My project race car (fiat 850 with a subaru engine and 915 trans)
The trans has always had issues. I never have gotten 5th and reverse to work but last season 1-4 were great. Last fall right before I put it into the shop I was driving it, tried to shift to third and it wouldn't go in.
Yesterday I take it out for the first time to test the brakes. 1st and second work great. Shift to 3rd, let out the clutch and I'm still in second. Now no matter what I do it stays in second. You can move the shaft in and out the full range but it stays in second gear. Any idea what might have broken it how to do more diagnostics?
I'm going to say shift fork - although it doesn't really matter, because you're going to have to take it apart anyway.
Time to put it on the bench and do an autopsy.
Porsche is very fond of having the shift forks clamp to the shift rod rather than having a bolt hole drilled through both the rod and the fork. From what you describe it sounds like a clamp bolt may have come loose, this would let the rod slide through the fork but not move the gears.
Looks like 12 or 12a and also 13 in this pic shamelessly stolen from Automobile Atlanta are the 1/2 shift rod and fork.
Anyone want a light (1450 lb) twin turbo rear engines smile till it tries to kill you machine with just upgraded brakes and new wheels and tires? Otherwise this thing might be shoved into the street and burned
Roll it into the back of the garage, throw a sheet over it and go choke down some brews. That's what I do when E36 M3 goes sideways.
Yup, walk away from it for a while, forget about it till next weekend.
Sometime in the wee hours of the night I will get it out. What's my next step for dissassemby and diagnoses?
Next is the phase where you pull it apart and stare at it, one of the most critical parts of any kind of project.
Knurled wrote:
Next is the phase where you pull it apart and stare at it, one of the most critical parts of any kind of project.
True. 
But it helps to know just WHAT you are staring at and how it's supposed to work. Contrary to popular opinion, manual transmissions (even Porsche boxes) are not full of poisonous snakes and voodoo. They are actually pretty straightforward.
Again drawing on my 901 experience, I'd start by removing the rear housing, this is the one that the shift shaft sticks out of on the 915. The shift shaft has a tab on it which engages with the shift rods inside the transmission. I have seen Mazda transmissions where the tab is disengaged because the shift shaft got flipped 180 degrees accidentally. Look for something silly like this first! If that doesn't seem to be the case, then the main transmission case will require removal. On these boxes, when you remove the main case the shafts etc will all stay in the diff housing (which you won't need to disassemble anyway). That means when you pull the main case all the shafts will stay together, at least on one end. It will look like this, sorta:
Closely examine the shift forks, look for one that is loose.
That black thing at the right side of this picture is the Porsche alignment tool. If you are meticulous and disassemble/reassemble only one part at a time, you won't need it. If for some reason you do, I believe you can rent it from Pelican Parts. You can see the shift fork clamp bolts in this pic, too. Those are the ones which squeeze the small end of the fork to hold them on the shaft and I suspect you have a problem with one of those.
Of course if you find one that's bent or broken, concentrate on that. Remember to check both the big ones and the small ones, the small ones are the ones that engage with the 'tab' on the shift shaft.
If you have to remove the shift shafts from the main case, there are spring loaded detent balls and more importantly interlock pins (these are what prevent the trans from going into two gears at once). Make very careful note of where they come from, they will be different lengths. This page http://porsche.wikidot.com/how-to:porsche-915-transmission-repair-tutorial-part-vii has a good tutorial and pics of the shift mechanism and detent balls etc.
Remember to watch out for the poisonous snakes! 
That page shows a removeable plate on the bottom of the gearbox that gives access to the shift mechanism for checking purposes. You might want to locate and remove this plate, then see what's happening before ripping it out and apart.
