[Walt Fricke]

The diode in this (and other relays which have one) is supposed to deal with "back EMF" (if I remember the terminology - maybe that is something different). A relay has a coil. When the relay is activated, current flows through the coil. When the relay is turned off (coil power or coil ground removed), the collapse of the electromagnetic field produces a voltage. Same as a spark coil. This stray voltage can harm electronic components.

I don't think it necessarily always does harm (many have substituted a standard relay for the fuel pump relay, especially if in a pinch, like a problem out on the road, without damage).

The sensor plate switch operates counterintuitively - when no air flows/plate in rest position, the switch is closed, completing a circuit. This disables the fuel pump relay coil, and the FP doesn't get current. When you open the switch, the FP relay can operate as normal. I can never quite remember how this does its magic. Relays can be used as logic gates, and Porsche has set this relay up as one. I've been able to figure out how this magic works, but then I forget, but I don't think it depends on the one way aspect of a diode.

Having a diode across a relay's coil can never hurt, though.