The OD. I have a 1989 740T M45 trans; OD kept cutting out or wouldn't engage. I hoped it was the relay (cheapest otpion first, right?). Nope, not it. The clods at the local dealership were pushing a new solenoid on me. That didn't feel right. And of course there is the possibility it is just the little sw/ on top of the gear shift.
If none of this has worked there is the oft overlooked and oh so inexpensive 4th gear sw/. My M45 trans has it positioned right-side of the trans aligned about where the shift lever enters the box. Jack the front end up and from underneath you can see a wire, reach your hand up on top of the case and you will find a 22mm nut with a male connector. Just slip an articulated wrench up there and unscrew it. It's like a fridge light switch for 4th gear. Since it is continuously clunked by the shift it wears down. After a point it won't register to the relay it is in 4th anymore so it won't engage the sloenoid. Hop this helps.