W126 Lock Problem: CANT LOCK TRUNK WITH KEY??
 

Guys, I've had a problem with my locks since day I got car. I've lived with it & would've continued to do so, except now it's worsened. Until today, if I locked driver's door, trunk and gas flap would lock also. Of course all other doors had to be manually pushed down as only driver door and trunk/gas flap would lock automatically. Now, when I lock drivers door, trunk/flap remain unlocked. WORSE is the fact that I can't seem to LOCK the trunk EVEN WITH THE KEY. It seems it SHOULD LOCK with the key. Right now...I just want my trunk to LOCK, will worry about rest later. Thanks in advance for help!!!

85 300SD, Astral Silver
226k miles

Does the trunk lock cylinder turn?

Sounds like vacuum line(s) to the doors leaked, and the leak has worsened. Some troubleshooting with a vacuum source (MityVac is highly recommended here, do a Search) will be in order.

The 85 SD should have an electrically operated vacuum pump in the trunk, by the spare, to make vacuum for the locks. Be sure it's running.

Does the key not turn in the trunk lock? I understand that the key will mechanically lock and unlock the trunk, as well as start the vacuum pump to pull the other locks open or closed.

Others will add to this I'm sure.

Best Regards,
Jim

Key Turns in Cylinder...but doesn't Lock!
 

Thanks for responses...yes key turns in the cylinder, but it still won't lock the trunk. I'm following locking instructions on owners manual, but won't mechanically lock. I'm perplexed! I've never tried to lock with key in trunk...it always locked when I locked drivers door. I can hear the vacuum motor/pump operating in trunk when I lock door.

on this topic
 

my rear lock does not turn.
and i also have always just lived with locking from the front doors
how would i remove/free the lock tumbler?
Ive tried to see how i could remove the thing but i dont see an easy way to remove the lock it self.

Sound like two problems, mechanical failure of lock cylinder in trunk lock and vacuum line leak(s) to door(s) and fuel door. Pick your first battle and let the games begin! ;)

I haven't worked on either, so I'm not going to be much help. I can just sound encouraging.

See what you can see of the trunk lock, and there may be an easy way to remove and check. I'll leave that support to others.

Jim

You probably are using the valet key.

Only unlocks the drivers door and starts the
car.

You can get the correct key from MB but
you'll need the VIN and proof of ownership
before they'll order one.

Louis.

The automatic locks don't lock all the doors?
My 300SD is soon to (hopefully) get a working door lock pump and was hoping it would lock all of them :(
BTW the trunk lock is easy to deal with...the lock cylinder and the key tumbler/button are two seperate parts that just sit next to one another..
Take off that black thing on the back of the car that holds the licence plate lights and whatnot) by snapping the plastic off the top lip there by the trunk front
Remove the 4 nuts (8mm I think?), take the black deal that has the lights off...
Then from inside there are two nuts 10mm that hold the lock cylinder on..
remove those..remove lock cylinder and fix.
If you take the tumbler out of the lock...make SURE you have the key in there first...else you'll be guessing on how the pieces go back in :)
It can be a pain *stares at drivers door with 1 bad leaf pair*
If you have any specific questions or need better details email me and I will explain it to you. I recent had to do this on my car to take the trunk lock in to see about getting a door key made...
Ended up getting it from the dealership and just gave them the lock to make sure it worked ok
My ignition key and door key are two different keys....This is correct?
Hope that helps!

EDIT:
That reminds me, when you go to remove the key buttom setup (after removing the 2 10mm nuts) you will have to remove a small screw from the back of this unit that holds the vac pod shaft to it...you can bend it and whatnot to get at it with a screw driver or whatever...just make sure you bend it back when you are all put back together.. :)
The vac line runs down the passenger side over to the gas door....yellow.

There's another thing you can do to get the vacuum to working in the trunk. On the passenger floor is where the vacuum line has a three way rubber fitting just plug one line at a time untill the lock start working and the pump shuts off this may make the trunk able to lock. then work on the vac lines when you have time. Alot of the time its the lock valve in the doors that are bad and need replacing.
Bill