Rear Shock Replacement on M-B W123 Chassis.
With the help of the M-B factory manual, I was able to change shocks
on the rear of my '82 300D in less than an hour. It took me longer
than that to write and edit this article. The original shocks
functioned perfectly for 16 1/2 years, travelling 136,000 miles.
They probably would have lasted longer, but they gave in to the
sub-zero temps in December and January that we experience in New
Hampshire. The top seals gave out, and the oil began to pump out
through the bad seals. Both sides failed within days of one another.
The fronts are still working well.
I bought a set of Bilsteins to keep the original ride quality.
According to the factory manual, there are three different brands of
shocks that were mounted on the W123 chassis, Bilstein, Boge, and F&S.
I do not know what shipped to The USA besides Bilstein, but that's
what I have. They (the Bilsteins) are differentiated by a single or
double white stripe of paint, perpendicular to the mounting lugs, on the
lower end of the shock. The Boge shock has a slanted white stripe, and
the F&S has one or two parallel (to the mounting lugs) white stripe(s).
The two-stripe models have higher compression/extension valving (for
countries with rough roads, I imagine).
Equipment required:
Floor jack
Two heavy duty jack stands
17 mm socket on 1/2"-drive ratchet
(2) 17 mm open end or box wrenches
5mm open end wrench
10mm socket with nut driver handle
First off, the manual advises that the shocks act as a deflection stop
for the control arm. You are advised to support the control arm before
removing the shock. My car's control arms have a travel stop on them,
but I followed the manual's instructions anyway, and supported the
control arm with the floor jack.
To access the top of the rear shocks, you must remove the rear seat.
Remove rear seat cushion by pressing "in" on orange tabs (one on each
end) along front-lower seat edge, and simultaneously lifting on seat.
Remove from car.
Remove seat back by removing the 10mm screws that secure each end of
the seatback, near the floor, and the screw in the center that is beneath
the armrest(total 3 screws). Lift/slide seatback off of mounting lugs
at top of seatback. Remove from car.
Pull back the ends of the insulation on the backrest wall until you
see two plastic plugs in the sheet-metal.
Remove the white, round plastic plug to expose the nuts that secure the
top of the shock.
Lift rear of car with floor-jack, and secure with jack-stands at the
highest level possible. This is so you will have the clearance needed
to remove shock from below control arm.
After jack-stands are in place, raise slightly, with the floor jack,
the control arm for the shock you will be removing. Loosen the nuts at
the top of the shock. The original had two nuts at the top of the shock,
and the replacements that I bought had one, self-locking nut. Remove
metal washer and rubber disk from top of shock shaft.
>From under the car, loosen the two mounting bolts from the bottom of
the shock. I sprayed these bolts with rust penetrant the day before
I changed them, to be sure they were easy to take out. After bolts
are removed, the shock will drop out through control arm opening.
Before installing new shock, be sure to have the new rubber rings
and metal washers assembled onto the shock like the original.
You will be using the floor jack to raise/lower the control arm
enough to insert and secure the shock.
Insert the new shock through the control arm opening and up into the
upper mounting hole. It is a little tricky to find the upper
mounting hole because of visibility, but a couple tries will do it.
Once the piston rod is through the upper hole, install one of the
lower mounting bolts loosely, to hold the shock in place while you
secure the upper mount. You may have to adjust the height of the
control arm to get the bolt inserted. The lower mounting lugs on
the shock are angled slightly, so be sure that the angle matches
up with the control arm prior to inserting the bolt.
Install the new rubber ring and metal washer over the top of shock,
and secure with the nut(s). The threads on the top of the shock
are cut so that you tighten the nut down until it will not turn.
This gives the rubber disks the right amount of preload. Apply
a counter-hold with the 5mm wrench to assist driving the first
nut down. If your replacement shocks come with two nuts, install lock
nut with counter-hold on main nut. Install the plastic plug and
replace the insulation.
Lower the control arm enough to draw the mounting lugs of the shock
onto the bottom of the control arm. Insert the other mounting bolt
and torque both bolts to 45 Nm.
Move jack to the other control arm and repeat removal/replacement of shock.
Remove jack stands. Lower car. Replace rear seat.
Drive and enjoy that original ride.
For civic-minded recyclers, you can release the gas pressure and
drain the oil from the old shocks:
Put on safety glasses.
Mount the shock in a vise with the shock piston rod pointing
vertically downward. Drill a 5mm hole into shock tube, 20mm
from bottom of shock. This releases the gas. On shocks
without the separating piston, pump oil out of shock by
compressing/extending shock. Oil capacity is 250-500cc,
depending on shock model. The shocks without the separating
piston (Boge or F&S) will have a rubber bellows seal at the
bottom of the shock. The units with the separating piston
will have a solid shock tube only.
Next, on shocks with separating piston, drill a hole in the
shock tube 60mm from the bottom. The oil will flow out of
this hole. Be sure to drill the hole to release the gas
first, else you will be showered with oil.
Good Luck!
Dave Masinick New Hampshire '82 300D MBCA
a Diesel-Benz subscriber on digest.net
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