B2 piston in transmission - PeachParts Mercedes-Benz Forum

#1 08-18-2002, 08:52 PM

Is there any prior warning that the B2 piston in the transmission is about to fail, or does it just go and trash the tranny?

Did these pistons fail early on or as the car aged? If the car has 99,000 miles on it, does it mean it will hold up?

Any thoughts appreciated - Ron

stevebfl · #2 08-18-2002, 09:13 PM

Which tranny?

Todd W126 · #3 08-18-2002, 10:39 PM

He may be talking about this article........

http://business.baylor.edu/Richard_Easley/autofaqs/b2piston.htm

Pretty much runs down the models that are allegedly affected, but only lists the tranny #'s as "specific numbers within the 722.XXX category". ( DOH! )

I had wondered about the frequency of failure with the B2 piston as well, being an owner of a W126.

The article also states...

"The W126 chassis (S body) is the only affected chassis in the Mercedes-Benz line that will permit easy removal/replacement of the B2 piston with the transmission remaining in the vehicle. "

I guess that's a good thing(??arrgh??) in my case. Thankfully, my car exhibits none of the symptoms he references in the article.......( knock on wood!

)

stevebfl · #4 08-19-2002, 07:51 AM

So we are talking 722.3/4 transmissions. The B2 band piston was redesigned early in the life of that transmission series. 126 cars would probably have the early band. The redesign was to the outer sealing edge. Not only does the late seal work better and cut wear in the bore, I think the valving in the piston was redesigned to aid in un applying the band (has to do with fourty gear flare).

The piston can be replaced in the car in most cases. It will be included in any competent rebuild. I don't think I would worry about it if the trans shifted good into 4th.

#5 08-19-2002, 10:45 AM

Actually, we are talking about the 722.3/.4/.5 transmissions.

Steve is correct in that the outer sealing ring was modified from a flat ring to a "T" ring, but there are other important modifications as well. The early style B2 piston's mold was changed to beef it up to reduce the potential for breakage. The sleeve in which the B2 piston operates was changed from metal to plastic to reduce the possibility of binding (and it is imperative that this sleeve be changed). [It is important to note that this is an important purpose for the improved "T" ring: there is more surface area to contact the outer bore, but even more importantly, the outer parts of the "T" seal effectively preclude piston-to-bore contact which could result in binding, not to mention wear of the outer bore's surface.] Finally, the release clearance specificaton for the B2 piston was changed by the factory to reduce delayed engagement complaints.

The B2's design was improved in the late 80s, so there are still a lot of MBs running around with the old-style B2. And, the addition of the "T" sealing ring purportedly did not occur until late 93/early 94 but as I am writing this, I seem to remember that the 1990 722.3 transmission that I rebuilt this summer had the "T" seal, though it could have been changed due to shift complaints sometime in its life.

Todd (DOH!), the reason for my listing of specific transmissions in the FAQ as being 722.XXX was simply an artifact at that time of wanting to get the FAQ online as quickly as possible for the benefit of others -- I had forgotten that this had never been changed. Though the clock FAQ that I produced with Jim Mahaffey has probably been useful to greater numbers of people, I am particularly proud that the B2 FAQ has saved dozens of MB owners many thousands of dollars in needless transmission overhauls. I cannot tell you how many individuals have e-mailed me to thank us for providing that FAQ -- which saved them from purchasing a rebuilt transmission. Note that the B2 piston is ~$100, so for less than $500 certainly, and likely $300 or so, your transmission is working properly again in most cases.

Related to the above, I would mildly disagree with Steve on the notion that one should not worry about it if the car is shifting fine into 4th gear and that it would be included in any competent rebuild. I certainly understand Steve's point, but we are talking about a potential catastrophic failure here that will absolutely leave you stranded if/when it happens -- and it may not be at a convenient time. And -- you never know when it will happen. I was lucky -- mine happened as I was leaving work and I was on a quiet residential street. Absolutely no symptoms until failure either.

I would strongly recommend preventative maintenance on this item if you have one of the affected cars and that would be roughly any pre-94 MB for the "T" sealing ring and pre-87-89 for the sturdier B2 piston. Note that if you have a catastrophic failure with a B2 -- and you are away from your trusted shop -- it is very unlikely that a shop will replace the B2 piston alone, particularly if it is a non-MB transmission shop. Nonetheless, I would recommend that you at least print out the FAQ discussed and keep it in your car. This way, if you haven't replaced the B2 and it fails, a shop might be more willing to do the replacement for you if it happens.

Steve's point about competent shops is also well-taken, and is great advice for someone that does not do their own transmission work. We know that Steve does great work and is very knowledgeable, but there are shops out there that will take your money and not give you what you need or want -- in this case all of the MB upgrades in your transmission rebuild as described below.

Just as one example, MB does lots of upgrades on their components through the life of the cars, as illustrated by the B2 discussion above. Another important upgrade on an MB rebuild is to include the automatically-adjusting B1 piston which was phased in during production. How many shops know that this is even an option? Not many if they are not MB-specific. Related, how many 190 transmissions have been rebuilt without the inclusion of a less-than-$10 part that will prevent catastrophic failure of that transmission? [You'll know that you're in trouble when your 190 locks in reverse.]

Many participants in these on-line forums like to save a buck, but along with this objective must come the knowledge to know when saving money is truly the wisest path to follow. Bottom line is that you get what you pay for -- if someone quotes you $500 -- 1000 to "rebuild" your MB transmission, well, don't say I didn't warn you. Have shortcuts been taken? Do the math: ~11 hours flat rate for the rebuild (not including R&R) and the B2 is around $100, not including everything else -- including profit. Do yourself and your MB a huge favor and patronize a shop and individual like Steve who knows these cars and what is required to maintain them properly.

Richard Easley
Waco, Texas

#6 08-19-2002, 02:39 PM

Thank you all for your input ,especially Richard. I can now at that to my long list of "to dos", but since I have a 124 with limited working space as compared to a 126, I'll probably have to have a shop do it.

Thanks again - Ron

Todd W126 · #7 08-19-2002, 09:33 PM

Excellent replies here from both Richard and Steve !

Firstly, Richard......please do note that the "DOH" was simply my expressing my own confusion ( quite vast, heh ) with respect as to whether my tranny was one of which may be affected. Your original article was EXTREMELY informative and I do indeed keep a copy of it in my car for the very reason(s) you mention.

That being said, I must admit, I'm still a bit confused as to whether I should have this replacement done.

to wit:

"I would strongly recommend preventative maintenance on this item if you have one of the affected cars and that would be roughly any pre-94 MB for the "T" sealing ring and pre-87-89 for the sturdier B2 piston"

While Steve relates........"126 cars would probably have the early band. "

My car is a 1990 420 SEL, would it have the sturdier B2 piston?

also: "I seem to remember that the 1990 722.3 transmission that I rebuilt this summer had the "T" seal, though it could have been changed due to shift complaints sometime in its life"

While you do qualify your statement in that it may have been changed " sometime in it's life", it still leaves me wondering about a few things such as.....

What trans would be in my car?

What's a fair price for a shop to do this procedure? B2 replacement and T-seal as well as the price for just the T-seal in the event I already had the improved B2?

and what would it cost me for a "mech" to open up the trans, only to say " you already have the upgraded B2 and/or T-seal"? Steve?

I've got NO problem doing the preventive maintanence stuff as long as it's needed. I'm not a mechanic but I have a trusted one so I would have to pay to have it checked/ repaired/replaced etc. I'm just trying to make intelligent repair decisions and stay within the family budget.

In closing, I just have to say that I've learned more about this car of mine in the last 4 weeks since I bought it than I thought possible. Thanks mostly to people like Richard and Steve. Thanks in advance for any and all info on this matter.

Have to tell ya too.........I LOVE this car !!! There's a certain appeal to owning such a fine automobile that cost $62K new and paying less than 8 grand for it, especially one in this nice condition! If I have to put another 2-3K into it to do all the preventative stuff and get it "just right" , it's no big deal to me.

Cheers, Todd- doh! haha

greenacres2 · #8 10-09-2002, 12:34 AM

I sent the following e-mail to Richard Easley tonight and thought some of you who have followed this thread might have some input as well:
Dr. Easley,
I found your B2 FAQ through the MercedesShop.com news group. I'm part way through the replacement and have a few questions. First, some background:
Car: 1989 560 SEL (126 chassis, 722.350 transmission, 100,000 miles)
Symptoms: Flaring 2-3 shift since we bought the car last December with 86,000 miles. Intermittent flaring/slipping in 3-4 shift, worsening lately. Last Thursday, no 3-4 shift at all. None since.
Action: Ordered new B2 piston and bushing. Disassembled tonight to replace B2 piston, ran into the following issues and questions:
1. Pulled old piston out and the push rod came with it, falling into the drain pan. The push rod has a groove on its shaft, nearer to one end than the other. On re-assembly, will the grooved end go in toward center of transmission or toward the case?
2. The bore for the piston shaft has what appears to be a teflon bushing toward the inside of the transmission, with what appears to be a black rubber lip seal on the casing side, no teflon or steel sleeve on the casing side. Lip seal is damaged. Seems impossible to replace the existing bushing without removing the valve body. Is the new bushing used to replace the rubber seal, with the phlange on the new bushing facing the case side of the transmission? If so, do we just pull the rubber seal? If not, do you know if the rubber seal is an available part that can be replaced with the tranny in the car? I have seen no mention of this in the FAQ or any newsgroup discussions.
3. The old piston came out in one piece, and initially looked fine. Then we noticed that the new bushing did not slide smoothly over the end of the shaft of the old piston. End of shaft is very slightly mushroomed and has one rough spot. Enough to restrict movement. Leads us to think we're on the right track. Might you concur?
Any thoughts you have would be greatly appreciated. My "helper" is a veteran wrench on American engines and transmissions, new to MB. This is my first look inside the transmission. Thanks for a great website and your giving spirit.
Earl McLain

Any comments or help would be most useful. Thanks.

#9 10-09-2002, 05:31 PM

Dear Earl,

Last month I had a trans failure in my 91 190 and fixed it myself with the help of Steve B and others around the world. These are doable projects and with some care have excellent results. Take pictures as you dissasemble, a trip to one hour photo or a digital camera can be life saving, Make match marks to help in reassembly. Keep everything clean, wrap parts in newspaper as you dissasemble and rinse in clean atf as you assemble. Rely on your sources. This group, others on the net and your dealer will get you through.

Probably about a million people repair automatic transmissions. Of that 20%have got to be real jerks. Now if two hundred thousand jerks can do this job so can you.

Good Luck,

greenacres2 · #10 10-09-2002, 09:16 PM

Randy,
Well said. Got a response almost immediately from Richard Easely, found a source for the piston seal this morning and should have it by Friday. Hopefully, by Saturday noon we'll have a verdict on the B-2 repair. On the upside, the fluid and pan were both very clean, no metal or grit. Will keep you all posted.

#11 10-13-2002, 05:20 PM

I called my "trusted" mechanic and inquired about replacing the B2 piston. His wife (she sounds knowledgeable) always answers the phone said that the transmission (87 124 300D- 99,000 miles) has to be pulled and since it is already out, it should be rebuilt.

Ever since he missed a tranny leak that I told where the leak was and he replaced some other seal, I have questioned his judgement. I took the car back and said the leak was where I originally said it was and he said you are right.

I have experienced no problems with the tranny, but I do not want to have the unit fail in the middle of nowehre, where things like this usually happen, in my case at least.

Is my mechanic's wife justified to say I should completely rebuild?
We are talking major bucks.

Has anyone replaced the B2 piston on a 124? Can the tranny be "persuaded" to move enough while still on the vehicle to get at the B2 piston?

Any thoughts appreciated - Ron

#12 11-10-2002, 05:15 PM

Finally got the B2 piston replaced.

I went to another mechanic in Denver and he was willing to work with a DIYer. He gave me suggestions on several areas to improve performance. He is from Germany and his knowledge appears to be good and trustworthy. He was willing to talk to me like I had some knowledge of cars. He listened when I said the seal on the pressure control cable to the tranny was leaking and he replaced it. Previous mechanic replaced another seal. Took it back, and he said, yea, it is the pressure cable seal leaking.

At any rate, he was able to replace the B2 piston on the 124 without removing the tranny. He said it was difficult, but he was able to do it. Other mechanic wanted to pull tranny and do a complete rebuild.

Now, that is one less thing to worry about.

Ron

BDBENZ · #13 11-10-2002, 07:22 PM

It was mentioned that pre 87-89 would need the B2 piston upgrade. Is there a specific chassis number cutoff or would an 87 560 most likely need the upgrade?

Thanks for any information.

BD

#14 11-10-2002, 08:19 PM

BDBENZ - I am not sure about the dates. I think that all 87 models have the problem of potential faulty B2 pistons. On the 126 like yours, there is sufficient room to get to the piston w/o removing the tranny. I have heard it can be a DIYer job with the 126.

My mechanic said that the B2 usually fails while driving in traffic/around town as the constant shifting up/down causes the faulty piston to fail.

Ron

amg280 · #15 11-10-2002, 08:19 PM

Hello.

Ive noticed on my 92 400E, my 3-4 shift under moderate acceleration slips a little bit. But, under normal acceleration, its nice and firm like the other shifts.

Ive tried boosting the pressure a little bit, and it helped, but the 3-4 shift is still weak in comparison.

Could this be the B2 piston failing?

How much clearance do you need to remove the B2 Piston? Could you "convince" the tranny to move enough without stressing anything?

Thanks for advice. Car has 120K on it.

Andy