No worries now...my son totalled it and we moved on to a W211 wagon.

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2004 E320 Wagon W211
2000 CLK430 Convertible W208
1985 300SD W126
1983 300D W123

Sorry for your loss

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C

I'm 100% good with it. Sold that car for $900 and picked up a 2004 E320 Wagon.

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2004 E320 Wagon W211
2000 CLK430 Convertible W208
1985 300SD W126
1983 300D W123

Haha I was reading the entire thread because Im pretty sure I have your solution until I got to the bit where you said the car is written off...Now I guess we will never know...
Between myself and my friends there are many W124's and they all have an option where you can set the central locking somehow to only unlock only the drivers door when unlocked using the key. When the engine is running the central locking operates all doors at the same time. This is a 'safety feature' that can be disabled and you probably enabled it by accident. Your description matches the functioning of this feature exactly.
I installed a remote system on my central locking so have never used this feature. It should be in the owners manual, if you dont have one look online.
Maybe someone else with a manual or knowledge of this feature can tell us how...

Fish--

What year is your 124? It's not listed in your signature.

The pump should work at all times (key in or out, engine running or not). This suggests that someone may have boogered your wiring. The pump should have power at all times from Fuse C (pin 3 of the rectangular connector at the pump should have a red/white wire and it should measure +12 Volts at all times.

Pushing or pulling one of the buttons (or turning a key in that door) not only locks/unlocks that door but switches a switch in the door that tells the pump to run, creating vacuum to lock all doors or pressure to unlock all doors. A round plug at the pump has three wires (blue=driver, green=passenger, yellow=trunk/tailgate). One of these wires could have broken where they go through the rubber accordion sleeve between the front doors and the A-pillars.

If you are losing pressure/vacuum you may have a bad diaphragm in one of the actuators. The fuel flap lock is often mentioned as a culprit. You can disconnect its line and test the actuator with a Mityvac to see if it leaks. Another option is to blank off that line (flap will no longer lock) and see if the system now works.

Many other options but try those first and report.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970