All 4 doors lock from the drivers side door with the engine running on my 75 280s
But when I shut the motor off and try locking with the key only the drivers door locks. Couple be a broken vac line or a master vac regulator maybe? Where do I start?
Thanks for your suggestions
You mean "lose" pressure?
You need to be methodical when it comes to troubleshooting vacuum central locking. It's actually quite easy once you know how.
First of all, you need a vacuum source, such as a MityVac. Without this you're wasting your time. If you don't have one, you need to get one as it's a valuable troubleshooting tool for any number of things on your car and for that matter, any other car you own.
1.) Check the vacuum reservoir. This is usually located in the area near the brake booster, and can be hidden in the rear portion of the front fender behind the wheel. You'll see a vacuum line going from a tee fitting to a round plastic tank or into a grommet in the fender wall.
If you can get to the reservoir, check the grommet where the vacuum line goes into the reservoir. These get old and dry out, and as a result will leak, allowing vacuum to leak out of the reservoir. The reservoir is critical to the system's operation, as without it you won't be able to lock or unlock the doors at rest. With the reservoir full you should get at least 3-4 cycles of locking/unlocking before you run out of vacuum.
Inspect the lines in this area as well - they can also crack and leak from heat and age. There can also be a check valve in this line which prevents vacuum from escaping back to the engine. It's a little disk shaped object in the vacuum line from the engine and is often two colors, like yellow and black.
2.) If the grommet and lines are intact, remove the line from the tee fitting and connect your MityVac to it. Pull a vacuum and see if it holds. If not, look for leaks. It's possible that the tank might be leaking. Rare, but it can happen. If it holds vacuum, disconnect the MityVac and reconnect the line.
You didn't say if your car is a W123 or W114, but this will apply to just about any MB of this vintage.
3.) Pull up the passenger side carpet so you can see the floor and plastic "gutters" that the vacuum lines and wiring run in. Pry the cover off the gutter in the floor of the passenger side. Once open, you'll see a bunch of yellow vacuum lines with colored tracers (lines) on the sides. There will be green (open/unlock), red (close/lock) and a third color that escapes me - I think the supply line color is brown. You'll see these lines connected to a bunch of tee fittings.
Here is a basic overview of an MB central locking system:
Vacuum from the engine is ported to a reservoir under the hood. A line from the reservoir goes to the door lock mechanism on the driver's door. The valve on the driver's door feeds vacuum to either the lock (red/yellow) or unlock (green/yellow) lines for the door actuators through a series of tee fittings located on the front passenger floor of the car.
4.) You can put your MityVac on the supply lines from the driver's door valve where they come into the tee fittings on the passenger floor. By opening or closing the valve on the driver's door you can see if you're getting vacuum to the appropriate tee fitting.
Understand that you may have to start the car occasionally to build up vacuum in the reservoir.
5.) So let's say that you're not getting vacuum to one of the tees. You'll have to check the lines back to the driver's door valve. If this is the case, you might have a cracked line at the door hinge in the rubber "accordion" that protects the vacuum lines and wires between the door and the body of the car.
6.) If you are getting vacuum to both tees, the fun begins! At this point, you'll want to systematically apply vacuum to each of the lines going from the tee fittings to the doors. Note that there will also be lines that go into the trunk for the trunk lock and fuel door lock.
As you apply vacuum to each line, you'll hear and see the respective door lock open or close, depending on which line you are applying vacuum to.
It's at this point in the process that things will get interesting.....
If all has been well up to this point, and typically it is, you'll find a door that has a leak in either the lock or unlock circuit. This will manifest itself by the inability to pull a vacuum on that line, as well as not getting the expected action from the door actuator.
At this point it's just a matter of isolating the leak by going to the actuator and applying vacuum directly to it. This means pulling the door panel and getting to the innards of the door if it's a door actuator. If it works it's good and you have a break in the line between the tee and the actuator. if it doesn't work then you'll need a new actuator.
You can also find the occasional door actuator that will hold a vacuum closed but not open. This is due to the rubber getting a smashed together and covering or sealing the break or crack.
Either way, if an actuator leaks, replace it.
For the interim a golf tee makes a good plug to seal off the leaking circuit until you can replace the bad actuator or repair the leak. The door or part affected won't work from the central locking system, of course, but this will at least let the rest of the system work.
Again, the central locking system is an example of simplicity. Once you understand the operation you have mastered the system for any MB with vacuum central locking....