Replacing Rear Subframe
Mounts on the 126 Chassis
The
technical material for this FAQ (Frequently Asked Question) was provided by
Richard Easley of Baylor University, Raymond Ryel of Portland, Oregon, and Stu
Ritter of Denver, Colorado, and is provided as a service to the subscribers of
the Mercedes-Benz Discussion List.
To
receive similar quality tips as described below on a daily basis, consider
subscribing to the Mercedes-Benz Discussion List, which is located at the
following site:
http://hsb.baylor.edu/html/easley/mercedes/welcome.html
Please
note that this procedure is specifically for the 126 chassis (S-body from
1981-1991) but is similar for at least the 123s (280Es, 300Ds 1977-1985), and
likely others in the MB line . . .
You
may be asking: “How do I know if the subframe mounts need to be replaced?” If you are driving an 80s model
Mercedes-Benz (or your MB has exceeded 100K miles) and the mounts have
not been replaced yet, then I can tell you that they need to be
replaced. Additionally, the rear
differential mount should be replaced at the same
time.
Replacement should make a
difference in ride height and will definitely make a difference in “feel.” And -- by extension, camber and toe of
the rear are affected because the car has an independent rear suspension. So --
if you are detecting wear on your rear tires, this is likely it. Keep in mind
that the entire rear subframe is depending on these 3 contact points.
Stu Ritter has the following
to say about replacement of the subframe and differential
mounts:
“Having
done hundreds of rear subframe mounts over the past 30 years, let me throw in a
few pennies..
As
the mounts collapse, they lose the ability to maintain the rear wheel tracking
ability or the ability to keep the rear wheels right behind the front wheels.
When the mounts reach the point, as Kevin Kenner’s did, where they are wearing
the inside edge of the rear tire down to steel while 70% of the rest of the
tread remains, you can rest assured that mount is moving the rear end of that
car around like Jello.
When
you replace the rear sub mounts, you are re-establishing the factory specs for
rear wheel steer. The Porsche 928 has some rear wheel steer designed into the
suspension system, while our Benz's do not. Once the mounts collapse or even get
soft for that matter, the rear end of the car starts to steer and when that
happens, the very relaxed, high-speed Benz feel goes in the
garbage.
Having
driven hundreds of resuscitations, let me tell you that it works. It snaps the
car back to where it had been when it was new. It ain't in your mind, it's in
your hands...you really feel it . . . ”
Assumptions
Using
the categories of mechanical ability from the Mercedes-Benz Discussion List
<http://hsb.baylor.edu/html/easley/mercedes/subscribe.html>,
you need to be at the level of "Medium Do-It-Yourselfer" at minimum, to
replace the rear subframe mounts. If you are below that level, you may want to
provide these instructions for someone who is at the medium level or
beyond.
Parts
needed:
1.
Subframe
mounts (note that these are specific for years; later models (1986->?) are
designated left and right)
2.
2
large subframe mount attachment bolts (MB part only, very large special bolt
with tapered end)
3.
Differential
mount
Tools
needed:
1.
Hydraulic
jack
2.
Jack
Stands
3.
MB
special tool (bearing puller), part number 126 589 00 33 00. Note: A ˝” threaded rod with nuts and
large washers will work in the place of the special
tool.
4.
MB
special tool (bearing installer), part number 126 589 01 33 00. Note: A ˝” threaded rod with nuts, large
washers, and a suitable-diameter ˝” drive socket will work in the place of the
special tool.
5.
Metric
tap and die in case either large bolt or mounting receptacle gets cross
threaded.
Replacing
the rear subframe mounts
1.
Jack
up the car and support it securely with jack
stands.
2.
Using
the hydraulic jack tension the subframe near the subframe
mount.
3.
Loosen
the 3 mount bolts and remove the "spider" plate.
4.
Relax
the jack. The subframe on that side will drop a bit -- no
problem.
5.
Using
a pry bar at the top of the subframe above the mount, pry the subframe down
enough to get "hand" clearance.
6.
Run
the threaded rod up through the subframe mount.
7.
Put
a washer on the rod at the top. Note: The size of washer is important since you
want the washer to go through
the hole in the frame assembly. Outside maximum measurement of 1.75" will fit
through the hole. Using several washers will increase the access to the nut as
you 'spread' to apply pressure to the rubber mount.
8.
Thread
2 nuts on the rod at the top, and add a washer on top of the uppermost nut. The
top nut/washer will put pressure against the unibody/frame. The middle
nut/washers against the bushing. Leave 1/2" thread exposed above the
washer. Note: Threading 2 nuts on the rod at the bottom and then 'jamming'
them tight allows better control of the rod and allows the use of a wrench on
the upper nut and a ratchet on the bottom. One nut/washer on top against the
frame; one nut/several washers on top of the rubber mount; two jam nuts on the
bottom to grip and use a socket/ratchet on.
9.
Run
the threaded rod up near or into the mounting hole.
10.
Using
your pry bar to ensure clearance between the subframe and the body, take the two
nuts by hand and increase the distance between them, ensuring that the threaded
rod is inserted in the mounting hole. [Note that the mounting hole has no
threads for about 1/2" so you are not hurting
anything.]
11.
Increase
the pressure on the subframe with the jack until the subframe mount is under
tension (the inner part of it will start "stretching" downward, away from the
outer part). You have now preloaded the subframe
mount.
12.
Using
a blunt-faced chisel or screwdriver, tap the side of the outer part of the
subframe mount. With the preload, it will immediately start releasing. If it
does not (or, you are in a northern area, or both), then you likely need to soak
the appropriate areas with a penetrating oil like KROIL. Note: It's not a bad
idea to soak with KROIL or other penetrant for several days prior begining this
job. Some individuals have used an air chisel to knock the bushing out.
After the bushing is one-third out, re-tension it by adding washers and
spread and retighten the nut again if necessary.
13.
"Walk"
it all the way out. It will probably fall out
easily.
1.
You
are going to "press" the new subframe mount in with the special
tool.
2.
Hand-fit
the new subframe mount to the subframe.
3.
Attach
the special tool’s cup to the subframe mount and run the threaded rod through
the mount.
4.
Attach
the nut to the other side of the mount.
5.
Press
the new mount into the subframe ensuring that it is
centered.
6.
Don't
forget to install the smaller rubber mount on the mounting hole above the
subframe before buttoning everything up.
7.
Jack
up subframe until it seats against unibody.
8.
Place
“spider cage assembly on subframe mount.
9.
Install
the large tapered bolt by applying constant, significant pressure to the bolt
head while turning the bolt with a wrench.
10.
Attach
remaining two smaller bolts.
11.
Repeat
on other side of car.
12.
Install
a new rear differential mount.
1.
You
are going to "press" the new subframe mount in with the hydraulic jack; make
sure that it is a “shop-sized" floor model, 'cause the "cup" is almost a perfect
fit with the outer edge of the new mount. Smaller jacks may work but you need to
protect the rubber from damage.
2.
Jack
up the subframe and place another jack stand under
it.
3.
First
coat all contact surfaces of the new rubber with silicon assembly grease
(SILGLIDE) or spray with silicon liquid. Press the new mount into the subframe
ensuring that it is centered.
4.
Don't
forget to install the smaller rubber mount on the mounting hole above the
subframe before buttoning everything up. Note: flat side up with 'cup'
down.
5.
Jack
up subframe until it seats against unibody.
6.
Three
ways to consider:
a.
Place
“spider cage assembly on subframe mount. Install the large tapered bolt by
applying constant, significant pressure to the bolt head while turning the bolt
with a wrench. Attach remaining two smaller bolts.
b.
Put
the spider cage assembly into place and hand tighten both of the small bolts
(with a drop of blue Loctite) then inserted the big bolt and tap it into
contact with a hammer. Use a wrench to get the threads started and a
ratchet to tighten it firmly into place, finishing by tightening the two small
bolts.
c.
Another
method is to use a bottle jack to get the bolt started by pushing directly onto
the head and use a 22mm spanner to turn the bolt with the weight on. Don't worry about cross threading
the bolt, that's what the taper on the end is for, it guides it into the
thread.
7.
Repeat
on other side of car.
8.
Install
a new rear differential mount.
Note:
To receive similar quality tips as described above on a daily basis, consider
subscribing to the Mercedes-Benz Discussion List, which is located at the
following site:
http://hsb.baylor.edu/html/easley/mercedes/welcome.html
©1999
Richard Easley, Waco, Texas and Stu Ritter, Denver, Colorado.
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