Are you sure it's not there? In my 116 manual there isn't a lot of
info, but after all these years, I finally stumbled across one diagram
that shows it all, though it's not easy to read. It was in the
section talking about the central locking system.
I also saw a guy on Ebay recently that was selling vacuum repair guides
for some models. Just looked and he doesn;t have one currently listed
for yours, but you could email him and see what he does have.
If that doesn't work, search on mercedes vacuum troubleshooting on
If you have the door panels off it should be easy to diagnose at least
the first part. I'm not familiar with that model, but just fixed my
116 80 300SD. I suspect it probably works similar. There is a vacuum
line that runs from the main vac line in the engine bay, through a
check valve located high up under drivers side dash and on to the
"vacuum switch" located in the driver door. The switch is a little
device with a plunger on it that gets moved by the key turning.
Center line is connected per above to engine vac source and reservoir.
The other two lines from the switch go to the locking vacuum elements
for the other doors, trunk, gas lid, etc. The vacuum switch applying
vac to one line opens, applying vac to other closes them.
With a vacuum tester that you can find at parts stores or Ebay, you can
generate and measure vac and figure out what's going wrong. If you
need a new component, look on Ebay. First thing to check is if vac is
present at that center hose on vac switch. It should also hold this
vac for days, so doors can be opened. I'd check where the vac lines go
through the firewall. Both of the ones I've had fail went right at
that point. Just pulling on them, while looking for the leak, they
There are guys selling new replacement vac elements etc for a lot less
than u can find them for elsewhere.