> After very carefully checking the level and draining and re-filling  just to be sure I am very confident this is not a low fluid problem.  The fluid I did take out was a little dark but still red and did not smell burned but not like fresh fluid either. I still have the problem  all backward, no forward in my 84S AT. I was reading the manuals and found troubleshooting guide which gave me some things to look at  including the B2 piston. It appears to get ot these parts I am going to need a special tool. To me it looks as if this special tool only presses in  the cover that's hiding the B2 "mechanism" so you can remove the snap ring and then get to the mechanicals. Is this something that a person  with no Automatic Trans experience can do or is it time for a  rebuilt/used trans? Anyone been there ??~M

Hi Mark,

I just read through some of the last digests, and saw both of your messages. The quick answer is YES, I have been there and done that, and you can too. I was a bit confused by the symptoms in your first message, but if you're now saying that you have only reverse and no forward, then you are definitely looking at the B2 brake band piston.

Attached below is the final message that I sent earlier this year after resolving my problem (apologies in advance for the length). You should also do a search of the RennList archives and you will find a few other messages (I just checked and they're all there) about my problem, and what I went through to diagnose and repair it (with the help of the great folks on this list). On the morning mine broke, I drove the car all the way back home in reverse, but that's a whole another story :-).

Unfortunately, there is not much of a knowledge base for the automatic transmissions (especially when compared to the other systems of the 928).
I remember seeing on one of the many websites that someone had archived my messages, but I can't remember which site it was. With all the things that could possibly go wrong on the AT, you may be quite lucky to have the identical problem to mine. Good luck, and please don't hesitate to contact me with any questions.

Best regards, Mark
Black 84 S in Atlanta
(shaken and stirred daily)

-----Original Message-----
Sent: Wednesday, June 18, 2003 10:40 PM
From: Reifenberger, Mark
Subject: Auto Trans Repair - Another 928 Back on the Road

Got home this afternoon to find a box of parts, right on schedule. Spent a couple of hours reassembling everything and then pumped it full of trans fluid. Dropped it down off the jack stands and it ran perfect. I'm pretty darn proud, especially that I didn't allow myself to be intimidated by such a complicated device.

Aside from the down time and my labor, here is what the repair cost me:
1072700432 - B2 Brake Band Piston = $87.29
0059977048 - B2 Piston Seal Ring = $3.00
1262771180 - Separator Plate Gasket = $1.89
1262770295 - New AT Filter = $10.09
1262711080 - New Pan Gasket = $5.89
TOTAL = $108.16

I had to do some searching, but I found a great web site for the Mercedes parts. No sales tax, and free shipping included. These prices are cheaper than any of the other sites I found, and the parts appear to be Mercedes OEM parts.

I want to thank everyone for all of the excellent assistance and guidance that I received on this job. As with most DIY projects, aside from saving some money, it is also very satisfying. This group is just the best, and I am very grateful to be a part of it.

Best regards to all, Mark
Black 84 S in Atlanta
(back in bwaaahhhsness again)

-----Original Message-----
Sent: Thu, 8 May 2003 11:20:54 -0400
From: Reifenberger, Mark
Subject: Automatic Transmission Problem

After being gone on travel a good bit lately, I finally got some time to dig into my transmission problem (original message attached below for reference).

I drained all of the trans fluid, and was very surprised at how good it looked (like brand new). Nice and red, with no discoloration at all, and no burnt smell. I removed the pan, and it was quite clean on the inside, with no residue or powder of any kind. I started to conclude that my problem was not due to contamination or crud, and was beginning to abandon my original plan to just try replacing the filter and fluid to see if it changed anything.

I read the service manuals on my flights, including the A28 transmission specific ones (thanks Theo). From everything I could understand (AMAZINGLY complicated device BTW), I kept thinking that it had to be a snapped B2 brake band. The failure was just too sudden and discreet, and there was no slipping or any other indication prior. This also conforms with reverse being completely unaffected and still operating totally normal.

The B2 brake band grabs the drum of the outer planetary gear set, which must be held motionless in order to move the car forward (in any gear).
in reverse, B2 is released allowing the B2 drum to spin, which would explain why reverse is still fine.

I removed the main valve body, the rear valve body, and the back plate, but that only exposed slightly more of the transmission innards. From what I could see (lying on my back looking up into it), nothing was broken. I decided to try to remove the B2 piston that actuates the band. It is accessed through a round cover on the side of the transmission housing (probably not ever intended to be removed with the trans still in the car).

Using special tool # 928-Reif-A28-A (which I had made a couple years ago to extract the front main seal) I was able to remove the retaining ring. The tool is a 3/16 flat blade Craftsman screwdriver that I had SLOWLY bent an inch of the tip into a right angle using a torch (amazing how many jobs since then that I've used that tool). Once I got the ring off and slid the cover out, Eureka! The top portion of the die cast piston was broken, and thus not actuating the brake band.

A picture of the part that broke is shown in the main workshop service manual on page 37-170 of volume 3. It is called the B2 brake band piston.
It has the following information cast into it:
MERCEDES-BENZ R 1262770538. So does anyone know where I can find a replacement part for this? Do any of the big three sell this (Wally? Jim?)?
I have not had a chance to make any phone calls yet, but I wonder if I will need to source it from a Mercedes parts supplier.

In hind sight, I didn't even need to remove anything except for the fluid, to get this part out, but hey, it was exploratory surgery on a device I've never worked on before. Along with a new piston, I'll also need to order a new filter, pan gasket, and valve body gaskets. I'm glad to have found the problem, but am not out of the woods until it all goes back together successfully. I am most nervous about the valve body. I removed it very carefully without disturbing anything, so I'm hoping that with a new gasket, I can get it back on properly.

Wow, sorry this got to be so long. If you've made it this far, thanks very much for reading it. Any advice will be very much appreciated. Please copy me direct, as I'm in digest mode.

Best regards, Mark
Black 84 S in Atlanta

-----Original Message-----
Sent: Friday, April 18, 2003 3:02 PM
From: Reifenberger, Mark
Subject: Automatic Transmission Problem

PATIENT: 1984 S Auto with 124k miles - Normally a rock solid daily driver.

SITUATION: Pulling out of a side street onto a main road first thing in the morning (1/2 throttle). Car starts to pull away like normal, and then transmission simply uncouples and free revs like someone bumped it from drive into neutral. Quickly pull over and play with shifter but no change.
Reverse is totally unaffected. Drove the car all the way home (1/2 mile) in reverse, luckily most of it within my subdivision. Parked it, and took one of my other cars to work.

STATUS: Went down this morning and started it, ran through the gears, and immediately checked the fluid level, and was surprised to find that it was indeed low. Took about 6 ounces to bring it back up to the bottom (20 degrees C) line. Didn't seem to help the problem. When I shift into D, I get no perceivable change in rpm, although after a minute or two sitting in D, I get a small rpm flare when I shift it back to N, as if sitting in D had slowly (slightly) loaded it. Also, if I put the shifter all the way down in 2, I can just get the car to drive away slowly, although still with LOTS of slippage, and some extra vibration. And again, reverse is totally unaffected and normal.

QUESTION: Waddayathink? Serious damage? Anything I can/should check?
Please copy me direct.

Thanks in advance, Mark
Black 84 S in Atlanta