What the hell kind of tranny is this and how do I fix it?

It sounds like the switch was added so the OD can be engaged in all forward gears ay any speed by creating a ground. With the switch off OD should engage in second gear is you let off the gas at 27-30mph. Second over is a handy gear around town. First over might work in traffic or hill to steep If you keep the switch hook a light to it to assist your memory.

 

Sounds like your governor isn't working... If everything is connected right, it should automatically drop out of OD below about 20 mph, which will allow you to get into reverse.

 

When you get it sorted, you can drive it like a 5 speed. 1st, 2nd, 2nd OD, 3rd, then 3rd OD. Or you can just use the OD for highway cruising and take it out of the equation for around town Driving. One of the things you need to check on is there should be a wire to the coil that momentarily grounds it during the OD upshift. This allows some slack in the drive line that eases the planetary gear engagement.

Note your car could still have fluid drive. This term is usually applied by folks who are not MOPAR people to the transmission. Which it is not. Fluid drives have the flywheel and clutch attached to an enclosed hydraulic power transfer unit. If so you should be able to let out the clutch in any gear with your foot on the brake, then just drive away by releasing the brake.

Don't know what you paid for the car but working Mopar r 10 ODs consistently bring $500.00 to 1200 when sold individually. A working sorted one is worth more. So don;t just give up on it. The OD is a 70% deal so it really adds to relaxed top gear highway driving.

You have gotten some good advice here, take your time get it sorted, and enjoy your bonus.

By the way, these old transmissions do not like new hypoid lubricants the anti shear additives are not kind to brass and bronze which makes up most of the synchronizer hard ware. Should use GL4 or older. Synthetics seem to be OK but like to leak more than conventional lubes.

 

Mopar man, here is a link to the best article I have ever read on the Borg Warner OD. I wish I'd found this years ago. http://www.fordification.com/tech/overdrive.htm