Got carried away with another post and figured I would make it a seperate post for any future searches.
Start at the begining for a lock problem in a W123.
One note, all yellow/red lines should connect to yellow/red lines. All yellow/green lines should connect to yellow/green or the system integrity is lost. It is the master controller in the driver door that controls whether vacuum is applied to the yellow/red or yellow/green. (Kerry, this is detailed for anyone's future reference, you have done much of this already)
Tools needed
1. A Mityvac.
2. Golf tees to isolate the components
3. A section of vacuum hose for working the system
4. A printout of the manual below, 80-260.
5. Patients, sometimes in short supply
6. A good check of the system using the Mityvac consists of, connect the Mityvac. Pump it up. You should hold a vacuum and/or have the lock operate and then HOLD a vacuum. If it leaks at ALL then you have a bad line or element leak/crack
7. Reference manuals below
http://skinnerbox.steaky.org/Service/W123/Index/Groups/80CentralLockSystem.htm
The Service Manual for the lock system ( another source)
http://benzbonz.biz/pdf/80-260.pdf
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Basic design of the system
A vacuum pump provides the power to the system (thinking deisels here). It is located at the front left of the 616/617 engine.
Vacuum is used extensively in the car.
At the firewall next to the brake booster, the lock system line enters the passenger cab. The line is Yellow, yellow/gray in older cars. Just before the line enters the firewall there is a check valve that should work. Suck on it to check it.
From there it goes to the master control valve at the driver door, located behind the door panel.
From that point, 2 sides of the sytem go to the rest of the locks. The Yellow/green is unlocking (think green-GO into the car). Yellow/red is locking (red-STOP, cant go into the car)
From the driver door control valve the red/or green lines recieve vacuum and this vacuum pulls the element unit to one side (lock or open). Vacuum is applied to all yellow/red lines to lock the car. Or all yellow/green lines to unlock the car. From the drivers floor. The yellow/red and yellow/green lines split to 1, the rear left passengers side. And 2, the right front passenger/rear passenger/ trunk lock and fuel flap side. Additionally a reservour is installed in the system to lock and unlock the doors while the car is not running. This is located above the fuel tank in the sedan and under the rear floor in wagon. It is routed to the the feed line entering the system before the driver door master valve.
To get started
1. Vacuum pump. Is it good, It should pull 22-25 inches when completely isolated. (Kerry by what you said it sounds OK.)
2. Remove all floor mats and carpets to check all 2,3 and 4 way connections and hard lines for cracks/age. OR do this one section at a time as you progress down the system.
4. At the driver door start by isolating the system down stream of the master control valve in the driver door and check the lock and unlock sides downstream. Do this under the driver floor carpet and mat. From under the driver carpet you have lines going to the right side of the car and ones going to the rear left passenger door. If the systems checks OK then its upsteam of/or the master.
5. If required, check your master in the drivers door, which requires removing the door panel. Make sure the seals are not leaking in the unit or the unit is not cracked. My daughter had this (leaking seal) in the 240D. Your system may have multiple leaks so checking the master valve may be prudent
6. If the leak is still down stream then go under the driver mats and carpet and check the left side rear passenger door.
7. If OK then at the driver floor, check the lines going to the front right passenger/rear right passenger/fuel flap/hatch or trunk side. If still leaking.
8. Then go to the passenger side floor and check the lines going to the right rear passenger and the fuel/hatch flap. If OK then its the right front passenger door area. If not then continue
9. Finally from there go to the fuel flap and hatch/trunk, check them seperately. You wil only find the Yellow/Red going to the fuel flap. The trunk/hatch element may or may not have the yellow/green line, depending on the year. The yellow/red only line elements are spring loaded to open.
10. It is possible that a line is cracked and causing the problem. Once you move along the system and get an OK reading isolate those down stream components and check the lines "only" going back up stream, this requires isolating the upstream element. That is where it could be. Now you must check the individual line, connection and element.
12. Additionally there is a vacuum reservior that may be cracked or leaking. In the sedan it is above the gas tank in the trunk and in the wagon it is under the rear floor. Gas models may or may not have an additional tank in the front wheel fender area (IIRC).
13. Depending on the year, you can rebuild the door element with a new diaphram. Or you just replace it.
Dave