355 Roof and Seat Switches

I just noticed something unusual about the roof and seat switches. The original WSM wiring diagrams showed 12 volts key power going through the roof switches at all times to the Roof ECU pins. I assumed it was an error and that the switches only send power to the Roof ECU when they are pressed.

However, after doing continuity tests on the switches, it looks like the WSM diagrams are correct.

I redrew part of the diagram below to show the WSM setup:

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The Roof ECU actually supplies 12 volts power to the switches on pins 1 and 2. I measured this on my own car. Oddly, the normally closed contacts of these switches already have key power on them. i.e. Both sides of the switch contacts have power on them with the switch not pushed, which seems nonsense to me, but this is how my car is wired up. I guess this results in zero or little current flow to/from the Roof ECU.

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When a switch is activated, ignition key power can no longer flow through this switch (except to illuminate the LED backlighting). The normally open contacts of the switch are now provided with an earth. The voltage provided by the ECU allows current flow to earth.

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Any ideas why this should be? Why not just leave the NC contact open?