door and trunk locks
 

All door locks and the trunk lock failed to operate at the same time? 1983 300sd

Is battery dead?
Could be possible vaccum failure in system?

door lock failure
 

They are controlled by vacuum, I assume you have a working vacuum pump otherwise the power brakes dont work. There is a resovoir. Check for broken hoses, check valves, etc. the vacuum switch is in the front door attached to the lock and controls the rest. A handheld vacuum tester is a great tool on diesels and early cars.

On the W126, the power locks operate off an electric vac pump in the trunk, under the plastic cover in front of the spare tire, I think. Smaller than a football.

Check the connections, and check for a bad fuse, too -- no electricity, no pump. A broken line in any door will cause them to fail, as well, as this system uses a single line -- vac to lock, pressure to unlock on the same hose. If the pump runs when you lock a front door, there is a leak or the pump itself leaks -- check with a MitVac. If the pump does not run, and the fuse it good and there is power on the main line to the pump, the pump is shot.

They are NOT operated off the engine pump nor manifold vac.

Peter

on my 94 e420 the gas tank lid does not always unlock. It has gotten stuck twice. Last time I unlocked the door a few days later and it opened but right now it is locked and will not unlock. Any ideas?

Pull back the liner in the trunk and find the manul override lever or door lock type button and unlock it by hand. After you get it open, check the locking tang -- on the W124 it's an extension of the door with a hole in it, often gets bent over so that the lock pin hangs in it.

Straighten it out if it's bent. May need to repaint it, too, as the paint will usually crack off when it gets bent over at a 90 degree angle...
Peter