When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
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!
I know it’s not the root cause, but where goes the hose from the pressure control valve #26 ? I think it is to the engine bay, but where exactly ? Merci 😊
Part No26 is the vacuum modulator that tunes the harshness of the changes. It is connected to the 5 way manifold at the back of the inlet manifold next to the MAF. If that system is not working you will get a harsh change on light load shifts. The line is a composite of hose, steel tube [along the torque tube], rubber connectors and nylon tubing.
Message from Theo Jenniskens, " Ok, all checks sound really ok. So the ATF pump runs, the TC spins, and all seems fine. The only thing that comes to mind is that there is no ATF pressure to actuate the valves. Maybe there is a leak, a crack or something that makes the oil seep out instead of pressurize the actuators.
(ps: a lot of talk about the filter… if the filter would not seal properly, the fluid level would not drop quickly when cold. This is when the pump sucks in all the fluid and it takes time to seep back until the ATF is hot.) "
I will remove the pan on next week. No time this week ...
The pan will be removed on next week to check inside.
Waiting for that, I have read on a MB forum
about 722.4 trans that it’s possible to check if the primary pump works fine (pressure) by removing the hose to the automatic transmission oil cooler. If there is no oil when the engine is spinning, the pump doesn’t work.
It's the lower hose on the front cooler but I don't think you're pump is the problem. Without the pump working, the fluid level would not change in the reservoir.
It's the lower hose on the front cooler but I don't think you're pump is the problem. Without the pump working, the fluid level would not change in the reservoir.
You’re right. The fluid change in the reservoir.
Video at cold. I start the engine, and R N D N R P.
The pressure in the oil cooler AFAIK is not very high and probably reflects the effort needed to drive the ATF through the cooler circuit back into the sump. For sure the pump is picking up the ATF or the level would not drop but then the real question is what is happening to the oil pressure that actuates the system? If there is a breach of the pressure circuit the pump will pump oil but it will not generate the pressure needed for the system to work. I am not familiar with the details and someone like GB will be able to amplify, but I suspect given the nature of the service the oil pump will be a positive displacement item [like the engine oil pump] and will generate whatever pressure is needed to displace the swept volume. If something within the pumped circuit has failed the system will not be able to generate the working pressure needed.
As to what has failed to cause this condition remains to be seen but that is why I suggested that you needed to eliminate that dink in the pan as a cause of such. That non of our friends has come up with a possible cause to me suggests that something quite unusual has happened.
The pressure in the oil cooler AFAIK is not very high and probably reflects the effort needed to drive the ATF through the cooler circuit back into the sump. For sure the pump is picking up the ATF or the level would not drop but then the real question is what is happening to the oil pressure that actuates the system? If there is a breach of the pressure circuit the pump will pump oil but it will not generate the pressure needed for the system to work. I am not familiar with the details and someone like GB will be able to amplify, but I suspect given the nature of the service the oil pump will be a positive displacement item [like the engine oil pump] and will generate whatever pressure is needed to displace the swept volume. If something within the pumped circuit has failed the system will not be able to generate the working pressure needed.
As to what has failed to cause this condition remains to be seen but that is why I suggested that you needed to eliminate that dink in the pan as a cause of such. That non of our friends has come up with a possible cause to me suggests that something quite unusual has happened.
I'm sticking with my post #21, on the 2nd of January.
Just a guess but has anyone checked the gear selector on the side of the trans? Have some one Move the shifter while observing if the selector is moving inside the box.
I'm sticking with my post #21, on the 2nd of January.
Greg,
Nothing wrong with your synopsis of course but the OP has not indicated any sign of imminent failure as you suggested was likely to be the case. Having changed the ATF 100 miles before the failure with no signs of debris is it feasible that this type of problem would occur in such a short usage interval?
The bit I find scary is your comment that a total failure can occur in a tranny that has covered as little as 60k miles in what is purportedly a bullet proof transmission that just happens to be fitted in my 928! Could this be a consequence of mounting such tranny in a transaxle configuration? MB never offered this tranny in a transaxle unit configuration as I am aware so one wonders whether there is something lacking in our application but then do many examples fail like this?.
Just a guess but has anyone checked the gear selector on the side of the trans? Have some one Move the shifter while observing if the selector is moving inside the box.
Somewhere near the top of the thread, until the pan is dropped not sure there’s much left to say.
Nothing wrong with your synopsis of course but the OP has not indicated any sign of imminent failure as you suggested was likely to be the case. Having changed the ATF 100 miles before the failure with no signs of debris is it feasible that this type of problem would occur in such a short usage interval?
The bit I find scary is your comment that a total failure can occur in a tranny that has covered as little as 60k miles in what is purportedly a bullet proof transmission that just happens to be fitted in my 928! Could this be a consequence of mounting such tranny in a transaxle configuration? MB never offered this tranny in a transaxle unit configuration as I am aware so one wonders whether there is something lacking in our application but then do many examples fail like this?.
The "bullet proof" versions of this transmission were in the earlier model cars. There are several aspects of the GTS vehicles, which made them less than the "ultimate" 928. Several.
The reverse clutch failure is super common in the GTS transmissions, well before 100,000 miles. I've seen earlier 4 speeds with 250,000 miles on them, without showing much reverse clutch wear. As I indicated, I believe that Mercedes "softened" up the lining material to make the shifts into reverse softer. In their next version of their transmissions (722/04), Mercedes apparently added another clutch disc, for increased wear duration. (I have never had any other Mercedes designed transmission apart, but the pictures I've seen indicate this.)
The reverse clutch failure mode is subtle and not everyone is aware of the progressive nature. There is always a delay when shifting from drive to reverse....the delay just gets longer and longer, the further the B3 piston has to travel. While I've seen a few of failures which actually slip in reverse, most failures are "Everything was perfect and then I couldn't back up."Several.
Without tearing the previous transmission apart and inspecting what was inside (a difficult, very "sharp" and easy to bleed from, task), it is entirely possible for all of the clutch debris to be sucked up into the filter...and there not be much of anything in the oil pan.
I "see" this, often.
The filters can "hide" huge volumes of clutch debris.
This failure, no reverse or forward drive, is interesting.
While I can imagine a reverse clutch failure that is severe enough to damage things badly enough so that the forward gears do not function, this is not something that I've seen, to date. (It would be possible for that "nub", which guides the B3 piston, to actually put a hole though into the oil pump, instead of the "nub" simply breaking off of the B3 piston. (This would cause the forward gears to not work.)
It is also possible that one of the C/V joints has failed/come loose and the OP is missing this....
Or the oil filter to have fallen off.....
Or a multitude of other failures. (There are transmission fluids which will destroy the internal seals.)
Guessing what people's problems are, over the Internet, is far from a science....it's sometimes an "educated" guess, at the very best.
(.....I never bother listening to people's recordings of noises. It's usually tough enough to isolate a noise, when the car is right in front of me.)
This failure, no reverse or forward drive, is interesting.
While I can imagine a reverse clutch failure that is severe enough to damage things badly enough so that the forward gears do not function, this is not something that I've seen, to date. (It would be possible for that "nub", which guides the B3 piston, to actually put a hole though into the oil pump, instead of the "nub" simply breaking off of the B3 piston. (This would cause the forward gears to not work.)
It is also possible that one of the C/V joints has failed/come loose and the OP is missing this....
Or the oil filter to have fallen off.....
Or a multitude of other failures. (There are transmission fluids which will destroy the internal seals.)
Guessing what people's problems are, over the Internet, is far from a science....it's sometimes an "educated" guess, at the very best.
(.....I never bother listening to people's recordings of noises. It's usually tough enough to isolate a noise, when the car is right in front of me.)
Hi all,
Thank you very much for your help ;-)
BTW, last maintenances are :
- ATF change, 100 miles ago. Change with MOBIL ATF-220 DEXRON 2D (around 8 liters).
- All C/V joints change, 700 miles ago.
- Differential oil change, 500 miles ago. Change with MOTUL GEAR 300 75W90 (1.9 liters).
- PSD DOT4 level, 600 miles ago.
01-07-2021, 04:11 PM
