Auto Trans FAQ Facts and Answers
12-01-2012, 01:16 AM
#17
Nordschleife Master
Thread Starter
Flares
This thread is a keeper for OB models! Fixed the problem on my 86.5. Ordered ther kit from my local MB dealer for a grand total of $20.
https://rennlist.com/forums/928-foru...rt-number.html
https://rennlist.com/forums/928-foru...rt-number.html
I would ask involved parties to elaborate here if possible (Tony?).
How to install...
http://www.benzworld.org/forums/w201...3-722-4-a.html
Last edited by G8RB8; 04-10-2013 at 01:50 PM. Reason: Add HOW TO
12-01-2012, 04:05 PM
#18
Nordschleife Master
Thread Starter
B-2 piston failure
B-2 piston failure
About That Infamous B2 Piston
Why it breaks
The broken B2 piston problem is a well understood and easy to fix transmission problem with certain Mercedes 722.xxx automatic transmissions found in 126, 123 and 107 body cars. Although the car is drivable with a busted B2, it's a bit tricky. There is a school of thought that says any pre 1994 MB with one of these should be replaced as preventative maintenance.
Busted B2 Piston
Broken B2 Piston
In this picture you can see two parts to the B2 piston. The problem is it is supposed to be one piece, not two!
There were a couple of design flaws with this part, common to most 80s and early 90's Mercedes cars.
The piston actuates the B2 band, that is it helps shift gears. It's about 3" in diameter and this piston has about a 3" stroke. There is a deal around the outside of the piston, much like a piston ring. This piston rides in a metal sleeve.
The seal was originally round, and the sleeve was metal. I saw was, because that design had problems that usually show up bewteen 100 and 200 thousand miles.
What happens is the piston is activated and pushed inward in its bore. But, the round seal sometimes gets stuck (for whatever reason) and hydrauic pressure builds up to tremendous force so eventually it does move but by now there's so much pressre that when it does move it does so with increadable fource and eventually it snaps' the 3" collar and the flat disc separate by breaking at the base. That's what happeneed to that poor piston above. it happens to most of them eventually.
Mercedes had several upgrades to these parts over the years and now we have new improves beefier B2 pistons that have a square seal instead of a round one, and a plastic sleve instead of a metal one. That seems to have fixed the problem. These new B2 pistons do not seem to fail.
The symptoms of a B2 piston failure are unmistakable:
Fails without any warning whatsoever
Slips out of gear in 1st with any load
Massive flaring going into second
3rd and 4th work normally
Reverse works normally
They fail anywhere between 100K and (in my case) 400K miles; chances are very high your first major problem with this transmission will be this one.
But, things could be worse, the car is drivable when this failure occurs, although it takes some patience. It's worse when cold, and is made worse by how much load you put on it. The object of driving a car with a bad B2 piston is to never give it much load in 1st and 2nd gears.
Think of parking on a hill facing down. You want to get the car into 3rd as soon as you can and 3rd and 4th are unaffected by even catastrophicB2 failure. You might even consider pulling the transmission vacuum line so it shifts way much too early into 3rd. I did. Seems to make driving in a car with this problem a little easier.
On level ground what you'll find is it will go into gear once you put it in drive but will pop out almost immediately after you give it even the smallest amount of throttle. But, then it'll go back into great and now you might actually be able to get going withvery light throttle. Or it may pop out again and you're ok now till it shifts into second and you get to hear it pop out again, then it'll go back in and you'll be ok now. It'll probably shift into 3rd and 4th quite normally, so floor it and don't stop. If you stop you have to go through this ordeal of starting the car moving again. Starting the car going uphill is a near impossability, but it can be done. It's unlikely the car behind you will have the same kind of patience you have though. Wave and smile.
Once the B2 piston has failed it's unknown how long you can actually drive it for before it pack in completely. I drove mine aroiund for 3 months till it warmed up enough outside to be able to fix it. It is not the end of the world.
The good news is the repair can be made while the transmission is in the car in the case of a 126 it's pretty easy - in other chassis the trans needs to be dropped just a little although people have reported success, with some effort, of doing it to 123 and 107 chassis with the tranny in place. It's not a terribly expensive repair either, the part is about $100 and it'll take you a couple of hours. While there's a few steps to this it's really only a little worse than changing a spark plug and you don't need of be a master mechanic to do this.
Here's how
Credit to: Richard Sexton
About That Infamous B2 Piston
Why it breaks
The broken B2 piston problem is a well understood and easy to fix transmission problem with certain Mercedes 722.xxx automatic transmissions found in 126, 123 and 107 body cars. Although the car is drivable with a busted B2, it's a bit tricky. There is a school of thought that says any pre 1994 MB with one of these should be replaced as preventative maintenance.
Busted B2 Piston
Broken B2 Piston
In this picture you can see two parts to the B2 piston. The problem is it is supposed to be one piece, not two!
There were a couple of design flaws with this part, common to most 80s and early 90's Mercedes cars.
The piston actuates the B2 band, that is it helps shift gears. It's about 3" in diameter and this piston has about a 3" stroke. There is a deal around the outside of the piston, much like a piston ring. This piston rides in a metal sleeve.
The seal was originally round, and the sleeve was metal. I saw was, because that design had problems that usually show up bewteen 100 and 200 thousand miles.
What happens is the piston is activated and pushed inward in its bore. But, the round seal sometimes gets stuck (for whatever reason) and hydrauic pressure builds up to tremendous force so eventually it does move but by now there's so much pressre that when it does move it does so with increadable fource and eventually it snaps' the 3" collar and the flat disc separate by breaking at the base. That's what happeneed to that poor piston above. it happens to most of them eventually.
Mercedes had several upgrades to these parts over the years and now we have new improves beefier B2 pistons that have a square seal instead of a round one, and a plastic sleve instead of a metal one. That seems to have fixed the problem. These new B2 pistons do not seem to fail.
The symptoms of a B2 piston failure are unmistakable:
Fails without any warning whatsoever
Slips out of gear in 1st with any load
Massive flaring going into second
3rd and 4th work normally
Reverse works normally
They fail anywhere between 100K and (in my case) 400K miles; chances are very high your first major problem with this transmission will be this one.
But, things could be worse, the car is drivable when this failure occurs, although it takes some patience. It's worse when cold, and is made worse by how much load you put on it. The object of driving a car with a bad B2 piston is to never give it much load in 1st and 2nd gears.
Think of parking on a hill facing down. You want to get the car into 3rd as soon as you can and 3rd and 4th are unaffected by even catastrophicB2 failure. You might even consider pulling the transmission vacuum line so it shifts way much too early into 3rd. I did. Seems to make driving in a car with this problem a little easier.
On level ground what you'll find is it will go into gear once you put it in drive but will pop out almost immediately after you give it even the smallest amount of throttle. But, then it'll go back into great and now you might actually be able to get going withvery light throttle. Or it may pop out again and you're ok now till it shifts into second and you get to hear it pop out again, then it'll go back in and you'll be ok now. It'll probably shift into 3rd and 4th quite normally, so floor it and don't stop. If you stop you have to go through this ordeal of starting the car moving again. Starting the car going uphill is a near impossability, but it can be done. It's unlikely the car behind you will have the same kind of patience you have though. Wave and smile.
Once the B2 piston has failed it's unknown how long you can actually drive it for before it pack in completely. I drove mine aroiund for 3 months till it warmed up enough outside to be able to fix it. It is not the end of the world.
The good news is the repair can be made while the transmission is in the car in the case of a 126 it's pretty easy - in other chassis the trans needs to be dropped just a little although people have reported success, with some effort, of doing it to 123 and 107 chassis with the tranny in place. It's not a terribly expensive repair either, the part is about $100 and it'll take you a couple of hours. While there's a few steps to this it's really only a little worse than changing a spark plug and you don't need of be a master mechanic to do this.
Here's how
Credit to: Richard Sexton
Last edited by G8RB8; 12-01-2012 at 04:59 PM.
12-01-2012, 05:02 PM
#21
Nordschleife Master
Lots of info in one place
Pretty much all the info you need in one place here on Theo's site:
http://jenniskens.livedsl.nl/Technic...T_TransAut.htm
Including an excellent B2 piston replacement write-up by rennlister Tails, with photos etc.
It also has various Merc bulletins for troubleshooting specific problems with the 722 boxes, as well as factory manuals etc.
http://jenniskens.livedsl.nl/Technic...T_TransAut.htm
Including an excellent B2 piston replacement write-up by rennlister Tails, with photos etc.
It also has various Merc bulletins for troubleshooting specific problems with the 722 boxes, as well as factory manuals etc.
12-02-2012, 04:41 PM
#22
Nordschleife Master
Thread Starter
Shifter Cable Replacement
Transmission, Automatic
Shifter Cable Replacement
>Can someone tell me (in general) what parts to remove to replace the shifter
>cable?
>
>928S Euro Auto 1981
1) Remove ground cable.
2) Remove shift handle and rubber boot.
3) Remove cover frame.
Put lever in "2" and push rear locking bar forward against stop with a scribe.
Put lever in "R", disconnect gate and push as far forward as possible.
Push front locking bar forward against stop with a scribe.
Put lever in "P" and carefully pop out left rear corner of the cover frame.
Put lever in "N" and remove cover and frame to the rear.
To Install:
Put lever in "3".
Push gate in frame forward all the way.
Insert cover and frame, and move lever to "P".
Lift left rear corner slightly and insert right and front of cover and frame, then push down on left rear.
Put lever between "R" and "P", disconnect gate on selector lever and push forward.
Push front locking bar toward rear until it locks.
Put lever in "3" and push rear locking bar back until it locks.
Reinstall gate, rubber boot and shift handle.
4) Pull bulb carrier from clips.
5) Mark selector base, then remove mounting screws.
6) Loosen intermediate muffler shield and push aside.
7) Disconnect selector cable from transmission lever and detach cable sleeve on brackets.
8) Take off ball head, hex nut and mounting parts.
9) Attach a strong wire to cable at transmission, and then pull selector base and attached cable forward and out.
10) Remove cable from base.
11) Reinstallation is in reverse order.
Wally Plumley
928 Specialists
Shifter Cable Replacement
>Can someone tell me (in general) what parts to remove to replace the shifter
>cable?
>
>928S Euro Auto 1981
1) Remove ground cable.
2) Remove shift handle and rubber boot.
3) Remove cover frame.
Put lever in "2" and push rear locking bar forward against stop with a scribe.
Put lever in "R", disconnect gate and push as far forward as possible.
Push front locking bar forward against stop with a scribe.
Put lever in "P" and carefully pop out left rear corner of the cover frame.
Put lever in "N" and remove cover and frame to the rear.
To Install:
Put lever in "3".
Push gate in frame forward all the way.
Insert cover and frame, and move lever to "P".
Lift left rear corner slightly and insert right and front of cover and frame, then push down on left rear.
Put lever between "R" and "P", disconnect gate on selector lever and push forward.
Push front locking bar toward rear until it locks.
Put lever in "3" and push rear locking bar back until it locks.
Reinstall gate, rubber boot and shift handle.
4) Pull bulb carrier from clips.
5) Mark selector base, then remove mounting screws.
6) Loosen intermediate muffler shield and push aside.
7) Disconnect selector cable from transmission lever and detach cable sleeve on brackets.
8) Take off ball head, hex nut and mounting parts.
9) Attach a strong wire to cable at transmission, and then pull selector base and attached cable forward and out.
10) Remove cable from base.
11) Reinstallation is in reverse order.
Wally Plumley
928 Specialists
12-02-2012, 04:42 PM
#23
Nordschleife Master
Thread Starter
Replacing Burned-out "Drive" Indicator Light
Transmission, Automatic
Replacing Burned-out "Drive" Indicator Light
>yesterday the light for 'Drive' [shift sequence] in the instrument cluster went dead.
>all the other pattern lights work except for 'Drive.' am i going to have to pull
>the cluster and replace a dead bulb? are there contacts under the manual
>shifter that turn these lights on? hopefully -- i don't know why porsche
>would do this -- it is something pathetically easy like replacing a burnt fuse.
Q1. Yes
Q2. No
Q3. No.
Drive bulb burns out, because it is ON all the time. Direct replacement bulbs are unavailable. They are 12V, however. Maybe you can look for one at a large electronic parts supplier. If the bulb is replaced with a different one with slightly different wattage, it may be dimmer (or brighter), than what you are used to. Instrument cluster must be removed from pod, then tachometer must be removed from cluster. Tachometer must be taken apart - only difficult part is pulling the pointer off without breaking it (good luck). To remove the pointer, you need the equivalent of a gear puller to grab the round part of the pointer and pull evenly, otherwise the shaft may be damaged. Once it's off, the circuit board can be unscrewed from the face assembly, the bulb removed, and a new one soldered in place.
Send it to Hollywood Speedometer (?), advertisers in Excellence. They can replace bulb for you. Or buy a used tach that has all the lights operational (contact 928 International). If you have the bulb replaced, better have all of them replaced at the same time, because PARK will be the next to go, maybe REVERSE. Replacing them all will assure that they all illuminate with the same level of brightness (and you don't want to have to pull the tach out again, if you can avoid it).
*Steve; 87S4/auto/GPW
928 Tips Home Greg's Home
Replacing Burned-out "Drive" Indicator Light
>yesterday the light for 'Drive' [shift sequence] in the instrument cluster went dead.
>all the other pattern lights work except for 'Drive.' am i going to have to pull
>the cluster and replace a dead bulb? are there contacts under the manual
>shifter that turn these lights on? hopefully -- i don't know why porsche
>would do this -- it is something pathetically easy like replacing a burnt fuse.
Q1. Yes
Q2. No
Q3. No.
Drive bulb burns out, because it is ON all the time. Direct replacement bulbs are unavailable. They are 12V, however. Maybe you can look for one at a large electronic parts supplier. If the bulb is replaced with a different one with slightly different wattage, it may be dimmer (or brighter), than what you are used to. Instrument cluster must be removed from pod, then tachometer must be removed from cluster. Tachometer must be taken apart - only difficult part is pulling the pointer off without breaking it (good luck). To remove the pointer, you need the equivalent of a gear puller to grab the round part of the pointer and pull evenly, otherwise the shaft may be damaged. Once it's off, the circuit board can be unscrewed from the face assembly, the bulb removed, and a new one soldered in place.
Send it to Hollywood Speedometer (?), advertisers in Excellence. They can replace bulb for you. Or buy a used tach that has all the lights operational (contact 928 International). If you have the bulb replaced, better have all of them replaced at the same time, because PARK will be the next to go, maybe REVERSE. Replacing them all will assure that they all illuminate with the same level of brightness (and you don't want to have to pull the tach out again, if you can avoid it).
*Steve; 87S4/auto/GPW
928 Tips Home Greg's Home
12-02-2012, 07:11 PM
#24
Rennlist Member
Master search thread is fine.
But what we need are step by step rebuild instructions.
That is where we fall short.
One guy did a great documentation job of disassembly, but quit on reassembly.
But what we need are step by step rebuild instructions.
That is where we fall short.
One guy did a great documentation job of disassembly, but quit on reassembly.
12-03-2012, 07:52 AM
#26
Instructor
Join Date: Oct 2005
Location: Finland
Posts: 137
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More about sift point adjustment
Found two other ways to adjust the 4-gear AT-box shifting points.
Govenor plastic adjustment screws:
https://rennlist.com/forums/928-foru...ed-only-2.html
on mine, it turned out to just be the govenor adjustment - 2 white plastic screws that we turned out 2 turns each. takes about 20 min to pull it out, 2 min to turn the screws, 20 min back. easy to do.
turning the screws 2 turns out on my old gal put the 1-2 shift consistantly at 6100, others are just above 6200/redline finally. the old gal rips now!! it's about 3mph faster (64mph) on my 600' driveway (and back to 0) yeehaa! can't wait to put it back on the track to see what that means in the 1/4mi
all shifts were higher, but 1-2 was noticably higher/now at redline from 4500-5400 before. other shifts at about 6200/a tad over redline but not enough to worry about on this motor, imo and it just seems faster overall as a result, but the track timers will tell the story better than my assometer
two screws are on the govenor body. it pulls out the side of the trans, and you can't miss'em - one on each side/white plastic.
I also did the trans bypass switch, but it only seems to affect shifting under part throttle (ie nicely drops down a gear when activated w/out any additional throttle input), and forces the trans into 1st while moving at low speeds w/no throttle input (impossible to do before).
I've also found some articles that indicated a low rpm shift (assuming everything is adjusted properly) could be from a dirty valve body circut or bad/spring or worn 1st gear clutch packs.
In fornt of the valvebody:http://avtopedia.ru/akpp/722-3%20722-4.pdf
Page 16. The arrow indicates the position of allen screw for full throttle control pressure. Turning clockwise results in earlier full load or kick-down upshifts. Turning counterclockwise results in later upsihfts.
I have seen both screws, but haven't touched them yet. Has anyone else used those before?
Some other related links and information:
More about WOT shifting issues and tuning: http://www.w124performance.com/docs/...pshift_RPM.pdf
More about modulator replacement and vacuum adjusting: http://www.benzworld.org/forums/w201...l-replace.html
Good S4 AT rebuild thread by atb:https://rennlist.com/forums/928-foru...ld-thread.html
About tranny pressures:
Hydraulic pressure controls Shifting.
-Hydraulic pressure is set by the primary pump and governor. This is NOT adjustable.
-Some pressure can be adjusted externally to control timing and quality.
-4 types of pressure work in concert:
a. Working Pressure: From the front driven primary pump, and its the basis of all pressures. Cannot be adjusted, but can be measured by a port.
b. Governor Pressure: Builds pressure with centrifugal force in a gradual proportion as the speed of the tranny increases. It works directly against control pressure to regulate the shift depending on vehicle speed. Cannot be adjusted, but can be measured by a port with the car driven a certain speeds.
c. Modulating Pressure: controls the working pressure, from partial to full throttle. Modulator can be adjusted to harden or soften the spring inside it. One way is constant, by turning the key inside it and the other way is constantly changing because is vacuum. This vacuum is also controlled by the VCV which leaks vacuum in proportion of the throttle position. Modulating pressure controls the shifts by varying the applied pressure to the bands on the clutches. Modulating pressure acts as a control for the working pressure in proportion to the torque of the engine. Can be adjusted by turning the key in the modulator and checking the pressure in the port close to it.
d. Control pressure: This is relative to the pedal position. Obtained from the modulator by means of the control pressure valve, which is influenced by the linkage or cable from the tranny to the accelerator linkage.
Govenor plastic adjustment screws:
https://rennlist.com/forums/928-foru...ed-only-2.html
on mine, it turned out to just be the govenor adjustment - 2 white plastic screws that we turned out 2 turns each. takes about 20 min to pull it out, 2 min to turn the screws, 20 min back. easy to do.
turning the screws 2 turns out on my old gal put the 1-2 shift consistantly at 6100, others are just above 6200/redline finally. the old gal rips now!! it's about 3mph faster (64mph) on my 600' driveway (and back to 0) yeehaa! can't wait to put it back on the track to see what that means in the 1/4mi
all shifts were higher, but 1-2 was noticably higher/now at redline from 4500-5400 before. other shifts at about 6200/a tad over redline but not enough to worry about on this motor, imo and it just seems faster overall as a result, but the track timers will tell the story better than my assometer
two screws are on the govenor body. it pulls out the side of the trans, and you can't miss'em - one on each side/white plastic.
I also did the trans bypass switch, but it only seems to affect shifting under part throttle (ie nicely drops down a gear when activated w/out any additional throttle input), and forces the trans into 1st while moving at low speeds w/no throttle input (impossible to do before).
I've also found some articles that indicated a low rpm shift (assuming everything is adjusted properly) could be from a dirty valve body circut or bad/spring or worn 1st gear clutch packs.
In fornt of the valvebody:http://avtopedia.ru/akpp/722-3%20722-4.pdf
Page 16. The arrow indicates the position of allen screw for full throttle control pressure. Turning clockwise results in earlier full load or kick-down upshifts. Turning counterclockwise results in later upsihfts.
I have seen both screws, but haven't touched them yet. Has anyone else used those before?
Some other related links and information:
More about WOT shifting issues and tuning: http://www.w124performance.com/docs/...pshift_RPM.pdf
More about modulator replacement and vacuum adjusting: http://www.benzworld.org/forums/w201...l-replace.html
Good S4 AT rebuild thread by atb:https://rennlist.com/forums/928-foru...ld-thread.html
About tranny pressures:
Hydraulic pressure controls Shifting.
-Hydraulic pressure is set by the primary pump and governor. This is NOT adjustable.
-Some pressure can be adjusted externally to control timing and quality.
-4 types of pressure work in concert:
a. Working Pressure: From the front driven primary pump, and its the basis of all pressures. Cannot be adjusted, but can be measured by a port.
b. Governor Pressure: Builds pressure with centrifugal force in a gradual proportion as the speed of the tranny increases. It works directly against control pressure to regulate the shift depending on vehicle speed. Cannot be adjusted, but can be measured by a port with the car driven a certain speeds.
c. Modulating Pressure: controls the working pressure, from partial to full throttle. Modulator can be adjusted to harden or soften the spring inside it. One way is constant, by turning the key inside it and the other way is constantly changing because is vacuum. This vacuum is also controlled by the VCV which leaks vacuum in proportion of the throttle position. Modulating pressure controls the shifts by varying the applied pressure to the bands on the clutches. Modulating pressure acts as a control for the working pressure in proportion to the torque of the engine. Can be adjusted by turning the key in the modulator and checking the pressure in the port close to it.
d. Control pressure: This is relative to the pedal position. Obtained from the modulator by means of the control pressure valve, which is influenced by the linkage or cable from the tranny to the accelerator linkage.
Last edited by simos; 12-04-2012 at 08:56 AM. Reason: Some additions
02-17-2013, 08:47 AM
#29
Nordschleife Master
Thread Starter
How to Properly Fill the Trans
https://rennlist.com/forums/928-foru...luid-help.html
A thread describing the location of the fill tube, and how to use it.
A thread describing the location of the fill tube, and how to use it.
02-17-2013, 01:37 PM
#30
Three Wheelin'
Here is an old thread for slow shift from park, seems like it could be worn clutches... My car has a delay going to reverse from park.
https://rennlist.com/forums/928-foru...o-reverse.html
https://rennlist.com/forums/928-foru...o-reverse.html