily to effectively force the upshift and also simultaneously eliminate the engine speed flare.