123/126 Chassis Central Locking Troubleshooting
(applies roughly to other models)
1.) Get a MityVac or other similar hand vacuum pump.
2.) Remove carpet from forward passenger footwell
3.) Carefully pry up cover from wire gutter on floorboard.
4.) You will find 2 sets of vacuum lines connected with a 4-way vacuum tee:
Yellow/Red stripe - locking
5.) Remove each line one at a time and apply vacuum. Every one should hold vacuum. You'll see a corresponding action from the related door/trunk/fuel door actuator.
6.) If you don't, or if a particular line will not hold vacuum, trace it down. It's pretty obvious where they go.
7.) The one thing that might make this a little tougher is the trunk and fuel door actuators, as they share the same circuit in the 126. If this line does not hold vacuum you'll need to pop the inspection cover off the trunk lock and remove the liner on the passenger side of the trunk to get at the respective actuators. Once exposed, remove the flexible lines from each actuator and test both ports. They should hold vacuum in both directions.
8.) If you trace the leak to a door you will have to remove the door panel to access the actuator. As in # 7, test both sides of the actuator. If it holds vacuum, plug the end of the line in the passenger foot well and attach your vacuum pump to the end in the door. If it won't hold vacuum you have a leak in the line, most likely inside the rubber bellows that protects the wires and lines at the door hinge. You can do a quick fix by removing the line part way and using a piece of flexible (rubber) line to "patch" the leaking area. Hard plastic vacuum line (clear, not yellow) is available as a stock item from most MB suppliers.
9.) Occasionally the rubber bushing that seals the line to the vacuum reservoir can dry out and crack. This will allow locking while the engine is running (on a 123) but no operation once it is shut off. Check the reservoir by applying vacuum from your hand pump to the yellow line in the engine compartment that has the gray stripe. It will take a fair amount of pumping to build up vacuum, as you're trying to evacuate a large reservoir with a little hand pump, so be patient. If after 30-60 seconds of vigorous pumping it still does not hold vacuum you can assume there is a leak in the line or the reservoir is leaking. Plug the line on one end and apply vacuum from the other. If it holds vacuum the reservoir or seal is leaking.
10.) On a 126 the vacuum is provided by a reversible pump that has a pressure operated snap switch to reverse operation when it reaches full vacuum/pressure. With the car turned off lock the doors from the inside or with the key. You should hear a quiet whirring noise coming from the trunk. If not, the pump may not be working. Life the spare tire cover and remove the spare. In the 4 o'clock position of the spare tire well is a metal cover. Remove the screw on the end of the cover and lift it off. Underneath is a large black foam "football" that contains the pump. Remove the pump and unplug it from the wiring harness. Plug it back in. If it runs and shuts off after 20-30 seconds it's timing out due to the inability to develop maximum pressure/vacuum in the system. (You have a leak!) Unplug and plug it back in again. If it runs and shuts off after the delay the pump is probably good. Unplug the vacuum line from the pump and plug the port coming out of the pump. Plug the pump in again. If it runs and shuts off after a few seconds each time you unplug and plug it back in it's good.
Keep in mind that an actuator may hold vacuum in one direction and not the other, so you MUST check both sides of the system to be sure to isolate the problem.
I had a problem with my 250 LWB holding vacuum for more than an hour or two. I spent approximately 10 minutes diagnosing the problem yesterday - a bad actuator in the rear driver's side door on the unlock side. Another 10 minutes to remove the door panel and check the actuator itself. Piece of cake.