Body and Accessory Electrical Systems
Concepts
Know the accessory circuits: door controls, wipers, horn, heaters/blowers, power windows, mirrors, and interior lighting. Understand relays, fuses, body control modules, and multiplexed networks. Recognize that many accessories are controlled by modules that communicate over a data bus, so a fault may be a network issue rather than a component issue.
Diagnosis
Isolate the circuit with a wiring diagram. Verify power, ground, and control signal at the component. For module-controlled accessories, check for communication faults with a scan tool. Use voltage-drop testing on high-current accessories such as blowers and heaters. Check relays by substitution or by testing coil and contact circuits.
Traps
Questions often present an accessory that works intermittently-usually a loose connection, chafed wire, or failing relay. Another trap: assuming a module is bad when the real fault is a poor ground or a blown fuse in a secondary circuit. Watch for shared power feeds that affect multiple accessories.
Rule of Thumb
"Power, ground, control-in that order." If all three are present at the component, the component is suspect.
Worked Example
Blower works on high only. The resistor block (or blower control module) provides the lower speeds; a failure there removes all but the high-speed path. Test the resistor/module before replacing the blower motor.