[Healeys] Overdrive problem #1

[warthodson at aol.com]

A friend's JB7 overdrive has stopped releasing (stopped going out of
overdrive). Electrically, everything is operating correctly. On rare occasions
it will release but 90% of the time it stays in overdrive. He knows to not
back up with it stuck in overdrive. When he stops the engine, we rock the car
backward & forward in gear until the overdrive releases with a bang. What is
the likely cause of this & are there any tests that we can perform of fixes we
can do with the overdrive/transmission in the car?
Gary Hodson