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Repairing the Front Power Seat Headrest
Mechanism
The headrests in the W140 are
electrically operated. They should never be moved manually (by
hand that is). It's
my opinion that the headrest mechanisms break due to being moved manually. The
parts used are not up to the task of withstanding forced manual movement.
When I bought my car, the head rests
were not working. I could hear a very slight hum when I used the adjustment
buttons to try and move the head rests. This told me that the adjustment buttons
were probably working, and that the motor was also working. The problem was
somewhere in the transmission of power from the motor to the head rest. The seat
back had to come off to see what the problem was.
Tools needed:
- Philips screwdriver
- Flat blade screwdriver
Parts needed;
- In this case, no MB parts were
purchased. 2 steel hose clamps were used for each seat.
Time needed:
- Less than one hour per seat.
| Notice the round
feature on the seat back. Pressing this area will release the mechanism
holding the head rest within the seat. To remove the head rest, push this
button in firmly and pull the head rest straight up and out.
You may have to tilt the seat
back some to gain sufficient clearance to remove the head rest.
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Move the seat to be worked on
forward enough to give you room to work from the backseat.
Remove two Philips screws from
the bottom of the seat back panel. There is one on each side at the
bottom.
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| Pull the seat back
straight down. The top of the seat back has a lip in it that fits
underneath the seat trim. |
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| The inside wonders of
the seat are now exposed. You will be able to see the seat bladders (if
your car is equipped with the adjustable lumbar and side bolster option).
On the right side of the seat, you will also see the motor that
drives the head rest. A flexible cable from the motor drives a rack and
pinion setup.
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| The cable drives a
pinion that is housed in a plastic assembly. This housing is mounted with
a steel clip to one of the seat supports (circled in diagram). This
housing broke on my car. As I mentioned, it doesn't seem up to the
challenge of handling the forces generated when the head rest is forcibly
moved, or perhaps even when the head rest stalls against it's travel
limits.
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| Here's a shot of the
rack (the blue part) with the pinion housing assembled as it should be.
The rack moves up and down when
commanded by the motor, and the headrest is driven by the rack. The head
rest post clips into the end of the rack.
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| Another shot... |
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| The head rest post
fits into the rack like this. |
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| This is what broke on
my car. The piece I'm holding should still be part of the main housing
with the pinion gear in it. The pinion was not held against the rack, and
therefore when it spun, it did not drive the rack.
I made sure that the motor,
cable, and pinion did in fact rotate when commanded by the seat adjustment
buttons. Everything worked.
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| To fix this, I simply
bought 2 stainless steel hose clamps. I re-assembled everything in place,
and used the hose clamps to hold the pinion housing together. The best fix
is of course to buy new parts, but in my opinion, they're going to break
again sooner or later. My fix works, and it's definitely less expensive.
Unscrew the clamps so you can fit
them around the support post. Tighten the clamps enough to firmly hold the
housing together, but NOT TOO tight. If the clamps are tightened too much,
the mechanism will bind and possibly cause damage. The head rest may not
move either.
Make sure the pinion is rotating
freely by using the seat adjustment button.
If everything is working, insert the
head rest back into the seat. Watch for proper engagement between the seat
post and the rack interface. The post should click into place.
Try moving the head rest again
with the adjustment button. You may need to re-adjust the tension on the
clamps.
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| Re-assemble
the seat back to the seat, and install the 2 Philips screws.
I would strongly suggest you NOT
drive the head rest into either fully lowered or fully raised travel
stops. Things could break again. As soon as you see you've reached the end
of travel, let go of the adjustment button. Keep the head rest somewhere
between the travel limits...do not let it stay at a travel limit.
Well done! Your head rests
should now be working again. Make sure you tell people using the car
to use the buttons, and not to move them manually.
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