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!
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:
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:
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