relay on for 1 sec after power loss

i'm running a carpc and my startup/shutdown controller just stopped working. i can use a relay to turn on the pc no problem, but turning it off is my issue. when i turn my ignition off, i only need the relay to initiate for about 1 second, then stop sending power. i figured i'd need a capacitor but what size? and someone correct me if i'm wrong please :)

a 10,000 mic cap paralleled across the coil of a relay should give you about a second.  You will also need to put a diode in series with the power wire, before the cap and relays coil.  The diode will prevent the cap from trying to power whatever is up line from the relay.