can't engage 2nd, 4th, or Reverse
 

I can't engage any of the gears on the lower plane of my 915 gearbox. I noticed the 1st to 2nd shift felt rubbier last time I drove it, so I think it's probably a bushing or linkage issue. I can move the gearshift into the 2nd, 4th, and reverse position, but when I let out the clutch, it's as if I'm still in neutral.

Where should I start?

I would start at the linkage under the access panel on the rear tunnel- maybe broken or loose?

Pull up the carpet on the hump between the rear seats. You'll see a hatch. Unscrew the screws holding it in place and take a look at the coupler. My guess is either that the bushings have disintegrated, or they are very worn.

That happened to me. The only think that I found was the linkage bushing bracket losed downside the shifter assembly. This is the bracket that use two identical small allen or hex bolts. I put some blue thread lock and tighten again the bolts and this resolve the problem.

Is that not solve the problem that you have. Maybe your 5th gear came out of his position in the transmission. This happend if you quickly take a gear from the foward position to reverse position. This happened to me 3 times.

Good luck!!

Happened to me very suddenly. The bushing in the coupler were so trashed, they actually fell out (what was left of them)......Took all of 15 minutes to pop the new ones in and put the cover back on.....

I finally got a chance to dig into this problem. The rear coupler bushings are fine. For some reason, when I pull back into 2nd or 4th, the gearshift pops out of the ball cup bushing. The bushing appears to be fine, if a little too greasy.
Should I just replace the bushing, or does this sound like a bigger problem?

did you try loosening your shift coupler and starting from scratch to reset it? you will need a couple of about 12 mm wrenches. put the shifter lever into neutral. now, loosen the coupler with wrenches and physically rotate the coupler in the passenger side direction while simultaneously pulling the neutral shift lever all the way to full left (between 1st and 2nd). in other words, you want the coupler orientation and the gear shift lever positions 'going away from each other' to the extent possible. tighten coupler and try to shift through the pattern. if it doesn't feel right, repeat the procedure with the shifter lever up in the 1st gear position, but still 'pulling the lever to the left', coupler to the right. re-tighten coupler and try again. it's an 'art' to properly adjusting a coupler..the 'sweet spot' will lie with the shifter lever being placed somewhere between these 'neutral' left and '1st gear' left positions. hope this helps.
ryan

Replace the bushing, same thing was happening to me last week. In fact replace them all, shift rod doughnut bushing, and the coupler bushings. My 915 shifts like butter now, I replaced them all, and then followed the alignment procedure in the tech section, easy repair.

I replaced all of the bushings about 3 years ago. The shift coupler bushings are in good shape. I'm just going to order some more bushings and replace them all. I was only concerned because the ball cup bushing "looks" OK, but I haven't physically removed yet. Thanks for all the help.