W124 Electric headrest problem

[millo777]

The passenger side electric head rest on my W124 seems to have given up the ghost, and after some investigation I'm stumped and need of help as to the where the problem lies.

I've taken the back of the seat off o.k, and the motor tested o.k. There is continuity across the terminals, and testing form each terminal to the body of the motor showed open circuits, so I assume there are no internal shorts.

The mechanism seems to be o.k, with no apparent damage to the plastic rack and pinion arrangment. when the switch is operated there is no noise from the motor.

I changed the electric seat switch on the door card with a replacement form mercman which hasn't solved the problem. All of the other seat adjustment functions work as they should.

Hardly a big deal, having a headrest stuck in the up position, but I'd like to get to it fixed all the same.

Does anybody have any ideas of other things to check/test before I throw in the towel and give it to a sparky to a deal with?

Thanks,
James.

 

[aircondoc]

Give it a shove whilst holding the switch?

 

[bh13coupe]

mine stopped working for 3 days then decided it would work again. No idea why. The car is getting to an age when some of the eclectric toys may start to play up. The passenger side would suggest an age realted issue rather than over worked.

 

[bolide]

Do you get volts across the motor when you operate the switch? If not the fault is electrical and you need to remove the door card & test the voltages coming out of the switch

I imagine it's wired with an X-switch where there are two feeds to the motor and the voltage applied by the switch is either - and + or + and - to get bidirectional movement. In other words there won't be a common earth

You should be able to fault find this is 10 mins with a meter

Nick Froome
the independent Mercedes Estate specialists

 

[millo777]

Getting back to this after ignoring it for a month.

I've eliminated the switch as the problem as I tested both of the pax side switches I have on the drivers side, and both operated the headrest with no issues (The headrest is the only seat opertaion that can be tested with a wrong handed switch).

I disconnected the flexi drive shaft from the motor, and when manually movign the headrest, this rotates normally, so no issues with the mechanical side of things.

I also tested the voltage across the terminals on the motor with the switch operated and nothing.

As per Nick's post, this would indicate an electrical problem.

I'm afraid car electrics are somewhat akin to witchcraft to me. Chnaging and biasing vacuum tubes is about the limit of ability with electrics.

With the above in mind, does anybody have an idiots guide to tracing the problem?

I can probably work out which are the appropriate wires coming from the switch, so is it simply a case of testing them at various points to check they still carry voltage when the switch is operated?

Are there any other obvious points to check? any relays in the chain etc

The heating element on the squab of the same seat is also kaput, so I'll be checking that at the same time, and probably be changing the pad while the seat is out, so any tips on that job are equally welcome.

Thanks,
James.

 

[millo777]

I guess my W124 has given me a succinct lesson in how to apply Occam's Razor. While testing the heat seater pad, I noticed the feckin connector plug for the headrest motor wasn't plugged in! The cable seems to have somehow got caught on the spring that holds the little flap of carpet in place at the front of the seat, so when the seat was moved fully forward, the connector came off. Anyway, all working now, and an important lesson learned.

 

[WDB124066]

Did you take a picture, this sounds interesting......

 

[millo777]

I'll take a pic tomorrow when it's light.

As an interesting side note, when I got the car the range of headrest height adjustment was maybe 3" from fully down to fully up. During diagnosing the problem I detached the flexi drive shaft from the motor, then before reattaching, manually moved the headrest to fully down, held the headrest down button until the motor stopped then reconnected the drive shaft. The headrest now has approx 6" travel to fully up.

So now I have to do the same to the drivers side.

 

[millo777]

Here are the pics. You can see the 2 pin connector for headrest motor clearly. The carpet flap at the front of the seat is clipped on the bar at the front by a small section of C shaped platic that is glued to the carpet. The springs at the other end clip on to a cross bar under the seat.

In my case, the spring had fouled the cable to the headrest motor. It's worth checking if you have a headrest problem as you don't even need to unclip the carpet, just peer around the edge with a torch!

For ref, the heated seats are connected via a 4 pin connector on the other side that I didn't get in the pic, so if you need to check these, the connectors are easily accessible.

The second pic shows the disparity in the range of height of adjustment since fixing the pax side. Both headrests are as fully up as currently go in the pic. I may have a go at adjusting the drivers side, but getting the upholstery re-fitted on the seat back is a bit tricky, so I'm no rush to take the back off the drivers side.

 

[WDB124066]

Thanks for this. I wondered what carpet piece you were referring too. My Cabriolets don't have carpet here, only a plastic cover. All is clear now and thanks for posting.

 

[rigel]

Greetings All,

I’m having much the same, if not a completely different problem with my headrests (both sides).

When I move the switches on the door I can feel the motors in the backs of the seat working but there is no movement either up or down by the rests?

After reading the above, maybe I must reset them to the full height first before they will respond?

Is it a big job to remove the seat back to access the mechanism and reset them? Can it be done with the seat in situ or must they be removed first?

Any pointers/pictures of the procedure would be greatly appreciated.

Thanks in advance.

 

[WDB124066]

You can gain access to the mechanism without having to take the seats out by removing the back panel of the seat. Sometimes the little plastic rack and pinion gearbox drive will come apart and fail. This could be the reason why you can hear the motor but are not seeing travel. They are available separately from MB and are an easy fit, once you have the back of the seat off all will be clear........

 

[rigel]

thanks for that W.

will be attempting the seat back removal when the car is SORNed next month.
is seat back removal a straight forward process? or are there little hidden clips/pins/fastenings that should be attended to first?

regards R.

 

[WDB124066]

Just the two screws at the base of the seatback cover and then starting at one bottom edge, pull it away from the seat toward the rear of the car. The side trim of the seat is folded into a groove in the seat back. All will be clear when you start, once you have one side off, the second side is easy.

 

[rigel]

Thanks again for the clarification W, backs are now off.

Took a leap of faith that I wasn’t going to damage anything, and a bit of fettling, but they did come free, and now I see how they are put together.

Looks like real leather on the side panels of the seats as well, which is nice.

The problem seems to be with the white plastic clip that holds the geared cog that the black plastic runner passes through to lift the head rest. It was fractured and part of it was found in the bottom lining of the seat.

Yet more money to MB Epsom.

Regards, R.

 

[WDB124066]

The good news is the replacements are designed slightly differently to prevent reoccurrence, and of course, that you can even purchase these as a replacement part in the first place.

I don't allow the head rests to reach full stroke now; stopping short, just in case!

 

[rigel]

i've superglued mine back together and it now works well without the load of the headrest to move.

will fit them back at the weekend and see how long it takes my kids to bust them again.

will keep you posted.

regards. R.

 

[WDB124066]

I tried that before I knew I could buy them new, then on the first operation they blew apart. You're running the risk of being as cheap as me, go buy some new ones...!!!!

 

[rigel]

Ok, W, i've bitten the bullet and taken yout advice.

just ordered them fro MB Epsom, well the cheque is in the post at least.

thanks for the persuasion. "if a jobs worth doing do, its worth doing properly", as my dad said to me in my teen years as i was about to dig out a large tree root in our garden.

regards. R.