Normal Operation
WARNING: This page is about a different car, the 2006 Mercury Grand Marquis and 2006 Ford Crown Victoria. However, it is still accessible from the selected car via links, so may be relevant.
When the digital TR sensor recognizes that the vehicle is in either PARK or NEUTRAL and the ignition switch is placed in the START position, it sends a signal to the mini ISO relay that, in turn, sends a voltage signal to the starter solenoid, which will engage the starter motor to crank the engine at a speed fast enough to permit the engine to start.