Vehicle Power
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
When ignition switch is turned to START or RUN position, battery positive voltage (B+) is applied to coil of PCM power relay. Since other end of coil is wired to ground, this energizes coil and closes contacts of PCM power relay. Vehicle power (VPWR) is now sent to PCM and various OBD-II systems.