Electrical and Electronic Systems
Electrical and Electronic Systems
This domain tests battery, starting, charging, lighting, sensors, relays, wiring, and basic electronic diagnosis. The exam expects measurement and circuit understanding.
Core rules
- Use a digital multimeter with the correct function. Measure voltage in parallel and current in series; never measure resistance on a live circuit.
- A healthy rested battery is about 12.6 V; a charging system typically delivers 13.8–14.4 V, but always compare with the manufacturer’s specification.
- Voltage-drop testing finds high resistance under load. A drop of more than about 0.5 V on a main cable or ground during cranking deserves investigation.
- For parasitic draw, allow modules to sleep, use a clamp meter or a fused jumper, and compare with the specified limit, often around 30–50 mA.
- Use wiring diagrams and test relays by coil and contact circuits; do not assume a new sensor is good.
Worked example
A vehicle cranks slowly and the dash lights dim. Battery voltage is 12.4 V at rest and 10.8 V during cranking. Measure voltage drop from the battery positive post to the starter terminal and from the battery negative post to the engine block. A 0.8 V drop on the ground side shows high resistance at the engine ground strap. Clean and tighten the connection, then retest. Replacing the starter would not fix a ground fault.
Traps
Testing resistance with power on, using a test light where a meter is needed, replacing parts before voltage-drop testing, and not waiting for modules to sleep before measuring draw. Also, beware of back-probing damaged connector seals.