New owner of an '81 300D. Will someone explain to me how the door locking system works (how to operate it)? Also, I can't seem to get my trunk open even when using the key.

 

all doors ,trunk,and gas tank lock with door key. key must be used to lock doors,you canot push door button down with door open ....[to prevent key being locked in car].car must be locked with key from door also locking trunk and fuel door.trunk lock will also loc and unlock door locks.i thhink thats it.they are vacuum operated so vacuum leaks must be repaired and vac pump must be working.

 

I must have a leak in the system then, because using the key only locks/unlocks the drivers door. All others must be done manually.

 

look near oill filter to see if vac lines are all pluged in...sometimes changing fillter can pull off vac hoses or cracked hoses.

 

What else is wrong with the car? Unless you got it for a song, it does not look good to me. MBZ lock works on vacuum and it is quite simple if you know how. You need a portable vacuum pump to diagnose the problem. There is quite a lot of write-up on the web about fixing the vacuum lock. The main distribution line is underneath the driver side carpet, under the black cable duct.

Can you open the trunk by pushing the button? If you cannot open the trunk with the key nor the push in button then it is not an easy job to open it from the outside.

Good luck.

 

87 300D mine will lock by pushing the button down and lock all locks. Also the darn thing open and close the trunk usally 3 times and it locks everything even when running. It did it to me 2 times until I got a extra key for my pocket.