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1988, my daily driver. Intermittent crunch from 3rd to 4th. Only after a long bit of driving. Repeatable. Goes away. Sometimes it comes back. 5th to 4th is fine..
Dropped the differential fluid last weekend. Clear. No measurable iron on the magnets. In fact, the least I've ever seen in all my years of differential fluid changes. No history of a previous fluid change.
The '87 to '95 5 speeds used the heavy duty steel synchros in 1st, 2nd, and 3rd gears. 4th and 5th gears got the bronze synchros.
Both the steel and bronze synchros have the same Moly coating on the friction surfaces, but the lower grade bronze synchros are more prone to distortion and overheating.
If you took this transmission apart, you would find that there is metal transfer between the Moly coating and the "cone" (attached to the gear, which the Moly "grabs" onto, to accelerate or de-accelerate the gear.)
Pretty common to have occur.
The "Borg Warner" synchro design (which these transmissions have a version of), has one inherent design issue, in that the synchros and cones are very isolated from a constant flow of fresh, cool gear oil, which is also being "slung" away from this area, by centrifugal force.
By the time the 993's were developed, Porsche figured out that by making the pinion and the input shaft hollow, with appropriate holes drilled into the needle bearings, they could get more fresh, cool oil to this synchro design.
On the 928 transmissions, we have gradually had to replace more and more of the "cones", as the years have passed. Cutting off the old cone (with
out damaging the gear, whatsoever, ) is time consuming and very hard on even the highest grades of carbide tooling....not fun!
I do have a couple of different gear oils which we have found to be superior to what you are using, which you might try. I'd be happy to share these, with you, offline.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
The '87 to '95 5 speeds used the heavy duty steel synchros in 1st, 2nd, and 3rd gears. 4th and 5th gears got the bronze synchros.
Both the steel and bronze synchros have the same Moly coating on the friction surfaces, but the lower grade bronze synchros are more prone to distortion and overheating.
If you took this transmission apart, you would find that there is metal transfer between the Moly coating and the "cone" (attached to the gear, which the Moly "grabs" onto, to accelerate or de-accelerate the gear.)
Pretty common to have occur.
The "Borg Warner" synchro design (which these transmissions have a version of), has one inherent design issue, in that the synchros and cones are very isolated from a constant flow of fresh, cool gear oil, which is also being "slung" away from this area, by centrifugal force.
By the time the 993's were developed, Porsche figured out that by making the pinion and the input shaft hollow, with appropriate holes drilled into the needle bearings, they could get more fresh, cool oil to this synchro design.
On the 928 transmissions, we have gradually had to replace more and more of the "cones", as the years have passed. Cutting off the old cone (with
out damaging the gear, whatsoever, ) is time consuming and very hard on even the highest grades of carbide tooling....not fun!
I do have a couple of different gear oils which we have found to be superior to what you are using, which you might try. I'd be happy to share these, with you, offline.
Good timing. I'm ready to fill the diff.
Last edited by Kevin in Atlanta; 11-09-2023 at 08:51 AM.
Another easy check... when was the last time you checked/changed your brake/clutch fluid? Cheap additional WYIT. I've become religious about 2-3 year flushes after a bad VW Bug experience that would have been avoided with simple maintenance being followed.
Just helped someone with a "doesn't like to engage when cold," and we started there.... a small burp when we bled the clutch slave, and the fluid was "ready" to be changed for sure. #problemsolved. This just happened this weekend, long shot that it's your problem, but it did solve this transmission issue that came on fairly "acute" in this scenario.
Another easy check... when was the last time you checked/changed your brake/clutch fluid? Cheap additional WYIT. I've become religious about 2-3 year flushes after a bad VW Bug experience that would have been avoided with simple maintenance being followed.
Just helped someone with a "doesn't like to engage when cold," and we started there.... a small burp when we bled the clutch slave, and the fluid was "ready" to be changed for sure. #problemsolved. This just happened this weekend, long shot that it's your problem, but it did solve this transmission issue that came on fairly "acute" in this scenario.
Fresh brake fluid and a GB clutch line in the last month. That changed nothing.
5 speeds have transmission and differential as one fill (two drains, however.)
Automatics have separate differential and the Mobil One is friendly to limited slips, so an OK choice.
5 speeds have transmission and differential as one fill (two drains, however.)
Automatics have separate differential and the Mobil One is friendly to limited slips, so an OK choice.
Patient is MT with LSD.
Last edited by Kevin in Atlanta; 11-09-2023 at 05:15 PM.
Following Greg's recommendation, I replaced the fluid.
The results are mixed. I need to drive the car more.
The day I swapped the fluid I drove the car for about 20 miles before the 3-4 shift complained. The second time I took the car out the issue took about 40 miles. In my shorter trips it never happens.
I would to explain in some detail what happens with the 3-4 shift. It is not the typical synchro grind. It's sounds a bit deeper.
And once it happens I can repeat the 3-4 one more time and it no longer complains for a while.
It is the strangest thing. It's like it takes two taps to move something.
Of other note, the issue where the shifter falls out of gear has not happened since the fluid change.
My life runs like this: At the moment I believe I have solved the issue and post it it comes back. So, sure as shooting the 5th gear will start popping out after this post. :-)
I yield to others with superior understanding of how the manual transmission to craft an explanation for the 'double tap'.
Following Greg's recommendation, I replaced the fluid.
The results are mixed. I need to drive the car more.
The day I swapped the fluid I drove the car for about 20 miles before the 3-4 shift complained. The second time I took the car out the issue took about 40 miles. In my shorter trips it never happens.
I would to explain in some detail what happens with the 3-4 shift. It is not the typical synchro grind. It's sounds a bit deeper.
And once it happens I can repeat the 3-4 one more time and it no longer complains for a while.
It is the strangest thing. It's like it takes two taps to move something.
Of other note, the issue where the shifter falls out of gear has not happened since the fluid change.
My life runs like this: At the moment I believe I have solved the issue and post it it comes back. So, sure as shooting the 5th gear will start popping out after this post. :-)
I yield to others with superior understanding of how the manual transmission to craft an explanation for the 'double tap'.
Kevin
Buy two 120-150 gallon Igloo ice chests. (Sam's club has the 150 gallon, right now, for $89.....we "stocked" up!)
Put the '88 into the first cooler.
Put the broken "core" GT transmission into the other.
I'll do the labor to rebuild the first ice chest, for the 2nd ice chest, plus if you need any of the big ugly priced "hard parts", I'll steal them, from the 2nd ice chest.
You buy the parts...
Buy two 120-150 gallon Igloo ice chests. (Sam's club has the 150 gallon, right now, for $89.....we "stocked" up!)
Put the '88 into the first cooler.
Put the broken "core" GT transmission into the other.
I'll do the labor to rebuild the first ice chest, for the 2nd ice chest, plus if you need any of the big ugly priced "hard parts", I'll steal them, from the 2nd ice chest.
You buy the parts...
Deal!
The second one is already in a cooler. :-)
I'll get back to you on when I can fill the first cooler and get that to you.
I have had luck with fluid changes helping with issues like this. I own a Corvette 6-speed swap 86.5 (done by PO). I'm chasing down several issues without going into any other details, a fellow member @Petza914, recommended using DRIVEN fluids, and it made a significant improvement in the T56. I've had the T56 gone through recently, had a new Torque Tube built, etc....all solved one issue or another; Pete's recommendation was based on his empirical data from multiple cars (including a similar swapped 928) oil analysis. The cost of the DRIVEN Fluids was similar to any other quality product, but the additive package seems to have been different enough to make it run smoother. The previous fluid was almost NEW (Amziol based on another shop recommendation). By all rights, it was the same "weight," etc...but I've had issues with fluids just not being "happy" before in a Harley and switching back to CHEAP DINO OIL was better than Synthetic for some reason (Blow by), so it was the cheapest starting point to swap it out.
Summary: After spending big money on big fixes before something basic has bitten all of us more than once.
The forum WINS again. It would be nice if the "secret sauce" was disclosed for the group...#IJS
The forum WINS again. It would be nice if the "secret sauce" was disclosed for the group.
I wouldn't count on that happening. Kevin's a great guy but if GB asks him for confidentiality, he'll honor that request. As would almost anyone here, BTW. It doesn't mean we necessarily agree with GB, but a promise is a promise.
11-07-2023, 01:25 PM
Crunchy 3rd to 4th shift
