Circuit Description

The body control module (BCM) monitors the inputs Off/Run/Crank, Run/Crank and the Accessory signals supplied from the ignition switch. The BCM uses the sequence that the signals appear and their voltage levels to determine the power mode called for by the vehicle operator and the ignition switch. A +5 volt regulated voltage source is supplied by the BCM to the ignition switch over a discrete circuit. The ignition switch uses this voltage for Off/Run/Crank signal discrimination and the anti theft reference voltage generation. A constant battery positive voltage is also supplied to the ignition switch to generate the Run/Crank, Accessory and Key In signals. These signals are transmitted to the BCM over discrete circuits. The Accessory signal is also routed to the vehicles electronic control unit (ECU) and is used as a digital signal reference. The Run/Crank signal in conjunction with being supplied to the BCM is sent directly to the Run/Crank relay control coil.