Calling on 915 experts - do I have to be concerned?? (long)

[ischmitz]

My car has an early 7-31 915 in front of a 3.6 engine with a stock shifter. The shift bushings are in good shape. I do not drag-race and usually shift gently. The shift-coupler is in excellent shape (no play) and the adjustment is perfect (from what I can tell). I never had so much as a hint of a shift-problem before. I took the 915 apart to install a one-piece bearing retainer six month ago. At the time the shift guide fork on the cover plate was in good shape and tightened down properly. I never touched any of the shift forks on the selector rods. With this being said here is what happened a week ago:

I didn’t notice anything unusual when put the car into a parking spot to drop something off. The 915 was probably a forward gear when I shut the engine down. Coming back pressed the clutch, started the engine, and put the shifter into reverse (still nothing unusual, felt like always). When I unset the e-brake and began to let out the clutch the car would gently roll back (slight slope) but not move due to my clutch action. Instead the engine stalled in the process.

First I thought maybe a CV joint failed or something similar. However, that wouldn’t quite explain the engine stalling when letting the clutch out but the car rolling with the clutch depressed. To my dismay it turned out the 915 was locked up!!! It must have been in two gears at the same time. Lots of thoughts started to shoot through my head – I suspected one of the shift forks came loose. Trying to shift through gears with clutch depressed and the engine running was nearly impossible – it felt funny. I couldn’t find the usually sloppy left-right play of the shifter in neutral but the shift linkage started binding. Back-forth movements of the shift lever had an equally wired feeling to it. The bottom line was that I could not find neutral or any gear. With several tries (try to find a gear, very carefully letting out the clutch) I found reverse and was able to back out of the parking lot. With more tries eventually found forth gear and I was able to limp home – still shifting was not possible. What a mess. Getting into stop-and-go traffic with only fourth gear and no neutral is NOT fun and shortens clutch life quite a bit.

Having the car in the garage I immediately removed the shift coupler cover to check if something had come loose. Negative. So I tried to work the shift lever carefully while rocking the car back and forth with the engine off. First the symptoms where the same: No neutral, no left-right play with most positions it felt the 915 was in two gears at the same time. After several tries all of a sudden to my utter surprise I found neutral. I was again able to shift into all four forward gears and into reverse. For every gear slowly letting the clutch out confirmed that everything was back to normal - the car made the right movements. This is when I had to pack it in.

Now here are the 10.000$ questions:
a) What happened? – again I did not apply any unusual force to the shifter

b) Why did it go away? – usually issues won’t fix themselves, right

c) What should I do next? - I am scared to drive the car and break down again or worse grenade the 915.

Any inputs are welcome
Ingo

[Porsche_monkey]

Free bump for a problem I would be very concerned about.

[gestalt1]

i'm not an expert on this but i have rebuilt a 915 before. the only things i can think of is either the shifter bushing, coupler or the shift rod forks in the transmission. it sounds like the coupling and shifter are fine, if they were wearing out the problem would have been gradual i would think. the shift forks in the transmission push and pull the sliders to select the gears, if one is loose or out of adjustment you could select 2 gears at a time or one gear would be selected and not de-selecting before the next gear is able to be selected. it's actually not that hard to adjust these but you do have to take the transmission out. i would also wait for more of an expert to give an opinion, i could easily be wrong.

[ischmitz]

gestalt, I have done my rebuilts myself as well and I am still short of being an expert. However, I doubt it being a selector fork that came loose because it wouldn't have put itself back into position.

John Walker has responded to a similar thread here saying it is the selector finger (for lack of a better description) that might have gotten into the wrong place. It is the thing hinged on the input selector shaft that rides in H-pattern and actuates the different shift rods. It left one gear (reverse?) engaged while shifting into another gear. If that was the case I for some reason was able to get out of this without having to do the backup light push trick but I have no idea how to prevent this from happening again in the future.....

[Jim Sims]

Did you get the detents reinstalled correctly?

[john walker's workshop]

detent position is suspect. easy to get them in the wrong position if you were not aware of their importance, when reassembling the cases.

[TRE Cup]

we have seen this in high mileage 915's (60k plus) The client is almost always in reverse when this happens. The trans end up in 2 gears at once. Sometimes the shifter will pop it back to normal position and it may not happen again for as long as you own the car. Sometimes the car gets towed in, we remove the guide plate off the bottom of the box and pry the shafts back into position ( I know- not as elegant as repairing the problem with a full rebuild, but thousands less!) . We had 2 clients that i remember that this would occur once a year or so.

[ischmitz]

Thanks for your feeback guys. Here are some comments:

Jim, John: I have done rebuilds on 915 at least three different times (mine twice, and MikeZ's) and to my knowledge every time I have installed the detends correctly. This transmission never had the slightest issue for about 10k miles after its last rebuilt. So I am quite confident that is has the detends in the correct position. However, I will check if maybe one plug fell off. You never know. Need to crawl under the car when I get back from Asia tomorrow and will update.

Dave, when you call a 60k plus 915 high-mileage then 95% of all 915 fall into that category, right So are you saying this is a total random thing and you are not sure what causes it or how to prevent it. Hm......

So should I continue to use the car and wait if it happens again. Or would you pop off the access cover and do some more checking? Still baffeled. Maybe I to a roadtrip down to TRE next week to drop off the Targa top - work is closed for the entire Thanks Giving week.

Cheers,
Ingo

[TRE Cup]

ingo
if the trans is operating correctly now, you won't see anything with the cover off.
i think what we see happening is that the worn shaft sleeves in the trans casings are allowing the shafts to foul each other. And that only seems to happen when engaging reverse or coming out of it.
One of the clients it happened to "regularly" was reversing and with the car still going backwards slowly, he would shift into first (clutch in of course) then it would get stuck. So based on this would not recommend using a 915 car for gymkhana style driving

See ya next week

[ischmitz]

Dave, that's what I am afraid off - I'd drain a load of perfectly good Swepco 201 only to find nothing. Oh well, I will see if it drives when I come back tomorrow. We'll see.

[Porsche_monkey]

Originally Posted by ischmitz

Dave, that's what I am afraid off - I'd drain a load of perfectly good Swepco 201 only to find nothing. Oh well, I will see if it drives when I come back tomorrow. We'll see.

Drain it and run it through a coffee filter, if it's clean re-use it.

[ischmitz]

Well, I drove some fifty miles today and had no issues whatsoever. First I shifted really careful but with time confidence built. So far it's fine. The shifts feel like always and nothing appears unusual.

However, in the back of my mind I somehow expect it will at some point rear its ugly head up again when I expect it the least.....

[ischmitz]

Update: both plugs on the housing that cover the channels for the detends are in place. So that is/was not the cause. Some more miles and still no recurrence or anything strange so far. Hope it stays that way....