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FCP Euro Service Kits

FCP Euro Kits

t1For a vehicle with such a prestigious heritage of chauffeuring, buying my Mercedes S500 with a stuck trunk was a bit of a disappointment. It would not open from either the button on the trunk or the button on the drivers door.  Apparently this is a common problem as these cars age. There is a manual override though which not all owners are aware of, the key can be turned a quarter in the opposite direction that you would expect to force the latch open.  This is not obvious as very often the mechanism has never been used and there can be a decade of dirt and dust holding it stuck.  Liberal use of WD40 or similar should free this up and let you get the trunk open.  To diagnose the problem further though we need to get into the lock mechanism.

Digging in

First thing to remove is the plastic around the lock - you'll need a Torx T20 bit to remove the 4 bolts.  The insulation panel is held on like a door card - plastic clips around the edges of the panel.  Just try to feel where these are and pull hard to detach them.  Unlike a typical door card none of my clips broke!  You may want to buy a few spares just in case though.  There is an inner switch attached to the panel, that can just be unplugged and the panel moved out the way.

Perhaps this is a little over-engineered?

The main lock is held on with 2 T30 bolts - but is very fiddly to remove due to limited access - ahead of that you can shine a light in and try to see if there any obvious problems that can be fixed with the mechanism still in place.  At this point you can see what is either a cool or crazy setup depending on your point of view.  The trunk unlock is controlled by an air pump applying air pressure to sealed diaphragm unit.  A piston then extends out of this unit which pushes a latch attached to a rocker which physically pulls the lock.  I don't want to question the wisdom of Stuttgart, but perhaps this is a little over-engineered?t2

In my case with the insulating panel removed and the button pressed I could hear an air leak from somewhere, so I knew the electrical side and pump were working.  Closer inspection found some plastic tabs had broken off the diaphragm unit and so air leaked out the at the joint rather than pushing the piston.  It's a safe bet the plastic was just weakened with age and the pressure on it - the air pump can actually supply a fair bit of pressure!

At this point you can buy a new lock mechanism or just attempt a backyard repair - liberal amounts of epoxy resin smeared around the joint and where the tabs should go has worked just fine for me so far.  If you have a stuck trunk and can't hear an air leak, unfortunately you'll have to diagnose further back starting with the pump fuse and then the upstream air lines themselves.t3

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About the Author: Bryan McPhail

IMG_2691Bryan is a longtime BMW enthusiast in Florida.

 

 


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Written by :
Bryan McPhail

Bryan is a longtime BMW enthusiast in Florida.


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