I recently built and installed my first ever carputer. Nice little machine, uses an M350 case, runs a Intel D525MWV motherboard (1.8 Ghz Atom dual core), 4GB DDR3 RAM, 320GB HDD, bluetooth, WiFi, and an M3-ATX-HV power supply, running Windows 7. Car is a 2007 Holden/Opel Astra H.
Anyway, I have the power supply connected to the power button connector on the motherboard via the included cable. When I start the car, the PC starts booting, as expected, and remains running for the entire time I have the car running, unless I shut it down. I've got the jumper on the PSU set to the P1 position, which should attempt to shut down the PC 5 seconds after I turn the car off, before doing a hard power off 1 minute after turning the car off. While it does the hard power off without any issue, it doesn't shut the PC down automatically. Its not a motherboard/BIOS/software issue - if I connect the case power switch to the motherboard and press it, the PC shuts down, however either the power supply isn't sending the shutdown signal, or for whatever reason the motherboard isn't doing anything with the signal. Any ideas?