Trunk stuck closed HELP 1979 350SE

[revilla]

before the locksmith, there's something you might try.  your locking mechanism might be stuck in the middle of lock/unlock position.  That explains the button going all the way in but no opening the trunk; but also explains the button getting stuck in the in position as it misses the correct stop hitting only the side and making the button get stuck.

Disassemble the B pillar trim on passenger side (easy, only 2 screws at bottom). Lift up the carpet under until you reveal 2 set of hoses (red with yellow stripe and green with yellow stripe).  You will also see 2 rubber 'T'.  One tip of the T comes from the front of car.  Another goes up through B pillar to the rear passenger door.  The 3rd connects to hose of same color going under rear bench in direction of trunk/gas tank lid.  This is the one you want to disconnect from the T.  NO NEED TO CUT THE HOSE, simply disconnect at mentioned T.  If you have a vacuum pump, that's great, but if you don't, like me, connect a regular bicycle pump to the red/yellow stripe hose.  Don't over do because you might pop up the rubber diaphragm from the locking element.  Not good.  Try one small pump at a time checking if the trunk unlocks each time. Easy on this procedure.  You can do a quick test first.  Blow through this red hose using your lungs-pump to test if there is resistance -there should be.  If there is not, then you have a massive leak behind that point possibly failure of a rubber diaphragm (a common failure mode). Then you can try the oposite, but this time you'll need a vacuum pump to create depression but this time using the green hose to move the locking mechanism into the unlock position.

red under vacuum = lock
green under vacuum = unlock

the oposite is also true:

red with positive pressure = unlock
green with positive pressure = lock

This is the trick with the bicycle pump but it's not how the system normally works as it works with vacuum coming from engine.

You'll see why once you reveal the vacuum elements and how they work.

I assume you already did the test with the car running and the driver lock at up position.  If you haven't please do before anything.

If nothing of the above works, then you have a trunk lock mechanism that is bent or has lost one of the secure pins which leave you with no other option than the locksmith likely drilling a hole to unlock.  I hope not.

Good luck

[Beastie]

Locksmiths don't know nothin' 'bout no vacuum system on a 40 year old car. I'd take it to a Mercedes specialist.

[rumb]

I would be surprised is a locksmith could do anything.  the lock appears to work, it is the mechanism that doesnt.

Here is a picture of the backside. the key and or the vacumm mechanism is suppose to align the big pin up with the latch release. for some reason yours is not doing that.

I would try turning the key at different angles and pressing the button in to see if you can get the alignment correct.

short of that a well placed hole drilled in the back sheetmetal and using a punch or screwdriver to push the magic spot would seem to be the only solution. Or use coathanger wire to manipulate the rotating portion into alignment. (I think that is what my PO did to get trunk open. )

the smaller pin is what the vacuum rod attaches to. if you vac pod  is broken it will keep the whole thing from rotating correctly and aligning as it should.

[marku]

If the lock barrel is turning then it is not engaging with release. I have no vacuum running on my 450 at the moment and just use the key in the lock. The previous owner had problems and solved it by drilling right through the barrel and removed it. The only way then to release the lid then was to put your finger in and feel for the release arm. Got a replacement barrel from a breaker and fitted it which works fine. The arm on the end of the barrel will not engage the release without the barrel being depressed but needs to be in the right position to do so. It is only secured by a small nut so perhaps this has moved. It is always in the locked position and although the barrel can be turned and the key withdrawn it is still locked I think that this is because of no vacuum. Have heard of accessing the interior through the first aid kit opening and unbolting the lid hinge on that side giving enough room to work on releasing the lock catch on the lid. Seemed to work but it sounds like a real pain. It may be that your only option may be to drill it out.

[revilla]

precisely; as rumb stated, alignment between big pin and latch release is the KEY.  That seems to be the source of your problem. I managed to align mine putting pressure in the red hose as explained in my previous post.  I would do anything before drilling a hole in your great looking car.  That's always the last resource.

[gavin116]

A tough situation you find yourself in...  I was doing a little internet research, and came across this thread.  I think it may be of some use.  Scroll down, and you will see the T-union that Revilla refers to.
http://ozbenz.net/viewtopic.php?f=25&t=12629
Good luck

[w116john]

hi i had a trunk that was difficult to open, there was only one spot around 1 oclock where i could open the boot.
like yours it didnt always spring back so it was trial and error looking for a sweetspot untill i found a catch and the boot opened , you would feel a bit of pressure on the key

long shot i know

[mercedes350se]

Thanks for all the help everybody, much appreciated! I am going to try out the possible solutions listed above this week sometime when I get a free chance and hopefully one of them will work. I really don't want to drill into the trunk, it'll be a shame to do it. I'll let you guys know if there's any updates. Thanks again!