Pneumatic seat lock mechanism post update

This topic has been truly fascinating. I have been contacted from W114 owners, near and far, thanking me for the diagram and explanation of the pneumatic seat lock system.  As there has been so much interest and several questions, I have updated the post to include some more information.
Please follow the link below:

Pneumatic seat lock mechanism

Check your check straps

When I think of the moving parts of a car that suffer the most amount of wear and tear, my thoughts are always directed to the drivetrain; engine, gearbox, driveshaft, differential, half-shafts, wheel bearings etc.  But when you actually think about it, none of these things work without you first having to open the door.

With this in mind, I considered how many thousands of times UGU's doors had been opened and closed in the past 43 years, and why they now showed signs of wear and would no longer hold open.

This used to annoy the life out of both my wife and I.  It was as if UGU was attacking us with her doors every time we tried to get in and out.  Even on a flat surface the doors would close hitting us on the shin or another part of the leg!  I had no idea why the doors would't stay open so I posted the question on the Facebook group W(A)114/115, and the guys there told me that my check straps were knackered.

For those that don't know what a check strap is, just like I didn't, the check strap is that flat metal bar that you see below the interior light switch in the door reveal which I have referenced in a previous post.  That metal bar connects the door to the main body structure of the car and serves two purposes: the first we have mentioned, to hold the door open, and the second function is to stop the door opening too far if caught be a strong wind for example.  Without the check strap the door would swing around and hit the front wing damaging the bodywork.

Check strap connection to main body towards bottom of image

I have not been able to determine if the /8 has different strength check straps. At this stage I do not believe that they do, even though the coupé doors are significantly bigger than the sedans, which I would imagine would put more stress on the check strap.  Perhaps this is why coupé's seem to all suffer from this problem.

A simple way to check if your car's check strap is broken is to try and wiggle the check strap.  It should not move from side to side; if it does, it is broken.

So, with the problem identified, what to do next? Get replacement parts...

I got on the phone and rang a few different MB parts suppliers.  Initially I was looking for new parts, but after being quoted $300AUD each I started looking for good second hand parts despite being told that I was wasting my time as they would just fail again.  Let me tell you, this is not true, second hand parts can work just fine!  Two years later I still have had no issues with my replacement parts.

I eventually sourced one unit in Arizona, USA via Ebay.  Even though I need two, I decided one was better than none, so when it arrived the next week I decided to attend to the passenger door first.  This made sense as only I ever open the drivers door and am more likely to forgive UGU for attacking me with her door - I find it hard to hold a grudge with her.

To access the check strap you first need to strip down the interior door trim; map pockets, arm rest, window winder and door card to name a few.  Despite their age, every single piece unscrewed and unclipped with ease. This was one of the first DIY jobs I attended to with UGU, and it was my first real appreciation for the renowned Mercedes-Benz build quality.

   

Passenger door with interior trim removed

With the trim out of the way, I fished about in the bottom of the door removing the dirt and muck that had gathered over the years. It was here I found the first part of the broken check strap; it was very broken, smashed to smithereens in fact.

Remnants of original check strap

This is what it is supposed to look like

To remove the check strap from the main car body, remove the c-clip on the bottom side of the retaining pin and pull the pin upwards.  At this point it is worth using something to hold the door open as it will want to close and it will keep hitting you, either tie the door to something, or prop it open. In true tyro fashion I used a wheelie-bin to prop it open, haha.

Next, undo the two bolts on the edge of the door and the single retaining screw on the inside face of the door. That is it.  Pull out the old check strap and have a look at what you have, and I bet it will be scrap.

Before installing the replacement part I would recommend cleaning it, re-greasing it, and if you are precious, painting the check strap the correct colour as it should be body coloured.

Whilst the door is stripped down it is also worth a good clean and investigation of the rest of the door inner parts such as the window mechanism and door lock.

To install the new part I retraced my removal steps. I loosely bolted and screwed the check strap to the door and set about connecting the metal bar to the car body.  Physics was never my strongest subject, but at this point I realised how the weight and momentum of the door makes opening and extending the check strap effortless. Doing this when you only have a tiny metal bar to pull on isn't so easy!  Only when you have finally extended the check strap to a point where it can reach the main body fixing point, whilst allowing you to fit between the car sill and door will you be able to line up the retaining pin. Push the pin all the way through the main body fixing and secure it with the c-clip you removed as one of the first steps.

With this done you will be able to open the door and fully extend the check strap (if you have not done so already).  If the new part is working the door will now stay open unassisted.

You can now tighten up the bolts and screws firmly, securing the check strap casing to the door.

Open and close the door a few times and make sure there are no grinding or groaning noises.  Also make sure the door is not hitting the wing when fully opened, and that it will stay open.  If all looks good, refit the interior door trim and that's it, job done.

Approximately 6 months past before I was able to attend to my drivers door as I struggled to find another replacement.  It was only after talking to a fellow member of my local MB car club that we realised that the check strap casing is the same as on a W108 which he coincidently had as a parts car.

The W108 check strap casing turned out to be the same as the W114, however the metal arm was a different length, shorter from memory.  This is an easy problem to overcome and this is how:

Strip down interior door trim, and prop open the door (wheelie bin worked for me)

Remove what is left of the existing check strap. It made sense that the drivers side had suffered more wear than the passenger side

This is one of the wheels from the existing check strap.  As you can see this wheel has not turned properly for years. Instead it has just ground up and down the casing creating these flat spots

Using a large flat head screw driver and hammer tap the top and smallest level of the spring out of the casing.  With this removed all pieces come loose and can be moved out of the way allowing the metal arm and connecting wheels to come free.

Top level of spring removed allowing all parts to become loose

The wheels on the arms are not physically connected to the metal arm, they fit onto a pin and once in the casing everything keeps in place.

Reassembled check strap (as mentioned before the metal arm is supposed to be body coloured but I like the shiny metal look)

Note: The metal arm has a specific curvature to allow it to open with the swing of the door.  If this is not put in incorrectly it will not work and you risk breaking the check strap and undoing all your hard work so please take care, and take note of the position of the original before you take it apart.  I learnt this the hard way and had to take it all apart again, but thankfully didn't break anything.

With the newly completed check strap, fit and test as per previous instruction.

UGU basking in the 38+C degrees with her door open unassisted, hurray!

I really hope that this will help someone else, and save them the annoyance of being attacked by their car doors, haha.

Tyro Part Mart - Item 2 Indicator lens

Following the rebuild of my headlights, I now have a spare indicator lens for sale.

This lens has been used, but is un-faded and in good condition.
Suitable for all European type headlights for W114 & W115 vehicles: 220, 250 & 280.
Fits both headlight units on left and right side
Quantity - 1 only
Part number: N/A

AUD $8 excluding postage.

Pneumatic seat lock mechanism

As with any new car, be that a new car, or a pre-owned car that is new to you, there are always different buttons, knobs and dials to play with; and UGU was no different.

Not long after UGU came home for the first time, I set about cleaning out the interior which was tidy, but grubby around the edges.  It wasn't long before my eye was drawn to hooks on both sides of each front seat.  Not knowing what is was for, I lifted the hook up noting that it had some resistance but when I let it go, it slowly fell back to the original position.  Stumped as to what it was for, I went on with my cleaning.

With everything wiped down, vacuumed and cleaned I thought it best to take the seats out to see what else lay beneath them having already unearthed fossilised apple cores, sweet wrappers and lots of fowl smelling dog hair, yuk!

To make it easier to get the back seat base out, I first removed the front drivers seat which is a very easy process of undoing four small bolts.  As I started to lift the front seat which, for the record, is very awkward due to the fact they fold, I saw a blue pipe was connected to the seat.  A quick look under the seat revealed the pipe was connected to a valve mechanism via a simple rubber connection.  I pulled it off and removed the seat.

   

Front seat out to help remove back seat base

Blue pipe that was connected to front seat

Under the back seat I removed an almost full bucket of sand, more sweet wrappers and even found evidence of the previous owners; which was exciting, but a story for another time.

I was intrigued by the blue pipe I had uncovered. I traced it towards the car's firewall and followed it up the side of the footwell to the back of the interior light door switch.  Looking at the light switch I could see that it too was a valve.

Interior light switch located in door reveal. note the valve in the centre

I went back to the seat I had removed and investigated the valve under the seat.  I lifted the hook I had seen previously, and as I did this the under seat valve was compressed.  Bingo!  I now knew what I was looking at.

With all modern 2-door cars, there is generally a lever that when lifted, pulls on a cable concealed within the seat back, which then releases the mechanism at the seat back pivot point allowing the seat back to fold forward so that passengers can access the rear seat.

I find it amusing that this simple concept was either not considered by the designers/engineers at Mercedes-Benz, or perhaps considered, but deemed too simple and primitive for a Benz.  For whatever reason they opted to use a pneumatic system instead of a cable. I wish they hadn't, as mine didn't work anymore, and with no rear seat belts there was little hope for me, the driver, in an accident as I would be crushed by a rear passenger.

With the front seat now back in the car and the blue pipe re-connected, I once again lifted the hook on the seat and watched as the valve within the interior light switch in the door reveal opened and closed with every lift of the hook.

                                                     

 

Pictures of the front and rear of the under seat valve with the seat in situ. The black rubber parts move in and out pushing and pulling the metal bars that can be seen 

So what to do now?

I spoke to a friend in the MBCCWA who had a W126 SEC - he told me that his car had a pneumatic system for the central locking in his car, and this had also stopped working. He advised me to follow the blue pipes through the firewall and into the engine bay to make sure the pipe had not just come off the engine, which is where the vacuum is provided.

It was easy to trace the blue pipes and as soon as I opened the bonnet I spotted them clipped to the firewall, before finally locating the connection to the engine. All looked to be intact.

Note the blue pipe coming along the firewall.  They come together at a T-junction connector just behind the right hand side strut.  A smaller pipe from the engine also connects into that T-junction connector.

I tried to do some research online, however this particular system seems to have very little easy-to-understand documentation.

The passenger door switch/valve was broken and the prime suspect for an air leak so it seemed like a logical place to start my process of fixing the system.  Luckily, I was able to source a replacement part and some additional lengths of blue pipe from a friend who has a coupe parts car.

At this point I should explain how the system is supposed to work.  With the engine running, suction is created, and with the doors closed the valves within the interior light switch are also closed creating an airtight loop.  The suction from the engine removes the air from the under-seat valves, which in turn pull in two rods which then activate a hook on either side of the seat base locking the seat back in place.  Still with me?

As soon as either of the doors are opened, the vacuum is broken, the seat hooks fall and the seat back can once again be folded forward. 

What impresses me is that the designers/engineers even considered what to do if the car is running, the doors are closed and a rear passenger wishes to get out.  A little black button located on the rear side panel, under the grab handle, can be pushed which also breaks the vacuum allowing the seat to be folded forward so the occupant can reach the door handle. Genius! 

I am conscious that this is hard to explain in words so I have drawn a diagram of the system so that I can share my experience. I hope it will help someone else.

Now that we understand the system, you will appreciate that it unfortunately only takes one leak for the whole thing not to work.

I replaced the passenger door switch/valve and crossed my fingers.  I started the car, pushed the switch in with my finger but the seat hook didn't move, and so began my process of elimination.

Unfortunately there is no easy way to do this; you are essentially chasing something you cannot see, so to increase the odds of success this is where the spare lengths of pipe become very helpful.  

I undid the connection at the engine and replaced it with a test pipe which I connected directly into the Y-Juction #2 as noted on the diagram.  In doing this I eliminated the drivers side of the car and also the existing pipework throughout the firewall as well as the rear release button leaving only the seat and switch/valve in the loop.  After starting the car, I again pushed in the switch/valve simulating the door was closed and hey presto the seat hooks lifted!!!  I was so excited.  I then re-introduced the back release button by reconnecting the original pipes between Y-junctions #1 & #2, and connecting my test pipe into the back of Y-juction #1.  Yet again, when tested the seat hooks lifted and with the door switch valve in the closed position, I pressed the back release button the seat hooks fell.  Fantastic!

Test pipe connected to the engine vacuum take off to help eliminate various parts of the system whilst looking for the air leak

The last bit to test on the passenger side was the connection through the firewall between Y-junction #1 and the T-juction within the engine bay.  I reconnected all the hoses inside the car and moved to the engine bay were I disconnected the passenger side pipe from the T-juction and connected it directly onto the engine vacuum take off.  With a final start of the car, the passenger side was now fully operational, so I moved onto the drivers side where the leak was eventually located after repeating the process.

Amazingly the leak turned out to be in the most unlikely place.  The pipe that connects the rear black release button to Y-junction #1 runs underneath the chrome sill trim, and this is where it was broken, a clean break in the pipe, weird!

Using my spare pipe I replaced the broken section and started the car.  I opened and closed the doors one at a time, climbed in the back and pressed the release buttons, and was very happy that it all now worked.

Like I say, there is no shortcut repair with this, but it is a very worthwhile exercise and gives me a bit more piece of mind on the rare occasion that I have passengers in the back.

Here is a video of the seat hook in action: