Shifting gears

Gear selection in a Tesla drive unit is done electronically. That's why so many electric cars have gear buttons instead of gear levers -- there's nothing physical that needs to move. But, in trying to keep the original BMW aesthetic, I want to use the stock automatic transmission shifter to send signals to the Tesla drive unit.

To that end, I've been playing with CAD and 3D printing in an effort to adapt the factory automatic trans shifter to my needs. The trouble is, without the detents in the auto trans itself, the original shifter was a floppy mess. I also need a way to convert the mechanical motion of the shifter to electrical signals I can send to the Drive Unit Controller of the Tesla motor.

The original US auto shifter had gear indicator lights on the steering column so I figured I could repurpose the rotary switch bolted to the side of the old auto trans. If they could make it illuminate light bulbs then I could use it to send 12v signals to the Tesla. But I also needed detents to hold the handle in place so I devised a rotary unit comprised of three sandwiched plates with a round disc in between that had dimples on its edge. Then, a ball bearing and spring could push against the dimples, providing distinct stop points. Skateboard bearings held the D-shaft going through the disc. I'd then have to devise a linkage and lever system to make the shifter spin the shaft, similar to how the original gear selector shaft in the side of the auto trans operated.

After numerous reprints to get the dimple depth right, it actually worked quite well as far as the detents were concerned. But when I tried to use it with the factory rotary light switch I realized that I didn't really understand how it worked. There are about 12 wires coming off that switch and a few hours with a multi-meter proved to me that there were no actual contacts in that switch. Instead, it appears to be a variable resistor (a potentiometer) with dead spots for the various gear positions? I eventually gave up and moved to plan B -- a linear, rather than rotary, system.

This one also took many reprints to get right. It operates somewhat like a player piano. There are 5 lever operated micro switches, one for each gear position (I will use "2" for sport mode but have no need for the 6th "1" position). "Domes" on a sliding plate, supported by 5 skateboard bearings, activate one switch at a time. the plate has dimples on the bottom edge, with the same ball bearing and spring mech from from rotary prototype provide detents. Linkage from the shifter arm attaches to the plate, making it slide in the housing, which is attached to the trans tunnel.

It took a while to figure out how far down the shift arm I needed to attach the linkage in order to get the proper "throw" to match the movement of the plate -- but it works perfectly. For durability, I milled the slider out of aluminum and cut an aluminum plate for the back but ABS works well for the rest of it.