DIY – Pictorial - W126 Front seat cushions in a W116 - PeachParts Mercedes-Benz Forum

Alec300SD · #1 03-17-2014, 05:36 AM

This is a great upgrade for anyone suffering discomfort from worn out front seat cushions on their W116 (or even a W123).
It is not hard to do and is well worth the time spent.
I have tried to be as complete as possible with photos and text to make this DIY job as smooth as possible.

Three stages are involved in successfully transplanting a W126 front seat cushion into a W116 seat.
1) Removal and disassembly of the donor seat to free the W126 seat cushion
2) Removal and disassembly of the original W116 seat to free the W116 seat cushion
3) Assembly of the W116 seat with the W126 cushion and reinstalling the seat in the vehicle

On 01/25/2014 I acquired ‘Desert Rose’, my new 1978 300SD Turbodiesel from Doktor Bert.
She is in great shape overall, but the Bamboo front seat cushions were very worn and needed replacing. (Fig.1 & Fig.2)

The front seat cushions of the W116s and W126s share the same spring box bases and they only differ in the number of springs.
The second generation W126 front springs boxes have two extra coil springs in the front and a bigger fifth rear spring (and the sixth rear spring is deleted).
The passenger’s side (PS) seat cushion and the driver’s side (DS) seat cushion are interchangeable as well.
The PS seat cushion almost always has less wear and is the best one to choose when sourcing a donor at the yard. Any W126 sedan up to 1991 is a potential donor.

I was fortunate and found very nice Palomino leather electric memory front seats from a 1987 420 SEL in the Oakland yard. (Fig. 3)
These seats also had the orthopedic backrests that I can use in my 1983 300SD Turbodiesel.

The following DIY shows how I replaced and updated her front seat cushions on 02/17/2014. (Fig. 4)

Alec300SD · #2 03-17-2014, 05:54 AM

1) Removal and disassembly of the donor seat to free the W126 seat cushion

Repositioning the W126 electric seats in the yard (to allow access to the 4 bolts that mount the chair rails to the floor) requires a 12 to 18 volt DC power source and a pair of leads. (Fig. 5 & Fig. 6)

Jumper batteries may be prohibited in the yard.
However, two fresh 9 volt batteries in series or the power pack for an electric drill can also work.

Move the seat forward to reveal the rear 10 mm mounting bolts.
The bolts are easy to round off with an open end wrench.
They are difficult to grip securely because of the awkward angle they insert into the floor.

A ¼ inch drive socket wrench with a universal (or wobble) adaptor can work.
I was able to borrow a 10 mm long-handled, swivel-headed micro movement gear wrench (I think it is the best tool for this job).
Remove the rear bolts. (Fig. 7)

Next, remove the plastic cover on the seatbelt rail guide anchoring bolt to access and remove the bolt. (Fig. 7)
Reposition the seat rearwards.

Remove the front plastic anchoring bolt covers (if they are still present and not broken off) under the bottom fore-aft seat adjusting track to access the 10 mm front anchoring bolts.
Remove the bolts. (Fig. 7 & Fig. 8)

Next, unplug the three chassis wiring harnesses that connect at the front right of the seat, below the seat cushion and near the front right anchoring bolt. (Fig. 8)
Trace the seat belt warning buzzer harness from the bottom of the seatbelt receiver to its chassis harness and also unplug it.
Also unplug the vacuum connector from the orthopedic vacuum supply line under the chair.

With all five anchoring bolts removed, and the four chassis wiring harnesses unplugged, and the orthopedic vacuum supply line unplugged, the whole seat can be removed from the donor car.

To save some money, if you only want the seat cushion, you can disassemble the seat to free the seat cushion in the yard.

To free the base of the seat cushion, you need to remove the two upper tracks that control the seat fore-aft position.
These are held in place with three large Phillips head screws that can be very tight. (The rearmost ones are visible in Fig. 7)

Position the chair upside down on some soft carpet or cardboard and wedge it against an immovable object so it won’t slide when you remove the three screws.

If the screws are stubborn, have the weight of your body directly over the screwdriver in the palm of your hand, and with your free hand grip the screwdriver handle with large adjustable pliers (ie Channel Lock brand of pliers) to give enough torque to break the screws loose.

The middle screw can only be accessed through a hole in the bottom part of the track assembly.
Use your battery pack (as needed) to expose the positions of the screws to allow their removal.

After detaching both rails, (Fig. 9) the next step is to unplug the three backrest motor harnesses. (Fig. 10)

Alec300SD · #3 03-17-2014, 06:02 AM

Wedging a small screwdriver tip into the slot at the base of the connector can let you pry out each plug without it breaking. (Fig. 11)

The seat base adjustment assembly should now be free.

If your donor seat has orthopedic controls attached to the seatbelt receiver, simply lift it up to detach it from the seat belt receiver.
[I did not realize this on the first donor seat and ended up unplugging the colored vacuum tubing from each vacuum connector and labeling the black connectors with tape (Fig. 15)]

Also withdraw the lower gray vacuum supply hard line that is in the spring box area.

Next, unclip the seatbelt warning buzzer harness where it is attached by two or three clips to the bottom of the spring box and withdraw the harness.

The backrest is attached to the seat cushion base with four 13 mm bolts that thread into the bottom of the seat hinges.
One of the four bolts is specialized for attaching the seatbelt receiver bracket to the spring box.

The seatbelt receiver is attached to the specialized mounting bolt with a spring clip retainer. (Fig.12)

With a large screwdriver, push the retaining clip back to remove it (Fig. 13) and then you can lift and thus remove the seatbelt receiver bracket from the specialized bolt. (Fig. 14)

Remove the seatbelt receiver bracket from the bolt.
After this is done you will have completely removed the seatbelt receiver assembly. (Fig. 15)

Alec300SD · #4 03-17-2014, 06:10 AM

Next, remove the specialized bolt.
Then remove the rear bolt with an open end wrench that will just fit under the plastic seat hinge cover. (Fig. 16)

This inward cover has enough give to allow the bolt to be removed without removing the inward cover.
However, the outward plastic seat hinge cover must be removed in order to access the two 13 mm bolts.

To remove a plastic seat hinge cover one must soften the four semi-circular prongs of the clip and bend the prongs inwards so they no longer grip the mounting hole in the seat hinge. (Fig. 17 shows the inward cover prongs)

A blow dryer with a styling tip can put out a small cone of enough heat to safely soften the plastic.

Remove the outward plastic seat hinge cover after heating, softening and gently bending the prongs inward with a flat screwdriver tip.
Gently push the cover off and set it aside. (Fig. 18)

Then remove the two bolts from the outward seat hinge. (Fig. 19)
At this point the backrest is still attached to the seat cushion by the three backrest motor harnesses.

If desired (to remove or inspect the hidden backrest components), you can remove the backrest panel retaining screws (Fig. 20) and slide off the panel. (Fig. 21)

Alec300SD · #5 03-17-2014, 06:18 AM

Remove the reusable cable tie. (Fig. 22)
Grasp the ear of the tie with some pliers, (tilt it back away from the tongue of the tie) and with your free hand push the tongue back out of the slot.
Withdraw the three backrest motor harnesses. (Fig. 23)

Success, you have liberated your new W126 donor seat cushion. (Fig. 24)

2) Removal and disassembly of the original W116 seat to free the W116 seat cushion

Use the manual seat height adjustment lever to raise the seat as high as it can go.
Then move the seat as far forward as it can go.
The two 10 mm rear anchoring bolts are now exposed.
Remove the bolts. (Fig. 25)

Lift a corner of the carpet covering the driveline tunnel in front of the rear bench seat.
This exposes the 10 mm bolt that anchors the seatbelt guide rail.
Remove the bolt. (Fig. 25)

Next, lower the seat as low as it can go.
Then move the seat as far back as it can go.
The two 10 mm front anchoring bolts are now exposed.
Remove the bolts. (Fig. 25)

Unplug the seatbelt buzzer wiring harness from its chassis harness connector.

Now remove the seat from the car.

Alec300SD · #6 03-17-2014, 06:18 AM

Remove the reusable cable tie. (Fig. 22)
Grasp the ear of the tie with some pliers, (tilt it back away from the tongue of the tie) and with your free hand push the tongue back out of the slot.
Withdraw the three backrest motor harnesses. (Fig. 23)

Success, you have liberated your new W126 donor seat cushion. (Fig. 24)

2) Removal and disassembly of the original W116 seat to free the W116 seat cushion

Use the manual seat height adjustment lever to raise the seat as high as it can go.
Then move the seat as far forward as it can go.
The two 10 mm rear anchoring bolts are now exposed.
Remove the bolts. (Fig. 25)

Lift a corner of the carpet covering the driveline tunnel in front of the rear bench seat.
This exposes the 10 mm bolt that anchors the seatbelt guide rail.
Remove the bolt. (Fig. 25)

Next, lower the seat as low as it can go.
Then move the seat as far back as it can go.
The two 10 mm front anchoring bolts are now exposed.
Remove the bolts. (Fig. 25)

Unplug the seatbelt buzzer wiring harness from its chassis harness connector.

Now remove the seat from the car.

Alec300SD · #7 03-17-2014, 12:10 PM

Remove the lateral backrest panels by removing the Phillip head screw on each panel (Fig. 26) and pry the post from the mounting clip. (Fig. 27 & Fig. 28)

A spring clip holds the post in the hole in the seat hinge. (Fig. 40)

Slide the rear backrest panel off the backrest. (Fig. 29 & Fig. 30)

This allows inspection of the backrest interior and makes it easier to reposition the seat during disassembly.

Alec300SD · #8 03-17-2014, 12:13 PM

Gently pry off the large black backrest angle adjusting knob with a large screwdriver. (Fig. 31 & Fig. 32)

Remove the outward plastic seat hinge cover by heating (with a hair dryer) the four semicircular prongs of the plastic clip and gently bending each of them inwards so they no longer grip the mounting hole in the seat hinge. (Fig. 33)

Gently push the cover off and set it aside. (Fig. 34)

The seat hinge to spring box bolts are now exposed. (Fig. 35)

Alec300SD · #9 03-17-2014, 12:16 PM

Remove the inward plastic seat hinge cover in the same fashion. (Fig. 36, Fig. 37 & Fig. 38)

I pried the backrest skin out of its track (Fig. 39) to expose how the upper part of the seat hinge was mounted to the back rest. (Fig. 40)

I was hoping to use the W126 orthopedic backrest on the W116 hinge…but it was held in place with rivets rather than bolts.

Alec300SD · #10 03-17-2014, 12:20 PM

Remove the large spring of the seat height adjuster mechanism with pliers. (Fig. 41)

Remove all six Phillip head screws (three per side) that mount the each of the seat height adjustment rails to the adjustment frame.

Remove the screws in pairs, (the front pair, the rear pair and the middle pair) and reposition the tracks with the seat height adjusting lever as needed to expose the screws. (Fig. 42 & Fig. 43)

Detach the outward plastic cover of the seat adjustment assembly (Fig. 44) by gently prying off the rear cover (Fig. 45) and then pushing the front plastic tab out of its metal retaining slot in the lower seat height adjustment rail at the front of the seat. (Fig. 46 & Fig. 47)

Alec300SD · #11 03-17-2014, 12:27 PM

Pull the back of the seat height adjuster from its locating tab (#1 in Fig. 48) in the back of the spring box.

Then pivot the adjusting lever up by the handle to release the lever (and the attached plastic cover for the seat adjustment assembly) from the grommet covered hole (#2 in Fig. 48) in the spring box.

Remove the assembly and set it aside. (Fig. 49)

Detach the inward plastic cover of the seat adjustment assembly in the same fashion.

Remove all six Phillip head screws (three per side) that mount the each of the seat fore-aft position adjustment tracks to the spring box.

Remove the screws in pairs, (the front pair, the rear pair and the middle pair) and reposition the tracks with the seat fore-aft adjusting lever as needed to expose the screws. (Fig. 50 & Fig. 51)

Alec300SD · #12 03-17-2014, 12:33 PM

Remove the wedge shaped bracket that normally lies between the adjustment rails.
Then remove the fore-aft adjustment lever and the fore-aft adjustment tracks.
Set them aside. (Fig. 52)

Remove the two bolts (four in total) that secure each of the seat hinges to the spring box. (Fig. 53 & Fig. 54)

The seatbelt receiver bracket is also mounted by the front bolt on the inward side of the spring box. (Fig. 55)

Alec300SD · #13 03-17-2014, 12:36 PM

Success, you have liberated your W116 seat cushion. (Fig. 56)

Here is a comparison of the new W126 and old W116 seat cushions (Fig. 57 & Fig. 58) and also the W126 production label. (Fig. 59)

3) Assembly of the W116 seat with the W126 cushion and reinstalling the seat in the vehicle

Bolt the original backrest to the donor seat cushion with two bolts (four in total) for securing each spring hinge to the spring box.

Don’t forget that the seatbelt receiver bracket goes in the front bolt on the inward side of the spring box.

Slide the seatbelt guide rail back into the slot on the seat belt receiver bracket. (Fig. 60 & Fig. 61)

Alec300SD · #14 03-17-2014, 12:41 PM

Lubricate the seat fore-aft adjustment tracks with grease (Fig. 62) before reattaching them (and the fore-aft adjustment lever, as well as the wedge shaped bracket) to the spring box with the six Phillip head screws (three per side). (Fig. 63)

Add the screws in pairs, (front pair, rear pair, and middle pair) and also reposition the tracks as needed to expose the mounting holes.

Reinsert the seat height adjusting lever into the grommet covered hole in the spring box and slide the rear portion of the seat height adjusting arm into the receiving tab in the spring box base. (Fig. 64)

Lubricate the seat height adjusting tracks with grease (Fig. 64 & Fig. 65) before reattaching them to the middle wedge shaped bracket with the six Phillip head screws (three per side).

Add the screw in pairs, (front pair, rear pair, and middle pair) and also reposition the tracks as needed to expose the mounting holes.

Alec300SD · #15 03-17-2014, 12:46 PM

Reattach the large spring for the seat height adjustment mechanism with pliers and reattach the plastic covers for each side of the seat adjustment assembly. (Fig. 66)

Apply heat (with a hair dryer) to the semicircular prongs of the plastic covers (Fig. 67) to the seat hinges and gently bend them perpendicular so they can lock securely in the mounting holes in the seat hinges. (Fig. 68)

Clip both covers in place.

Reattach the large plastic seat adjustment knob. (Fig. 31)

Slide the rear backrest panel back in place. (Fig. 69)

Clip both lateral backrest panels back in place and reinsert the Phillip head screws as well.

The seat is now reassembled. (Fig. 70)

Place the assembled seat back in your vehicle.

Install both 10 mm front anchoring bolts first.

Then install both 10 mm back anchoring bolts.

Finally, install the 10 mm anchoring bolt for the seatbelt guide rail.

Success, now enjoy your updated seat. [|ó-{)