“”the w123 vacuum locking had one major design flaw over all the later systems, it used engine vacuum stored in a tank, needing to be entirly leak tight to work. even just a tiny air draw would exhaust the vac tank over night, meaning unlocking next day would not work. later systems, eg w124, used a vacuum/pressure pump that ran when the vacuum/pressure was needed, and so could cope with considerable leaks and still work.
the place to start with yours is under the bonnet. at the o/s bulkhead you will see various vacuum pipes. the two yellow ones are for the locking system. one is yellow with a black trace – this goes to a vacuum storage tank at the rear. the other pipe is plain yellow and this goes to the valve in the driver’s door.
when the drivers door is locked or unlocked it moves the piston in the valve. you will note that the other two pipes connected to the valve are yellow/red and yellow/green. the red trace denontes the locking circuit and the green trace denotes the unlocking circuit.
from your drivers door the pipes then go to a junction point under your drivers foot mat, near the o/s of this area under a plastic duct. this is the easiest place to isolate seperate parts of the circuits for diagnostic purposes. you will note that each of the two pipes coming from the driver’s door branch into three seperate pipes, one goes across the chassis to feed the n/s doors through a further junction in the passenger footwell, one goes to feed the o/s/r door, the other feeds the fuel flap and boot/tailgate. note there are two pipes to every lock mechanism, but on later models the filler cap has only a locking pipe.
this job is all about following the vacuum through the system, starting at the one way yellow valve and two pipes under the bonnet and working back. you would not usually remove the drivers door valve, rather testing the system and blanking off parts to trace leaks. it is best done with a vacuum gague/pump, such as a mytivac, but can be done with no special tools at all, just some golf tees to blank off the pipes.
usual causes are leaking diaphragms and boots on door cells, and now they are getting very old we see cracked pipes too. it may be as simple as the pipe at the front being disconnected or the one way valve being broken. the main vacuum feed is from a grey pipe connected to the intake manifold. this will also feed purple pipes for the head light level adjusting and possibly some other features – can’t remeber, it’s been a while!!
