Ford Focus mk3 horn system description
DESCRIPTION AND OPERATION
The horn output is controlled by the Body Control Module (BCM). The Body Control Module (BCM) energizes the horn relay, sounding the horn.
The horn switch consists of 2 sets of contacts separated by springs. The lower set is connected to ground and the upper set is connected to the horn signal circuit. When the driver airbag is pressed, it pushes down on the upper set of contacts, collapsing the springs and allowing the contacts to touch. When the contacts touch, it completes the circuit and provides the ground signal, which is routed through the clockspring to the Body Control Module (BCM). The Body Control Module (BCM) grounds the horn relay coil to energize the relay. When energized, the horn relay provides voltage to the horn, enabling the horn to sound.
B1323:11 Horn Switch: Circuit Short To Ground
B1323:23 Horn Switch: Signal Stuck Low
B1C55:11 Horn Relay: Circuit Short to Ground
B1C55:13 Horn Relay: Circuit Open
B1C55:19 Horn Relay: Circuit Current Above Threshold