Another AT trans question about first gear... maybe B2 piston?
08-17-2010, 03:24 AM
#17
Rennlist Member
The manual and auto gearboxes are kind of the last frontier.
Not many people have fixed them at home.
The pros are sharing limited information, nuggets here and there.
Up to us to start demystifying these ourselves I guess.
Not many people have fixed them at home.
The pros are sharing limited information, nuggets here and there.
Up to us to start demystifying these ourselves I guess.
08-17-2010, 05:28 AM
#18
Burning Brakes
Thread Starter
Think I wil just start with a B2 piston replacement and possibly K1 (read on the MB forum that it can also be replaced with trans in the car).
How hard is it to remove the aluminium sleeve of the b2 piston with the trans in the car?
Want to replace it with the plastic liner off course.
How hard is it to remove the aluminium sleeve of the b2 piston with the trans in the car?
Want to replace it with the plastic liner off course.
08-17-2010, 08:27 AM
#19
Rexpontius,
I you check my past post you will find 3 pdf files on how to change the B2 piston with the auto still in the car.
Usually when you have a B2 piston failure you loose all forward gears, but still retain reverse gear, so I would suggest that you have not got a B2 piston failure.
The WSM give you a very good fault and diagnostic procedure that you can recognise the problem or at least narrow it down to a couple of issues.
At the present time my gearbox files and manuals are currently stored away and I don't have easy access to them, however if you purchase Jim Moorehouse's Technical CD (Roger at 928sRus has the CD for sale and he will be putting them on his web site for free down load - www.928srus.com) and there is a instruction manual on the auto box included).
Earlier model 928 change down to first gear when stopping, however, the S4 and the GTS models will only change down to 2nd gear and if you wish to select 1 gear when starting off you either "floor" the accelerator (via the kickdown switch) or change into 2nd gear and the car will start off in 1st gear.
There are various scanerios devised by many auto box drivers to control 1st gear selection if the box is designed not to change down to 1st gear. You will need to investigate what your model is designed to do. Go and speak with a Mercedes Benz auto workshop and they should be able to help you.
The search function on this web should find the MB related model cars fitted with the 722.3 MB box.
Tails 1990 928S4 Auto
I you check my past post you will find 3 pdf files on how to change the B2 piston with the auto still in the car.
Usually when you have a B2 piston failure you loose all forward gears, but still retain reverse gear, so I would suggest that you have not got a B2 piston failure.
The WSM give you a very good fault and diagnostic procedure that you can recognise the problem or at least narrow it down to a couple of issues.
At the present time my gearbox files and manuals are currently stored away and I don't have easy access to them, however if you purchase Jim Moorehouse's Technical CD (Roger at 928sRus has the CD for sale and he will be putting them on his web site for free down load - www.928srus.com) and there is a instruction manual on the auto box included).
Earlier model 928 change down to first gear when stopping, however, the S4 and the GTS models will only change down to 2nd gear and if you wish to select 1 gear when starting off you either "floor" the accelerator (via the kickdown switch) or change into 2nd gear and the car will start off in 1st gear.
There are various scanerios devised by many auto box drivers to control 1st gear selection if the box is designed not to change down to 1st gear. You will need to investigate what your model is designed to do. Go and speak with a Mercedes Benz auto workshop and they should be able to help you.
The search function on this web should find the MB related model cars fitted with the 722.3 MB box.
Tails 1990 928S4 Auto
08-17-2010, 08:58 AM
#20
Drifting
If you can get first gear manually using the shift lever... it's not the B2 piston. Just like other's have indicated, if you loose the B2 you loose first gear entirely. The fact that it works some of the time I'm pretty sure means it’s okay.
I have an 87 auto that will shift to first on it's own with about ½ throttle when depressed quickly at very low speeds (less than 5 mph), but will upshifts just about as quick to second if you don’t get on it more before 3.5K rpms or engage the kick down switch to hold it to redline. Howver, if I slowly roll to full throttle but short of kick down, my will not grab first... even from a stand still.
That leads me to believe that it’s somehow related to a vacuum modulator. Not exactly sure how, but something about a spike in vacuum gets my to 1st no problem. Also, total speculative, it seem like the first gear grab started working better after my last fluid exchange (I run M1 synthetic) when I included a can of Trans-X. Also a few tanks before I (or should I say AAMCO did the pan drop), I added an off the shelf transmission cleaner product.
Mine was leaking pretty bad at the time. I was adding about a half quart a month so finding room for the cleaner wasn’t a problem. Then after adding the Trans-X (recommended by someone on this forum), not only did my transmission start shifting better, but it quit leaking. I think that was about two years ago now.
Also for what it’s worth, mine won't hold 1st past redline or bump off the limiter in 1st, it overrides and shifts to second. I know for sure the rev limiter works in second, but never been able to hit in 3rd… not that I haven’t tried.
I have an 87 auto that will shift to first on it's own with about ½ throttle when depressed quickly at very low speeds (less than 5 mph), but will upshifts just about as quick to second if you don’t get on it more before 3.5K rpms or engage the kick down switch to hold it to redline. Howver, if I slowly roll to full throttle but short of kick down, my will not grab first... even from a stand still.
That leads me to believe that it’s somehow related to a vacuum modulator. Not exactly sure how, but something about a spike in vacuum gets my to 1st no problem. Also, total speculative, it seem like the first gear grab started working better after my last fluid exchange (I run M1 synthetic) when I included a can of Trans-X. Also a few tanks before I (or should I say AAMCO did the pan drop), I added an off the shelf transmission cleaner product.
Mine was leaking pretty bad at the time. I was adding about a half quart a month so finding room for the cleaner wasn’t a problem. Then after adding the Trans-X (recommended by someone on this forum), not only did my transmission start shifting better, but it quit leaking. I think that was about two years ago now.
Also for what it’s worth, mine won't hold 1st past redline or bump off the limiter in 1st, it overrides and shifts to second. I know for sure the rev limiter works in second, but never been able to hit in 3rd… not that I haven’t tried.
08-18-2010, 03:38 PM
#21
Rennlist Member
hijack! Kurt, I just changed my vacuum modulator out w Andrew because I was experiencing a similar issue. Regardless of bowden cable adjustment, I could not start in 1st even with selector in 2nd. From a dead stop, if I depressed the accelerator half way down or floored it, the car would shift to 1st gear (thats issue #1) and then, the shift to second would always be too soon at 4500rpms (issue #2). Issue 3 is car lurches or hard shifts when selecting a forward or reverse gear while stopped. Issue 4 is car would not down shift when manually moving the gear selector (D to 3 or 3 to 2) unless accelerator was depressed.
I have not checked pressure between vacuum line and modulator yet since my guage would not measure pressures/vacuum that high.
But.... the new modulator has improved all things above. the 1-2 shift is more like 5200rpm. Not redline but closer. If adjusting the 't' on modulator improves this (dont believe it will but im going to try), i will let you know.
Back to the original question asked by the poster of this thread...
For the record, if it was not clear on earlier posts. the mercedes auto trans used in our 928's is designed NOT to start in 1st gear when in D or 3 unless the WOT/kickdown button is actuated (acc pedal to the floor).
Your A/T 928 will always start from a dead stop in second gear when in 3 or D If the gear selector is in 2, the car will start in 1st. To manually shift to second when gear selector is in 2, quickly shift up to 3 then back to 2. Now you'll be in second gear. You can wait for the trans to redline to go from 1st gear to 2nd gear when in the 2 position. Your choice.
Hope this helps.
Brian
I have not checked pressure between vacuum line and modulator yet since my guage would not measure pressures/vacuum that high.
But.... the new modulator has improved all things above. the 1-2 shift is more like 5200rpm. Not redline but closer. If adjusting the 't' on modulator improves this (dont believe it will but im going to try), i will let you know.
Back to the original question asked by the poster of this thread...
For the record, if it was not clear on earlier posts. the mercedes auto trans used in our 928's is designed NOT to start in 1st gear when in D or 3 unless the WOT/kickdown button is actuated (acc pedal to the floor).
Your A/T 928 will always start from a dead stop in second gear when in 3 or D If the gear selector is in 2, the car will start in 1st. To manually shift to second when gear selector is in 2, quickly shift up to 3 then back to 2. Now you'll be in second gear. You can wait for the trans to redline to go from 1st gear to 2nd gear when in the 2 position. Your choice.
Hope this helps.
Brian
08-18-2010, 03:49 PM
#22
Supercharged
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Brian... the earlier 4speed 928s do start in first gear in D or 3. The black one in my garage most certainly does this (should have pointed that out to you). There was a running change in '87 to a newer version of the trans. That trans behaves as you suggest. Always starts in 2nd from D or 3.
08-18-2010, 04:45 PM
#23
Burning Brakes
Thread Starter
Brian
I think you might have helped my one step further to solving the issue.
When I bought the car a few months ago the upshifts were dramatically hard. After some reading here on the forum, checking vacuum to the modulator and replacing oil+filter, I decided to adjust the modulator. I took about 4-5 full turns counterclockwise to solve the harsh shifts.
But if the vacuum modulator also affects how/when to select first gear, I think maybe the modulator itself just might be broken.
How hard was it to change the modulator? Can it be done with the trans in the car?
Really hope so :-)
Cheers
Michel
I think you might have helped my one step further to solving the issue.
When I bought the car a few months ago the upshifts were dramatically hard. After some reading here on the forum, checking vacuum to the modulator and replacing oil+filter, I decided to adjust the modulator. I took about 4-5 full turns counterclockwise to solve the harsh shifts.
But if the vacuum modulator also affects how/when to select first gear, I think maybe the modulator itself just might be broken.
How hard was it to change the modulator? Can it be done with the trans in the car?
Really hope so :-)
Cheers
Michel
08-18-2010, 04:51 PM
#24
Supercharged
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Yes it can be done int he car. There are two 5mm allen bolts that hold a bracket that keeps the modulator in place. They are hard to get at and almost blind but should be at rougly 9 and 3 o'clock positions. Once the bolts are out the modulator can come out.
BE CAREFUL: There is a 3" long nylon nipple, for lack of a better word, that sits on the modulator and goes into the trans. This was actually broken on Brian's car, luckily we had a parts trans that we were able to steal a different one off of.
BE CAREFUL: There is a 3" long nylon nipple, for lack of a better word, that sits on the modulator and goes into the trans. This was actually broken on Brian's car, luckily we had a parts trans that we were able to steal a different one off of.
08-18-2010, 04:56 PM
#25
Rennlist Member
I think you need to drop the exhaust (easy on these cars) and the heat shield, too.
There are different modules that have some degree of interchange, yours is probably red in color? Look for a like-replacement.
There are different modules that have some degree of interchange, yours is probably red in color? Look for a like-replacement.
08-18-2010, 05:25 PM
#27
Drifting
Interesting stuff about the boden cable and modulator. I read the earliest 4 speeds automatically started in first, then I think they switch to may drop to first if pushed (through mid 87???), after that sounds like the only way to get first is move the selector.
Mine doesn't shift perfect, but not too far off to warrant messing with it now. The main thing I don't like about mine is it wants to up shift too quickly under medium acceleration... it goes 2nd, 3rd, 4th rather quickly now. I might try a kick down switch just for fun.
Anyhow not sure if these are relevant, but attached are couple misc guides I've picked up along the way.
OOOPS, the good one was too big to upload... anyhow; following are some links I pasted together with some decent info.
PM me your email and I can forward another PDF files specific to our transmissions.
_____________________________________
Edit... Coverted PDF to word DOC and it fits.
______________________________________
http://forums.rennlist.com/rennforum...d.php?t=461390
http://forums.rennlist.com/rennforum...d.php?t=449831
http://forums.rennlist.com/rennforum...d.php?t=462655
https://rennlist.com/forums/928-forum/447460-automatic-trans-adjustment-write-up.html
https://rennlist.com/forums/928-forum/410690-transmission-problem-question.html
https://rennlist.com/forums/928-forum/429605-question-about-b2-piston-and-b2-band.html
https://rennlist.com/forums/928-forum/424255-looks-like-the-82-trans-needs-help-update-warning.html
https://rennlist.com/forums/928-forum/402994-auto-transmission-has-adjusting-vacuum-modulator-caused-early-upshifts.html
https://rennlist.com/forums/off-topic/387535-how-hard-is-it-to-rebuild-an-auto-trans.html
https://rennlist.com/forums/928-forum/430016-at-trans-failure.html
https://rennlist.com/forums/928-forum/444764-s4-a-t-rebuild-thread.html
https://rennlist.com/forums/928-forum/421142-b2-brake-band-replacement-adjustment.html
https://rennlist.com/forums/928-forum/333680-transmission-help.html
https://rennlist.com/forums/928-forum/260643-strange-automatic-leak.html
https://rennlist.com/forums/928-forum/249966-so-how-do-you-change-auto-trans-fluid.html
https://rennlist.com/forums/928-forum/238442-auto-trans-slips-in-all-gears.html
https://rennlist.com/forums/928-forum/191040-automatic-trannsmision-skipping-2nd-gear.html
https://rennlist.com/forums/928-forum/133501-achtung-achtung-autotrans-rebuild.html
The primary pump, supplies pressurized fluid to the hydraulic circuit. This pressure applies bands, clutches and lubrication for all moving parts. The most hydraulic circuits are found in the valve body. As the fluid moves into the valve body it has the potential to actuate every single valve. When the valves move they redirect the pumps pressure to control the application of bands and clutches thus providing up and down shifts and different ratios through the planetary system. The pumps working pressure is always the highest pressure in the hydraulic system.
All other pressures are derived from this maximum pressure and is reduced and controlled by regulating valves. The primary control valve is the pressure regulator valve. Mercedes calls this valve the control valve working pressure. It controls and regulates the amount of working pressure in the transmission. This pressure varies depending on which way the valve moves. If the valve moves towards the spring, which sometimes breaks, we have low working pressure. If it does not move, or moves a little, we have high working pressure. There are other pressures that balance and regulate the pressure regulator but when we have 210 pounds of working pressure in a vehicle in drive, the problem is usually the pressure regulator.
The governor is the road speed input from the transmission. It has no regulating pressure from a stop and does not control a first gear start in any valve body. In a Mercedes first gear start valve body, the 1-2 command valve is stroked in the first gear start position; by design once the car is started, commanding the B2 band on and the F, one way clutch locked. If the valve body were a second gear start, then the one two command valve would be stroked in the 2nd gear start position commanding the B1 and B2 bands on.
The description of operation in that PDF is for a 1996 C22, which Is a 722.4 transmission that starts in second gear.
What they are referring to by the engagement dampening circuit, is the accumulator cutting in oil circuit wish is a dampening system designed to soften shift engagements from a dead stop (not at road speed):
The dampening circuit just cushions the selected gear engagement.
N to D = B1 and B2 = 2nd Gear
N to 3 = B1 and B2 = 2nd gear
N to 2 = K1 and b2 and f is locked = 1st gear
N to R = K2 and B3 = Reverse
In the neutral position there is no power flow between the engine and transmission, no bands or clutches are applied. This is also the towing and push start feature of the car.
By moving the transmission lever from neutral to drive at road speeds, you are going from a pressure less system (neutral) to a high pressure system (drive). At that high pressure you are probably freeing up whatever is stuck in the valve body.
I think the problem is in the valve body. If that car was at my shop I would change it.
In any 722.3-4-5 MB or Porsche 928 if you move the selector lever manually to second, the transmission will start in first gear. This is engine breaking. Try that as a test.
How a transmission shifts is based/ derived from a balance /unbalance of pressures
The main plays are
Governor, road speed input oil.
TV Bowden cable / road load input
Modulator pressure, load and speed dependent.
Working pressure, pump
Shifts happen when governor pressure moves the shift / command valves in the valve body (control center) in opposition of TV, Bowden cable/ rod, control pressure, making the connection between main pressure and the appropriate band and clutch drums
Mine doesn't shift perfect, but not too far off to warrant messing with it now. The main thing I don't like about mine is it wants to up shift too quickly under medium acceleration... it goes 2nd, 3rd, 4th rather quickly now. I might try a kick down switch just for fun.
Anyhow not sure if these are relevant, but attached are couple misc guides I've picked up along the way.
OOOPS, the good one was too big to upload... anyhow; following are some links I pasted together with some decent info.
PM me your email and I can forward another PDF files specific to our transmissions.
_____________________________________
Edit... Coverted PDF to word DOC and it fits.
______________________________________
http://forums.rennlist.com/rennforum...d.php?t=461390
http://forums.rennlist.com/rennforum...d.php?t=449831
http://forums.rennlist.com/rennforum...d.php?t=462655
https://rennlist.com/forums/928-forum/447460-automatic-trans-adjustment-write-up.html
https://rennlist.com/forums/928-forum/410690-transmission-problem-question.html
https://rennlist.com/forums/928-forum/429605-question-about-b2-piston-and-b2-band.html
https://rennlist.com/forums/928-forum/424255-looks-like-the-82-trans-needs-help-update-warning.html
https://rennlist.com/forums/928-forum/402994-auto-transmission-has-adjusting-vacuum-modulator-caused-early-upshifts.html
https://rennlist.com/forums/off-topic/387535-how-hard-is-it-to-rebuild-an-auto-trans.html
https://rennlist.com/forums/928-forum/430016-at-trans-failure.html
https://rennlist.com/forums/928-forum/444764-s4-a-t-rebuild-thread.html
https://rennlist.com/forums/928-forum/421142-b2-brake-band-replacement-adjustment.html
https://rennlist.com/forums/928-forum/333680-transmission-help.html
https://rennlist.com/forums/928-forum/260643-strange-automatic-leak.html
https://rennlist.com/forums/928-forum/249966-so-how-do-you-change-auto-trans-fluid.html
https://rennlist.com/forums/928-forum/238442-auto-trans-slips-in-all-gears.html
https://rennlist.com/forums/928-forum/191040-automatic-trannsmision-skipping-2nd-gear.html
https://rennlist.com/forums/928-forum/133501-achtung-achtung-autotrans-rebuild.html
The primary pump, supplies pressurized fluid to the hydraulic circuit. This pressure applies bands, clutches and lubrication for all moving parts. The most hydraulic circuits are found in the valve body. As the fluid moves into the valve body it has the potential to actuate every single valve. When the valves move they redirect the pumps pressure to control the application of bands and clutches thus providing up and down shifts and different ratios through the planetary system. The pumps working pressure is always the highest pressure in the hydraulic system.
All other pressures are derived from this maximum pressure and is reduced and controlled by regulating valves. The primary control valve is the pressure regulator valve. Mercedes calls this valve the control valve working pressure. It controls and regulates the amount of working pressure in the transmission. This pressure varies depending on which way the valve moves. If the valve moves towards the spring, which sometimes breaks, we have low working pressure. If it does not move, or moves a little, we have high working pressure. There are other pressures that balance and regulate the pressure regulator but when we have 210 pounds of working pressure in a vehicle in drive, the problem is usually the pressure regulator.
The governor is the road speed input from the transmission. It has no regulating pressure from a stop and does not control a first gear start in any valve body. In a Mercedes first gear start valve body, the 1-2 command valve is stroked in the first gear start position; by design once the car is started, commanding the B2 band on and the F, one way clutch locked. If the valve body were a second gear start, then the one two command valve would be stroked in the 2nd gear start position commanding the B1 and B2 bands on.
The description of operation in that PDF is for a 1996 C22, which Is a 722.4 transmission that starts in second gear.
What they are referring to by the engagement dampening circuit, is the accumulator cutting in oil circuit wish is a dampening system designed to soften shift engagements from a dead stop (not at road speed):
The dampening circuit just cushions the selected gear engagement.
N to D = B1 and B2 = 2nd Gear
N to 3 = B1 and B2 = 2nd gear
N to 2 = K1 and b2 and f is locked = 1st gear
N to R = K2 and B3 = Reverse
In the neutral position there is no power flow between the engine and transmission, no bands or clutches are applied. This is also the towing and push start feature of the car.
By moving the transmission lever from neutral to drive at road speeds, you are going from a pressure less system (neutral) to a high pressure system (drive). At that high pressure you are probably freeing up whatever is stuck in the valve body.
I think the problem is in the valve body. If that car was at my shop I would change it.
In any 722.3-4-5 MB or Porsche 928 if you move the selector lever manually to second, the transmission will start in first gear. This is engine breaking. Try that as a test.
How a transmission shifts is based/ derived from a balance /unbalance of pressures
The main plays are
Governor, road speed input oil.
TV Bowden cable / road load input
Modulator pressure, load and speed dependent.
Working pressure, pump
Shifts happen when governor pressure moves the shift / command valves in the valve body (control center) in opposition of TV, Bowden cable/ rod, control pressure, making the connection between main pressure and the appropriate band and clutch drums
