Principles of Operation
The Battery Junction Box (BJB) supplies the switched and control voltage to the horn relay (part of the Body Control Module (BCM)). When the driver air bag module is pressed, the horn switch (which is integrated into the driver air bag module) is closed and ground is supplied to the BCM which is routed through the clockspring. The horn relay then energizes, sending voltage to the horn, enabling the horn to sound.
The horn switch is not a single component, but consists of contact points located on the steering wheel and driver air bag module. The contact points are separated by springs. One set of contacts is part of the steering wheel itself and the other set is part of a metal plate mounted to the rear of the driver air bag module. Springs mounted to the metal plate are isolated by plastic seats on the steering wheel. When the driver air bag module is pressed against the steering wheel, the contacts on the driver air bag module and the steering wheel touch to complete the circuit.
Anti-Theft Horn
On vehicles equipped with perimeter alarm, the vehicle has a separate anti-theft horn. Refer to ANTI-THEFT - PERIMETER , for information on the perimeter alarm and anti-theft horn.
Normal Operation
When the driver air bag module is pressed, the normally open horn switch (which is part of the driver air bag module) is closed and ground is routed through the clockspring to the horn relay, energizing the horn relay. The Battery Junction Box (BJB) supplies the switched and control voltage to the horn relay, which is integrated into the Body Control Module (BCM). The horn relay then provides voltage to the horn, enabling the horn to sound.
This pinpoint test is intended to diagnose the following
- Fuse
- Wiring, terminals or connectors
- Steering wheel harness
- Horn
- Clockspring
- Driver air bag module
- BCM
When the driver air bag module is pressed, the normally open horn switch (which is part of the driver air bag module) is closed and ground is routed through the clockspring to the horn relay, energizing the horn relay. The Battery Junction Box (BJB) supplies the switched and control voltage to the horn relay, which is integrated into the Body Control Module (BCM). The horn relay then provides voltage to the horn, enabling the horn to sound.
The Tire Pressure Monitor (TPM)/Vehicle Security Module (VSM) is also wired to the horn relay coil controlled circuit. The TPM / VSM sounds the horn during TPMS sensor training.
This pinpoint test is intended to diagnose the following
- Wiring, terminals or connectors
- Steering wheel harness
- Clockspring
- Driver air bag module
- BCM
- TPM / VSM