Accessories and Auxiliary Electrical Equipment
Accessories and Auxiliary Electrical Equipment
This domain covers wipers, washers, horns, windows, central locking, blowers, instruments, audio, auxiliary sockets and body-control functions.
Core rules
- Most accessories are controlled by relays or a body control module (BCM), not directly by the switch.
- Wiper motors use a park switch to return the blades to rest after the switch is turned off; two-speed and intermittent operation use additional brushes or electronic control.
- Window motors are reversible permanent-magnet motors; anti-trap systems sense current rise or use position sensors.
- Central locking uses solenoid or motor actuators and may be controlled by the BCM, remote key or door switch.
- Blower motors draw high current; a seized motor or blocked cabin filter can overheat the resistor pack or module.
- Auxiliary sockets: current = power/voltage; a 120 W load at 12 V draws 10 A, so a 15 A fuse and adequate cable are needed.
Worked example
A horn circuit uses a relay with an 80 Ω coil. Coil current at 12 V = 12/80 = 0.15 A. The horn itself draws 15 A. The dash switch therefore carries only 0.15 A, while relay contacts 30–87 carry 15 A. If the horn sounds weakly, check the horn earth, relay contact voltage drop and battery voltage under load before replacing the horn. A 0.5 V drop at 15 A wastes 7.5 W as heat.
Traps
Adding accessories directly to an existing circuit; fitting a higher-current horn without a relay; assuming a BCM fault when a door actuator is seized; ignoring park-switch faults when wipers stop in mid-screen; damaging window mechanisms by forcing them; overlooking parasitic drain from a stuck relay or boot lamp.