I've owned cars with the BW overdrive units off and on. I have always read that you have to take the od out before backing up. What damage can be caused when leaving it in and then backing up?
Actually in a stock unit there is a rod internally on the reverse linkage that lock up the planetary unit so that you can backup in reverse. When you take it out of reverse, the rod moves in and unlocks the planetary. You don't have to do anything other than put the trans in reverse. When you take it out of overdrive by pulling the cable out, what you are doing is locking the planetary unit in a similar manner. The sun gear is slid back into internal teeth in the planet carrier effectively locking the gears together.
If it's still in factory trim, the transmission takes care of that internally when you shift into reverse. So you don't need to manually disable the OD when backing up. If it's a conversion onto a trans not normally fitted with an OD, then you should disable it by pulling out the overdrive cable. It's also a good idea to shift into reverse when parking so the car can't coast away.
Edit. Everyone types faster than me. Sorry. Most factory transmission/overdrive combinations had a mechanical link inside. Shifting to reverse disengaged the o/d. But most answers you’ll need are here. The original factory manual. http://www.oldwillysforum.com/forum/TechData/BWOverdriveManual.pdf
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