Yeah, I was on yahell lists before. I was not particularly impressed. This
IS the place for information on ALL Mercedes, not just diesels (although a
good deal of us are from an old diesel list). Here is some B2 info....
http://transmission.articles.mbz.org/b2/why/
http://transmission.articles.mbz.org/b2/repair/
I sifted through about two dozen links and that is all I found. Here is
another one, although it has nothing about b2 piston failure.
http://www.dieselbenz.info/
Here is what I have from my archives from Allan Streib.
Here is my summary of the repair, as promised.
Despite what you may have heard or read, it is possible to do this
repair on a W123 without removing the transmission. My thanks again to
Dave S. who helped me at several points along the way with this.
Read the articles at http://transmission.articles.mbz.org/b2, and
its links to the two articles at Richard Easley's site. My repair went
pretty much along those lines, using all the parts listed in the mbz.org
article. Where my experience deviated from the existing writeups was:
Before you start, get yourself a set of picks. These look like
dental tools. I got a set of three at the local True-Value store, which
included a 90-degree pick, a "double offset" pick, and a "hook" pick.
The cost of the set was about $9, and absolutely worth the price for
this repair. I used the hook shaped pick the most, so if you can find
that on its own it would probably be sufficient.
At the hardware store, you may also want to get a "brad puller" to
help with removing the seal at the back of the main bore (more on that
later). This is like a nail puller but in minature. Should have a long
shank (at least 7 - 8 inches) and a bent tip. Alternatively you can do
what I did: take a longish screwdriver, heat the end in a torch until it
is a dull red, then hammer it over so there is about a 15 or 20 degree
bend at the very end.
In the W123, clearance is tight. To gain the necessary access,
support the transmission mount with a floor jack and remove the four
bolts that hold the mount support plate to the bottom of the car. When
I removed these bolts, one of the lock washers split in two. The others
were squashed absolutely flat, so I decided to replace them. I picked
up a package of four 3/8" grade 8 lock washers, which had the same
gold/cad color and are a very snug fit, like the originals. I used a
scribe to mark the exact position of the plate on the underside of the
car before I loosened anything; I'm not sure if this is necessary but I
did not want to risk getting the drivetrain out of alignment.
Also unbolt the center driveshaft bearing, which is about 6 or 8
inches rearward of the transmission mount. Again, I used a scribe to
make some marks so I could line everything up exactly as it was upon
reassembly. Unbolting this bearing will allow the transmission to drop
a bit farther than unbolting the transmission mount alone, which is
essential to gaining enough clearance to get the new piston installed.
With the mount and drivshaft bearing unbolted, gently lower the
floor jack, and wedge the transmission over to the left as described in
the above mentioned articles. Be sure the hood is up since the front of
the engine will rise as the transmission is lowered.
Clean the B2 cover as well as you can. I used CRC Brake Cleaner
spray, which is not something you want to breathe a lot of. Take a
break at this point and let the fumes clear.
The cover is held in place by a retaining ring. There is a gap in
the ring, which should be located where you see the raised round area
near the outer edge of the cover. This was at about the 2 o'clock
position on my car. Press the cover in, and use your hook-shaped pick
to get under the end of the ring and pull it out. Then work around the
ring until it pops free. I was able to depress the B2 cover with hand
pressure; I did not need any kind of homebrew "compressor" tool as
described in the mbz.org writeup.
You may need to wiggle the cover by pressing alternating sides,
tapping it, etc. but it should pop free once the retaining ring is out.
About a cup of ATF will get dumped at this point, so be prepared.
Assuming your B2 piston actually is broken, it will come out in
pieces. There is a small pushrod in there -- try to notice which end is
in and which end is out. Mine fell out onto the floor, so I don't know
if I reinstalled it the same way -- it does not look like it would
matter, but it would be best to keep it in the original orientation if
you can.
Once the piston is out, you have to deal with replacing the
aforementioned black seal at the rear of the bore. I tried pulling it
with the picks but it absolutely would not budge. Using your brad
puller or modified screwdriver, approaching horizontally (parallel to
the floor) from the rear, place the tip between the seal and the alloy
bushing immediately behind it. Tap the tool under the seal to dislodge
it. Once it has been dislodged, you can use your pick tools to pull it
free. It may still be a bit tight; I had to grab the pick handle with a
pair of pliers to get a good enough grip to pull the seal, but it did
come out. Note you will damage the face of the bushing in the process
of doing this, but that's OK because there should be a new platic
replacement bushing if you ordered all the parts mentioned in the
mbz.org article.
Once the seal is out, the old alloy bushing comes out easily.
Install the new plastic (teflon?) bushing and drive the new seal in to
hold it in place. I used the butt end of a screwdriver handle to do
this; you want something that won't mar or dent the metal surrounding
the seal. Be sure the seal is driven in flush with the case and is
holding the bushing down tight.
Replace the large red O-ring that seals the B2 piston cover.
Remove it with one of your pick tools, gently, so you don't scratch the
case. (Note that one of the Easley articles cautions against doing
this, saying it is nearly impossible to replace the O-ring with the
transmission in the car. However I had absolutely NO problem getting
the new one in place.)
Before you install the new piston, carefully inspect the bore for
any dirt or other foreign matter. Wipe it out carefully with a clean
lint-free rag and be sure it is absolutely clean.
Install the little pushrod into the indentation in the band at the
rear of the bore. It should stay in place, though it will be sloping
downward a bit. One of the Easley articles suggests using a dab of
heavy grease to help it stay in place, but I did not find that necessary
nor am I sure that sounds like a very good idea to introduce heavy
grease into the transmission works (though maybe the ATF would just wash
it away once it started circulating).
Due to the close clearances, I draped a piece of plastic wrap over
the new piston to keep it clean as I installed it, because it's almost
impossible to avoid brushing some part of the chassis as you work it
into position. The plastic can be slipped off once the piston has
cleared the chassis and is ready to be inserted. I used Saran wrap,
which is pretty clingy and thus was a bit awkward to remove. Regular
visqueen or maybe even waxed paper would probably be better. You
definitely don't want have it tear and leave a bit of plastic wrap in
there (I checked to be sure the wrap was intact once I got it out).
Everything I read about this repair stressed the need for cleanliness so
I didn't think I could be too careful here.
As you slide the new piston in, be sure the pushrod engages into
the piston. You won't really be able to see this so you kind of have to
visualize it. Mine worked on the first try. You know it's in place if
when you depress the piston you get a spray of ATF out of the bottom of
the valve body.
Replace the cover, reinstall the snap ring, and you're done.
Remeber when installing the snap ring that the gap in the ring straddles
the raised round extrusion on the edge of the piston cover. I put mine
back in the original 2 o'clock position, though I'm not sure it really
matters.
That's about it -- from here on it's just a matter of putting
things back together. I also did the K1 spring upgrade (described at
mbz.org) before I put the pan back on, since I had everything opened up
anyway and it only added about $10 to the parts cost. That was a very
easy install; nothing to add to what's already written up on that.
Don't overtorque the pan or it may warp and leak. Buy or rent a
low-range torque wrench for this, it's important. Justify the purchase
by the amount of money you are saving on this repair (I was quoted
$1,500 by one local shop).
Hope this all helps anyone who might undertake this repair on a W123.
Allan
--
1983 300D
1966 230
__________________________________________________
Rick Knoble
'85 300 CD
'87 190 DT
