Electrical and Electronic Systems
Domain focus
This domain covers batteries, starting, charging, lighting, wiring, grounds, sensors, and electronic control systems. The exam favours voltage-drop testing and circuit reasoning over guessing from a fault code.
Core rules
- Ohm's law relates voltage, current, and resistance. A circuit needs a power source, protection, switch, load, and return path; a poor ground is a common cause of ghost faults.
- A rested lead-acid battery should read about 12.6 V. Charging systems typically raise battery voltage to roughly 13.5–14.5 V at speed; a permanent-magnet stator produces AC that the regulator/rectifier converts and controls.
- Test voltage drop under load rather than only measuring resistance. More than about 0.5 V drop on a main power or ground path warrants repair.
- Use a test light or meter appropriate to the circuit. Do not probe CAN bus, ABS, or ECU pins with a low-impedance test light.
- Repair wiring with crimped and sealed terminals, proper gauge, and heat-shrink. Soldering is acceptable only where it will not flex and break.
- LED turn signals may need a compatible flasher or load resistors; LED headlamps must match the housing and beam pattern.
- Disconnect the battery before welding and observe polarity when jump-starting or charging.
Worked example
A motorcycle will not keep its battery charged. At 3,000 rpm, battery voltage is 12.4 V instead of the specified 13.5–14.5 V. The technician measures AC voltage across the three stator leads at 3,000 rpm and finds it within specification, and checks that no stator lead is grounded. That isolates the stator as good, so the rectifier/regulator or its wiring is suspect. A voltage-drop test from the regulator positive output to battery positive, and from regulator ground to battery negative, finds a corroded ground. Cleaning and retesting restores 14.2 V.
Traps
Testing resistance on a live circuit; checking battery voltage immediately after charging without removing surface charge; assuming a 12 V battery should read 12.0 V; probing CAN wiring with a test light; reversing polarity; using pliers instead of a crimper; leaving a ground loose; and fitting an LED flasher without checking lamp compatibility.