At 06:17 AM 7/8/01, Repo wrote:

>Hi all, what does the "x-relay" actually do?


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