Trunk lock problem-W126 300SD

[ytr1903]

I have a relatively small, but irritating problem with locking and unlocking the trunk lid on my 84 300SD. I have searched the site and I have looked at my repair CD, but the solution still is not in sight. The key operates the trunk lock mechanism correctly. Counterclockwise locks, clockwise unlocks. At first, I thought that I may have only valet keys, but I have watched it operate with the trunk lid open and can see the proper movements of the latch. The central locking system works perfectly from the driver's side door location and properly locks and unlocks everything including the trunk. But when I try to unlock the trunk with the key after having locked the system at the door, the trunk will not unlock, nor does any of the other locations unlock. I thought at first that there might be some sort of electrical switch at the trunk that actuates the central system, but the CD diagram and my visual observation only reveals a vacuum diaphragm at the trunk lock. Any thoughts or solutions?

Tom

[TX76513]

I feel for you : I have a thread here, do a search "300sd trunk lock failure" and there are some pictures on a related thread. I now understand how that mechanism works in a 126, but it's hard (for me) to put into words - so I took lots of pictures and will share with you if you get stuck just PM me.
You may read a thread about a "magic" cable under the back seat - it does not exist (on the 126). Entry through the first aid kit and license plate light are not options either.

[DieselJim]

The problem has to be the trunk lock. Mine works just fine. I haven't taken that area apart. I suspect the trunk key must unlock the trunk via the central locking system as opposed to the door key mechanism which pulls up on the locking mechanism and causes the other doors to lock. My locking mechanism is getting sluggish so I suspect I have a small vacuum leak somewhere but other than the drivers side door key not turning mine works great. Good luck. I imagine there will be an easy solution. The worst case is you have to get a salvaged trunk lock and have to carry around an extra key.

[RockinWagin]

Originally Posted by TX76513

I feel for you : I have a thread here, do a search "300sd trunk lock failure" and there are some pictures on a related thread. I now understand how that mechanism works in a 126, but it's hard (for me) to put into words - so I took lots of pictures and will share with you if you get stuck just PM me.
You may read a thread about a "magic" cable under the back seat - it does not exist (on the 126). Entry through the first aid kit and license plate light are not options either.

Actually my 84 300SD does have the "magic" cable under the rear seat on the driver side. However, I can pull as much as I want and nothing happens. Is there a magic phrase I am supposed to say at the same time?

Actually, to respond to the question, I thought that I was having the same problem and did not try to do anything about it until my battery went dead one day and only the driver door would unlock with the key. I tried the trunk and it did not unlock the first time so I tried again (one definition of insanity is that you keep trying the same thing with the same result) and found that if it turned the key and pushed the button in at the same time, the trunk would unlock. It has worked the same way since then.

[TX76513]

Originally Posted by RockinWagin

Actually my 84 300SD does have the "magic" cable under the rear seat on the driver side. However, I can pull as much as I want and nothing happens. Is there a magic phrase I am supposed to say at the same time?

Actually, to respond to the question, I thought that I was having the same problem and did not try to do anything about it until my battery went dead one day and only the driver door would unlock with the key. I tried the trunk and it did not unlock the first time so I tried again (one definition of insanity is that you keep trying the same thing with the same result) and found that if it turned the key and pushed the button in at the same time, the trunk would unlock. It has worked the same way since then.

I read a hundred threads and wished for magic words when mine wouldn't open - had the back seats out anyways but no magic cord on the 85SD. It ended up being that black plasic cog that pushes the main latch up - you could hear the vac and plunger activate but the side was broke

[lietuviai]

I've got a strange problem with the trunk lock in my 300D too. While it does lock the problem is when you have the lock turned to where you can open the trunk by just pushing the button, it can be opened when the rest of the car is locked but stays locked when the car is unlocked. Seems like something is bassackwards.

[TX76513]

Originally Posted by lietuviai

I've got a strange problem with the trunk lock in my 300D too. While it does lock the problem is when you have the lock turned to where you can open the trunk by just pushing the button, it can be opened when the rest of the car is locked but stays locked when the car is unlocked. Seems like something is bassackwards.

I would examine the black plastic cam on the lock shaft.

[jbaj007]

RockinWagin,

No magic phrase; but some might help

The owner's manual describes using the cable "loop" handle as a two man job. One pulls the loop, the other pushes the button on the trunk latch.

My '84 has the magic cable.

[TX76513]

Originally Posted by jbaj007

RockinWagin,

The owner's manual describes using the cable "loop" handle as a two man job. One pulls the loop, the other pushes the button on the trunk latch.

My '84 has the magic cable.

I probably have the only 126 without the magic cable Probably was a $10 add on the PO didn't want. However, I did create a magic cable - because I don't ever want this to happen again!

[pmckechnie]

I'm going to give you a few facts about my 84 500SEL concerning the trunk lock.
My car had an alarm system which had been disabled except for the trunk lock. The trunk lock is an electric solonoid (sp) that was operated by the alarm system. When it was in the lock position, nothing would open the trunk. When in the unlocked position, then the key worked as normal and the trunk could be locked or unlocked with the key. However when it was in the locked position the only thing that would move it to the unlocked position was a key in the drivers or passengers door. If the battery was dead and the trunk locked, then forget about getting in to the trunk. My car also has the reclining rear seat and I have never been able to find any evergency cable. Since the vacuum pump was missing and a replacement was not available at a resonable price because the car also has the orthopedic seats which share the pump with the door locks, I converted the door locks to electric and used the trunk lock also, so if the doors are locked, so is the trunk.
Check under the sub pannel in the right front floor board and see if there is a small box with i believe 1 plug with 10 or 12 wires. That would be the alarm system, and is the unit that handeles the trunk lock.

[TX76513]

Originally Posted by pmckechnie

I'm going to give you a few facts about my 84 500SEL concerning the trunk lock.
My car had an alarm system which had been disabled except for the trunk lock. The trunk lock is an electric solonoid (sp) that was operated by the alarm system. When it was in the lock position, nothing would open the trunk. When in the unlocked position, then the key worked as normal and the trunk could be locked or unlocked with the key. However when it was in the locked position the only thing that would move it to the unlocked position was a key in the drivers or passengers door. If the battery was dead and the trunk locked, then forget about getting in to the trunk. My car also has the reclining rear seat and I have never been able to find any evergency cable. Since the vacuum pump was missing and a replacement was not available at a resonable price because the car also has the orthopedic seats which share the pump with the door locks, I converted the door locks to electric and used the trunk lock also, so if the doors are locked, so is the trunk.
Check under the sub pannel in the right front floor board and see if there is a small box with i believe 1 plug with 10 or 12 wires. That would be the alarm system, and is the unit that handeles the trunk lock.

I will have to take a peek and see what's there. I am guilty of not taking pictures of this (mess) when it was out. A good photo montage would be helpfull to everyone. Unfortunately there does not appear to be much uniformity in the years.

[RockinWagin]

This thread got my curiosity going.....not too hard to do.....so yesterday when the trunk happened to be open, I pulled on the cable and heard a click at the trunk lock. So something was happening. I closed the trunk and used the key to lock the front door. The trunk locked. I pulled the cable and nothing happened.

I was able to unlock the trunk with the key--turning counter-clockwise and pushing in the button at the same time. If I don't lose the key, I am OK.