1984 300SD (W126) trunk won't open even with the key
 

I just bought a 300SD, and the seller said the trunk won't open. She has some items inside and would like them back. I can insert the key and turn the cylinder to the left and push in, and nothing. Pushing the lock in at any position will do nothing. The only way I can think of to get in is to bust out the tail light, and replace it once I have gained entry and the latch fixed. Any ideas?

you can try drilling out the cyl but I dont know if that'd work

can you get in through the speaker holes behind the back seat? Or the first aid box hole?

I don't really want to drill a cyl that I have the proper key for.

If I could, what would I push/pull to open the trunk? Even if I busted out the tail light, I have no idea what to move that will open the trunk. I think the locking mechanism is different than a 123 (I have one that I could practice on), so not sure what to do once I have access (tail light, first aid kit hole etc).

Hmmm.

I just started driving my 85 SD, its trunk lock and electric door locking functions work perfectly...
when you turn to the left, do you feel anything click?
on mine, I feel the tumbler disengage the lock, and actuate the door unlock pump.

Mityvac on your yellow/green line should insure your trunk lock vacuum actuator is at least unlocked too.

Mine is an '84. I have not even looked, so I am not sure if it has the vac/pressure pump or just vacuum. Either way, turning the key to the left and pushing in should pop the trunk. If I had a photo of the inside of the trunk, I could possibly figure out what to push or pull from either the first aid kit hole (first choice), or the tail light. I could go to my local wrecking yard, but have no spare time to do so.

If all else fails, cut a hole behind the Reg plate, under the lock, with a hole saw and pop-rivet a patch on it when you're done.

You might ask the Lady if any of Her keys ever opened the Trunk.

None of my Keys open the my 123 Trunk because I do not have the Master Key. I have the Valet key.
I do not know if it is the same for 124s but some day I need to get down and have a Master Key made before it gets too expensive or the stop making them.

If the Lady was never able to open Her Trunk with the Key She may have been in the same situation I am in.

Try putting the palm of your hand on the pop buton and smacking it in really hard over and over while pulling upward on the lid, I bet youwill get it to open .
I had the same problem with my 85 SD. when I got it. After I started using it more often it started working better.

This smack it with your hand (or rubber mallet) worked for the locksmith Triple A sent for me. Once it was opened he adjusted the latch (had developed an overbite)

Before breaking, drilling hitting etc try one thing first. If the car has a factory alarm system, the trunk lock is electric, not vacuum. First check and REPLACE the alarm fuse (sorry, don't remember which one it is) and if good, unlock the drivers door and the passenger door with the key. If that doesn't work, look under the false floor under the carpet on the passenger side front. Make sure the alarm module is plugged in and the contacts in the plug are clean. Try the key in the front doors again. I had this problem when I first got my 84 500SEL. Trunk wouldn't open. Turned out someone had unhooked the alarm system and the trunk was locked at the time I guess. After reconnecting the alarm system, and unlocking the doors to try to get the alarm system to reset (it was bad), the trunk would open and the key would lock and unlock it as it should

Paul

It appears that the lock cylinder rotates.

You can try funola's (aka MacGyver) bamboo and coat hanger trick. Maybe the 300SD is similar. If someone can assist you by taking a pic of their trunk latching mechanism, that would probably help.

http://www.peachparts.com/shopforum/diesel-discussion/194141-breaking-into-123-trunk-w-o-breaking-anything.html