Transmission sticking in 3rd. Why?

[cstreit]

My racecar has a 915 gearbox, custom gears (factory) and a Carrera case.

It was recently rebuilt (2 hours on it) and while racing Sunday it started getting difficult to get in 3rd gear, and then got stuck in third requiring a lot of "wiggling" and waaay to much force to get it back out of gear. It shifts in and out fine with the engine off.

I've torn down the gearbox and everything looks normal. All syncros, dog-rings, etc.. are new, no appreciable wear, burrs, galling, etc.. NO indication of any issue. Detents work fine, shift forks aligned, etc...

I'm in a real bind here because I can't figure out why this would happen. THe trans mounts look good and the shifter action is good, no obstructions.

[bell]

my tranny was stuck in second....everything looked fine but the slider sleeve was fighting against a slight wear point.....keeping it from moving. but it wouldn't come out even if the engine was off.....
is the clutch releasing all the way? iirc christian had shifting problems and it ended up being a broken pin which holds the clutch pedal solid on the shaft (his was spinning)

[cstreit]

Clutch releases fine. I tested it the day after by depressing the clutch and engaging reverse (per factory recomendation) and no grinding...

It IS perhaps coincidental but it seemed less apt to get stuck when shifting at 5000 ROM vs. 7200 normally... However since the clutch IS disengaged, torqueing of the drivetrain should be irrelevant.

[cstreit]

Well I tore down the trans and there was no visible sign of any issues (suspected dog ring pulling from gear again) except for some more play on the syncro-ring itself. I suspect this:

Originally posted by Grady Clay

In the image below (this is for a 901 but a 915 is very similar),
to the left the sliding sleeve is just starting to contact the
syncro (red arrow). The sliding sleeve dogs aren’t fully
engaged with the gear dogs until the two green arrows
are aligned. Note the concave contour of the sliding sleeve
and the convex contour of the syncro ring.

When the gear is fully engaged the spring action of the convex
syncro ring inside the concave center of the sliding sleeve helps
keep the gear engaged. When you pull the sliding sleeve to
disengage the gear the syncro ring must compress to allow the
sliding sleeve to move (to the left in this illustration.)


In the right illustration below, I have exaggerated wear on the
inside of the syncro ring (blue arrow) from the brake band. This
allows the brake band to stick in its expanded position
preventing the syncro ring from compressing when attempting
disengaging the gear. As you might imagine it doesn’t stick all
the time.
"
"
© Dr. Ing. h.c. F. Porsche A.G.

[john walker's workshop]

are there any notches worn into tips of the 3 arm hub?

[Grady Clay]

Chris,

If it is this, you should be able to see it. Be sure and keep everything in its installed orientation (particularly the brake bands). What does the back side (inside) of the syncro ring look like?

John is absolutely correct. The condition of the 3-prong hub and the mating torque surfaces of the sliding sleeve are suspect.

Take some good images of what you see (clean off the oil).

Whatever the issue is, we want to find it today.

Best,
Grady

[cstreit]

Thanks Guys,

I can't take pictures because I already overnighted the entire assembly to Powerhaus. I've been working with Roger there.

The was significant play parallel to the shaft in the syncro ring itself, evidence of warpage or "dishing" in the snap-ring retainer. Other than that everything looked damn good. No gouging, nicks, or unusual wear on the hub or selector sleeve.

[Grady Clay]

Chris,

Have Roger give me a call (303-756-7308). I'll go look also. It is about an hour for me depending on traffic.

Best,
Grady

[cstreit]

Thanks Grady... I think his full-time guy was on vacation this week too, so he might enlist you for more work, be careful.

Roger's been a pleasure to work with so far! The stuff should be there by 10:30 today...

[Grady Clay]

Chris,

Roger has been working on your stuff for at last the past 1 1/2 hours while Stan & I have been bench racing (solving the world’s Porsche problems). Great fun.

I suspect you will get a call soon and possibly your stuff on its way today. That is GOOD service.

Best,
Grady

[cstreit]

...and here we all thought he just answered the phones. (joking)

I'm very impressed with their service and willingness to discuss all things transmission. Give him a hug for me.

[zotman72]

Roger (and Power Haus II) is a great supporter of our annual PCA club race in Pueblo --which is happening next week. Maybe Roger might even be racing with us this year.... Power Haus II is another one of our fantastic Front Range Porsche assets (such as Grady, Dart Auto, Storz, Eisenbud etc.). I even got a tee-shirt from Roger to prove my point...