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I may be looking at the same issue with a broken centering spring. I've been experiencing a lose of first gear and trouble at times finding other gears when shifting. There are times everything is shifting fine. At Frenzy most I talked with thought it was the rear bushings. But, Hans checked out my shifter and said he thinks my centering spring is broken, since it's not centering and is floppy. I guess I better get it looked at by our indy before I do more damage.
I may be looking at the same issue with a broken centering spring. I've been experiencing a lose of first gear and trouble at times finding other gears when shifting. There are times everything is shifting fine. At Frenzy most I talked with thought it was the rear bushings. But, Hans checked out my shifter and said he thinks my centering spring is broken, since it's not centering and is floppy. I guess I better get it looked at by our indy before I do more damage.
It's a fairly straightforward inspection once you have the transmission drained. look for metal on the drain plug and fish around inside with a magnet probe. The tip of the spring generally breaks off.
So you should be able to find the pieces in the bottom of your case most likely stuck to the magnet on the plug. What is left of the spring rides on a threaded pin. One early cars, the spring was held on the treaded pin by a C-clip. On later cars the C-clip was omitted and relied on the spring tension to keep it held one. This is problem if the head breaks off then the spring is no longer retained and can fall into the bottom of the case. Someone smarter than me can say when this change happened. I belive it was sometime between 1985-1987.
@WyattsRide find out if your car has the c-clip that will guide your next steps if you find parts of the spring head in your case.
Also, be aware the reverse lock-out spring can also break, but the piece is small and can be removed via the drain plug.
It's a fairly straightforward inspection once you have the transmission drained. look for metal on the drain plug and fish around inside with a magnet probe. The tip of the spring generally breaks off.
So you should be able to find the pieces in the bottom of your case most likely stuck to the magnet on the plug. What is left of the spring rides on a threaded pin. One early cars, the spring was held on the treaded pin by a C-clip. On later cars the C-clip was omitted and relied on the spring tension to keep it held one. This is problem if the head breaks off then the spring is no longer retained and can fall into the bottom of the case. Someone smarter than me can say when this change happened. I belive it was sometime between 1985-1987.
@WyattsRide find out if your car has the c-clip that will guide your next steps if you find parts of the spring head in your case.
Also, be aware the reverse lock-out spring can also break, but the piece is small and can be removed via the drain plug.
Thanks for the info Michael. If I can get to draining the transmission (and refilling it) before taking the car to my mechanic I'll report back. My mechanic is only 4 miles away, so I planned on driving it there but not anymore until then. He knows 928's well.
BTW, I changed my gear oil a year ago and didn't find much of anything on the drain plugs. To me my shifter centering hasn't changed from then to now (or ever in 10 years) So I really don't know if the CS is broke or not.
Got to meet Mike in person tonight on his way up to BC. Great guy with a really cool S4. Safe travels tomorrow and have fun tearing into that tranny with Colin!
It was a fairly uneventful drive up to Canada, mechanically speaking. I stopped in Seattle on the way and was able to meet fellow list member, Nate @GT6ixer . We had a good time talking about previous cars all things 928, and various list member characters. I suspect our conversation was not unlike may have the ones you all have had at various meet ups. It was great to meet Nate and he has some really great ideas on how 3-D printing can help us repair and improve our 928s.
I arrived at Colin’s place in Canada around 10 AM and we quickly got to work I’m dropping the rear end of the car. That’s Colin an Michael getting busy.
We ran into our first snag while trying to drop the rear shocks one of the bolts was stripped but we were able to get it out with some creative leverage. Take a look at the stretched bolts on the shock tower. It looks like they were significantly over torqued that time they were reinstalled when I had the crossmember replaced (https://rennlist.com/forums/928-foru...uspension.html)
With the transmission removed, the gears were inspected and the centering spring was removed. Fourth gear had one tooth with a nick in it but all the other gears on the top and visually spear to be in good condition. We had not inspected the gears on the layshaft yet but we suspect damage on those as well.
Here is the centering spring. As you can see the head had broken off, however there wasn’t any evidence of the head in the case. Look at the damage it is really quite unbelievable.
Drive Shaft replacement
We observed some damage to the splines on the driveshaft from incorrect attachment to the transmission and considering the bearing wear we decided to replace the driveshaft and rebuild the TT.
However removing the TT posed an additional challenge because the clutch release arm was preventing us from accessing the top bolts on the TT. Subsequently the clutch had to be removed as well. The problem with the interference was caused, as Colin suggested, by the improper mixing and matching of GTS throw out bearing with S4 release lever. The GTS throwout bearing is longer and the release lever is shallower than he S4. so when you install the GTS throw out bearing into an S4 with the stock release arm it pushes the assembly rearward towards the torque tube. The clutch was replaced by one of the POs back in 2005.
Pilot bearing replacement
After removing the clutch , Colin identified the pilot bearing as being original and requiring replacement.
by this time we had burned up most of the hours in the day and it was time to call it a wrap. So Colin and his helper Michael cleaned up and went home to their families and I booked a hotel room and had a wonderful night out on the town. Tomorrow will get back into it by rebuilding the TT and putting everything back together. Hopefully we’ve ended our discovery of things that were broken or needed to be repaired and it’s just a matter of reassembly
stay tune for more drama and lots of photos tomorrow. Wish us luck!
Last edited by Michael Benno; 10-04-2019 at 11:04 AM.
There are three versions of the shift spring, although Porsche only ever had two, in the spare parts program.
The 1985 transmissions had a spring with the lower pivot portion thinner, with a spacer. This version, which used the circlip, was a great idea, except when the spring needed to be replaced. Getting that circlip on and off with the transmission assembled was....extremely difficult/impossible. This part was never offered as a spare part, by Porsche.
In 1986, they made the spring with a wider lover pivot, and deleted the spacer and the circlip. This "1986" version was available as a spare part, up to about 15 years ago. It could be used in the 1985 and 1986 transmissions with no other changes.
In 1987, they made the version that had the wider upper "detent" area. This part was the only one that was available, for many years, after the 1986 version was NLA. Using this part in the 1985 and 1986 transmissions also required changing the "roller" lever to the 1987 version that had a wider slot. Failure to do this would result in the "detent" area of the shift spring being "levered" in the narrow groove of the 1985/1986 roller and break the detent portion off very quickly. Many of the later shift springs "died" from not performing this "update".
I've been nagging Porsche Classic to remake both versions of these shift springs.
Drive Shaft replacement
We observed some damage to the splines on the driveshaft from incorrect attachment to the transmission and considering the bearing wear we decided to replace the driveshaft and rebuild the TT.
This ruined torque tube shaft and the resulting damage to the inside of 5th gear is caused by the harmonic absorber loosing contact with the torque tube, due to the rubbers degrading. The frequency issues are normally absorbed by this piece, but once the rubbers loose tight contact with the torque tube, the harmonics are not absorbed and the end of the torque tube shaft vibrates terribly inside 5th gear.....it literally just sits inside the gear and "hums". No amount of tightening of the clamp will keep this from happening....this harmonic vibration actually wears everything (including the clamp) and the bolt will continuously come loose.
The "updated" harmonic absorber used in the later model torque tubes with the different rubber "donuts" is the best solution. I also modify the original dampers to accept the later rubber pieces.
I keep all of these pieces in inventory at all times, for the people that are smart enough to figure out the issue.
928 International has reproduction rubber pieces for the original harmonic absorber. I have not tested these pieces. The hardness of the rubber is of paramount importance and I'm not sure how much "science/reverse engineering" went into these rubber pieces.
On this car, the wear on the TT shaft was most likely caused by incorrect installation.
The pinch bolt had maybe 20ft lbs torque to remove.
All 6 of the TT to transmission bolts were dead loose. A couple of them I could have removed just with my fingers.
And then there was the upper shock bolts that were put on way way too tight......
Either way, Michael is back together now. With the bolts properly torqued.
I own a Porsche repair shop and have built 100's of trans and motors was also involved with racing in the 80's IMSA, father in-law won 12 hours Sebring, but back to transmissions, I always recommend to use the factory fill oil none synthetic, every time a customer would buy the most expensive oil the trans would come back with problems when switching back to the normal fill the problems go away, go figure I cant explain it but thats what i've seen in the last 37 years of builds
The car is done and driving again. Colin and his crew installed the rebuilt TT with a replacement driveshaft bolted to the tranny and re-assembly went pretty smoothly as did Colin's extra special shakedown drive, which included some well appreciated track driven tips of rev matching shifts :burnout
With everything back together, I packed up and made the 6hr drive back home in mostly pouring rain, which was mildly stressful. Driving impressions. It's nice to have the centering spring again, the added force to get into the R-1 gate is a nice mental reminder. Shifting on the transmission is noticeably more notchy (and more force) getting into and out of gears than the previous transmission. I used STA gear oil in this transmission, vs Porsche gear oil in the old one. I'll see if it changes over time, but I am going to take a little break from the 928 for a few days.
Some thank you's are in order. Firstly to the forum for all of your advice, suggestions, and opinions. I always appreciate your constructive input and patience as I learn what many of you know all too well. Thanks to Roger and his daughter Mercedes to rushing out the parts I needed to make this repair/roadtrip happen.
But most of all, THANK YOU to Colin and his staff for their ever positive attitude, willingness to help me through problems and getting me fixed. They were super professional and took the greatest of care of my car the same way I would. Thanks again Colin, hope to see you soon.
PS, here is a recap of the parts we replaced
- G28/13 gear box with LSD, (STA 75/90 gear oil)
- Rebuilt TT with drive shaft
- Clutch pilot bearing
- Shock tower upper mounting plate
- Refurbished ABS harness
- front shifter ball socket
10-01-2019 | 05:22 PM
