[W126 Coupe] Central Locking Relay on 83 500SEC

[James Fletcher]

"Does anyone know if an electrical relay forms part of the central 
locking (CL) system for a 1983 500 SEC?
  I think my CL vacuum pump is OK; it cuts out in about 2 seconds when I 
use the key to unlock the driver's door, and all the vacuum actuators 
now open and close in about 1 second. The problem is that when I lock 
the driver's door, the vacuum pump is not cutting out.
My Mercedes mechanic recently solved  the problem of my apparently 
'dead' CL pump by installing a supplementary earth wire. The driver's 
door vacuum actuator recently sprang a 'leak', but everything worked 
fine for a while after it was replaced. This history makes me wonder if 
there is a dodgy electrial relay somewhere in the system.
Does anyone know if the W126 series CL system includes a relay, and 
where it is located?"

Mark Hunter
Australia