Just checking that we are all on the same page, I only have experience with W114, W116 and early W126 central locking systems and may be confused as to what's installed in your car.

It is my thinking that the W114 and W116 are vacuum driven from engine manifold or engine driven vacuum pump. There is a reservoir either in the drivers left fender or under the package shelf. In the drivers door is a switch over valve which controls which of 2 lines has vacuum. These 2 lines are routed to each door to operate vacuum driven locking mechanisms. There is no locking mechanism in the drivers door, since you are locking it with the key or knob. I don't remember if the trunk and fuel door are 2 lines or 1 with a spring. Since the 450SEL's system had so many leaks, I installed electromechanical actuators in each door and didn't bother with the trunk and fuel filler door.

In a early W126 like my 300SD, the system uses both vacuum and pressure provided by a pump in the spare tire well. The drivers door has a switch module that triggers the pump. There is no vacuum line in the drivers door. The switch is "latched" or maintained, meaning it stays set in which ever position the key or knob has put it. Locking the door will cause the pump to pull a vacuum on the system, locking the other three doors, the fuel filler door and the trunk. Unlocking the drivers door triggers the vacuum pump to apply pressure to the system unlocking everything. There are sensors in the pump that check for the correct vacuum at the correct time. If the vacuum doesn't reach setpoint, the pump will reverse, unlocking everything then try again. It will cycle several times then give up. The 300SD was recycling a lot until I lubed the linkage for the rear doors. They were slow to react and would lock at the last moment, causing the vacuum level to change and tricking the pump into thinking something hadn't locked properly.

My reading leads me to believe that the later W126 cars had additional switches in the passenger door and the trunk and that the drivers door had a vacuum/pressure actuator as well as the switch. This would let you lock or unlock the entire car from either front door or the trunk.

Since my 300SD is an early car with only one switch, a remote keyless system with one actuator in one door takes care of the whole car.

I can't find service manuals for the W123 so I don't know how the central locking works with that chassis but am willing to learn.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

Service manual has a nice diagram of w123 central locking system.

And yes they did modify the central locking on later w126's. Mine has an actuator in the drivers door.


Basically the 123 system works like this:

-powered by either engine vacuum on gas models or engine vacuum pump on diesels.

-2 circuits: one locks the second one unlocks, both work on vacuum and the actuators have 2 lines going into them.

-the vacuum reservoir is above the fuel tank on the sedans

-No actuator in the drivers door

-The "Central locking valve" is only located in the drivers door so central locking only works on that door. If lets say you try to lock the car from the passenger door then only that door will lock but none of the others.

-Fuel door has only one line going to it. It's kind of a fail safe so that if you have a leak the fuel door will just become open over time by the small spring it has.

The bolded is the W123, mine anyway.

My only other experience with this is the remote system I put in my '86 SDL, 2nd gen W126, I was able to control all 5 locks from the module in the tire boot.

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1986 300SDL, 362K
1984 300D, 138K

Could you share with us the brand and model actuator that you used to make yours work? Better yet, did you get it from a boneyard wreck?

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1986 300SDL, 362K
1984 300D, 138K

My take away from cmac2012 and christuna's replies is that the W123 is essentially the same as the W116.

From that, if you want to fix the system and keep it all original, without remote control, you will need to find and fix all vacuum leaks and possibly replace or repair all actuators on the car, there are 5 on a sedan, coupes have 1 in each seat for the folding seat backs, I don't know if they use the same vacuum reserve as the locks. There are check valves that separate the vacuum system into at least 3 sections. Climate control, brake assist and central locking. There may be a 4th subsection, engine control but I'm not sure here as it changes year to year and model to model. Finding vacuum leaks can be a huge undertaking, there are a ton of plastic and rubber hoses and rubber diaphragms. You might want to look into getting a smoke machine.


If you want to keep the vacuum system intact and want remote control, you will need to piggyback a electromechanical actuator to the drivers lock knob to mimic the action of pulling it up and pushing it down. This will trigger the switch over valve in the drivers door, the vacuum system will take care of everything else. In a W126, there is room for the electric actuator near the outer arm rest mounting bolt with the rod passing between the sheet metal and the central locking switch.

If you don't care about the vacuum system or it's so far gone that it's too expensive to repair, you can replace the drivers door switch over valve with a 2 or 5 wire electric actuator, replace the vacuum actuators in the other doors with 2 wire actuators and either replace the fuel filler door and trunk vacuum actuators with door lock style electrics or just use the key on the trunk and ignore the fuel door. The difference between 2 and 5 wire actuators is that 2 wires allow lock/unlock only while 5 wires provides input to the control module to allow central locking with a key if the remote has gone missing. It might even be possible to add a 5 wire module to the trunk and passenger door and get the same functionality the later W126 cars are rumored to have.

I'll give you links to what's in the 300SD but I've not powered it up beyond testing to make sure I didn't get stuff that's DOA, since the vacuum system works on this car so I don't know how well it works.
Actuators
Remote system

I installed an older version of this in the 450SEL, I did not use the alarm portion so could have saved money with this package I don't think my key fobs were as clunky looking as the current version. The actuators are beasts, sticky mechanisms are not a problem. They are kind of chunky though.


Edit:
You will see units that say they will control a pneumatic system. As far as I was able to determine, they will only work on later W126 cars with the electric pump in the trunk. They won't work with the early W126 electric pump and not at all on purely pneumatic systems with out adding actuators as mentioned above.


Michael

__________________
Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

https://www.ebay.com/itm/323635719483

You need this to trigger the vacuum switch at the driver door for the w123 or earlier w126. You do not configure your keyless remote as pneumatic configuration. It is configured as polarity triggered. Fixing vacuum is easy if you have a vacuum pump. It is divide and conquer to locate the leak.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.