transmission piston

[mbtjc]

I saw somewhere that W126 transmissions have a flawed "B-2 piston" and it said to replace it right away with an improved part or your transmission could fail and you'd be out a tranny. It seems like if this was a huge problem I would hear about it more often. Does anyone know anything about this?

[stevebfl]

You call it a flawed piston. I would call the new piston an improved version and I'm sure thats the way MB sees it. The problem that occurs is that during the release of the B2 band (in all 722.3 trannies not just 126 versions), pressure is exerted on the back side of the piston to assure prompt release. This is happening at the same time a clutch pack is using the same pressure source to inflatethe cluth pack. The piston is about 3+ inches in diameter and originally had a teflon piston ring installed similarly to the steel rings of an engine piston. It was about the thickness of an engine piston ring also.

The new piston uses a teflon ring that is built like an umbrella, its surface area to the cylinder is about a quarter inch instead of less than an eighth inch and the umbrella effect allows the pressure to apply the sealing surface instead of opposing it.

The end result is that the fluid losses occuring in the release of the band are reduced which keeps the clutch filling to the original specs and reduces the tendency of 4th gear to flare.

There is absolutely no reason to do anything to an old tranny untill flaring becaomes a problem. By the same token there is no reason to rebuild a tranny and not change this piston.

[mbtjc]

Thanks for the detailed response. Just fyi, I personally wasn't calling it flawed, that is just the impression I got when I heard about it. I understand what you are saying though. But do you think you could explain to me the flaring a little bit more? What will I notice if this happens or what are the symptoms? Thanks again!

[stevebfl]

Some would call flaring, slipping. Its is not! The term refers to an increase in rpms during the shift as if there was slippage.

To understand flaring one must understand a little about how auto transmissions work and the term shift overlap.

Because in the mechanical world absolutely nothing moves instantly, shift timing must be part of the design concept. In the case of fourth gear a band is released and a clutch is activated. The end result is that all the guts of the trans are locked as one and spin together making high gear. As it turns out it takes very little time to release a band. Applying the clutch is a different matter; there is a stack of discs that must be squeezed together to apply. This takes a finite amount of fluid and has an exact fill rate due to oriface size and original freeplay of the pack.

The problem occurs in this tranny when the piston causes a fluid loss and the clutches are wore or the seal is leaking in the pack or in the case of early cars the clutch packs were assembled with too much play. Now what happens is the band releases which was holding the outside of the clutch drum stationary related to the trans case. This caused a gear reduction as the output came off another element of the planetary gear set (much too hard to explain with words - need a picture). Anyway at the moment of release the group is able to freewheel and the engine is unattached and revs. Normally the release action is retarded and the clutch fill action is early engaged to give what is called shift overlap or shift timing. In theory both items are applied at the same time. Sort of like "just in time" inventory control the clutch which is started to engage first gets there just in time that the engine is normally not released - no flare.

Wear in the pack and fluid losses both in the piston and clutch seals changes the timing equation and causes this problem not slipping.

[mbtjc]

Well...let me study that for a few minutes...just kidding. I know a lot about how engines work, but I only have basic knowledge of transmissions, but I think I get basic point of what you are saying and I think I could tell if it was messing up. What would it cost to replace that piston at your shop for example? Thanks again for the well-written reply.

[stevebfl]

I imagine it would be close to $200. I forget the price of the piston but the labor is the tough part. Unfortunately it is likely one would normally need to remove the valve body to guide the flexible pin into the cup of the band. This can be seen with the VB removed. With experience and some tricks of the trade (heavy soluable grease used to support the limp probe) it can regularly be done without removing the pan. I am not sure what we would charge for labor as doing the job with VB removed is probably worth 4 hours labor. We probably estimate this quantity and then charge a couple hours if we are successful from the outside (which probably takes less than an hour).