Electrical and Electronic Systems
What this domain tests
This domain covers battery, starting, charging, lighting, wiring, sensors, actuators, networks and electronic control. The exam tests measurement discipline: voltage, voltage drop, current and resistance, plus safe work around SRS and CAN systems.
Core rules
- Ohm's law: V = I × R. A circuit needs supply, ground and load; a high resistance joint causes a voltage drop, not necessarily a zero-volt reading.
- A healthy battery at rest is about 12.6 V; below 12.4 V it needs charging or testing. Charging voltage is typically 13.8–14.8 V, but follow the manufacturer.
- Voltage-drop test under load is more reliable than an ohmmeter on a live circuit. A starter positive cable should generally drop less than 0.5 V while cranking, and a ground strap less than 0.2 V.
- Use a fuse in the meter lead when measuring current, and never measure resistance on a live circuit.
- Do not probe SRS yellow connectors with a multimeter; use the specified scan tool and disconnect the battery with the key off, waiting the stated time.
- CAN bus faults need a scope or network test; a 12 V test light can damage a data line.
Worked example
A car cranks slowly and the battery tests at 12.5 V. During cranking the battery falls to 10.4 V, which is acceptable. The voltage drop from the battery positive to the starter positive is 0.3 V, but from the engine block to the battery negative it is 0.9 V. The reasoning is that the ground path has excessive resistance, so the starter receives less current. Cleaning the engine earth strap and the battery terminal reduces the drop to 0.1 V and cranking returns to normal. Replacing the starter would not have fixed the cause.
Traps
Do not replace a battery because it reads 12 V; load-test it. Do not measure resistance with the circuit powered. Do not use a test light on airbag or CAN wiring. Disconnecting the battery with the key on can damage modules. A DTC does not prove the sensor is faulty; test the circuit first.