Cheers,
Graham
Good question, as the "X" relay causes a lot of confusion. The positions of the relays in the 928 are specified by Roman Numerals, and the easy (but erroneous!) assumption is that Relay "X" is the relay in position X, or that there is some relationship. As far as I can remember, Relay X is never in position X.
Electrical circuits are organized into buses. A bus is a connection or set of connections that are powered together. As an example, let us look at the ignition switch.
One bus from the ignition switch powers the ignition system, the ECUs, etc. Another bus powers the accessories. Another bus powers the starter.
This arrangement allows organization of the electrical system. Thus, if you want to install an accessory that is always powered, you want to find a bus that is always hot. If you want the accessory to go off with the engine, you need an ignition or an accessory bus. One of our customers wanted to install an intercooler pump that would run only when the engine was running. This only required that we find a bus that was powered only when the engine ran, and using that to trigger a relay.
Relay "X" furnishes power to the "X" bus. That is, this relay furnishes power to a particular set of connections for various purposes. Bus X could perhaps be better called the "retained power bus". This is the bus that makes the windows, sunroof, etc., work between the time that the ignition switch is turned off and a door is opened. It is an accessory bus, but one that remains hot after the ignition switch is turned off but you haven't opened a door yet.
Yet another verbose answer!
Wally Plumley
928 Specialists
To add a bit more verbosity:
There is a DIN standard for terminal designations. 30 is direct positive voltage from the battery. 15 is switched positive, seems to be used for spots where voltage is present with the ignition on. There doesn't seem to be a designation for terminals that have voltage when the switch is in "Accessory" position, so some schematics use "X" for those. That's my theory about why it's called the "X" relay.
And if your car is pre-85, don't go looking for it, it ain't there. Big improvement, though perhaps not up there with the extra 16 valves.
-John White-
'85 928 S