Elective Units: Vehicle Systems Diagnosis and Repair
What the elective units cover
Elective units apply core knowledge to specific vehicle systems: starting and charging, lighting, body electronics, instrumentation, HVAC and accessory circuits, plus diagnostic method and customer handover. Questions reward a logical diagnostic sequence over guesswork.
Systems you must understand
Charging: alternator output, regulator control, belt condition, and how to test for undercharge versus overcharge. Starting: battery condition testing (state of charge and load capacity), cranking circuit voltage drop, starter solenoid operation and immobiliser interaction. Lighting and body electronics: globe types, relays, flasher units, central locking, power windows and CAN/LIN network basics - know that a network fault can disable several unrelated-looking functions. Instrumentation and HVAC: sender units, gauge circuits, blower resistor packs and compressor clutch control. Batteries: lead-acid, AGM and EFB differences, charging requirements and memory/reset procedures after replacement.
Diagnostic method
Follow a repeatable process: confirm the customer complaint, gather symptoms, check for technical service information and fault codes, form a hypothesis, test with a meter or scan tool, then verify the repair and clear codes. Use voltage-drop testing under load rather than resistance testing alone.
Traps to watch
- Replacing a starter when the fault is a poor earth strap or low battery.
- Ignoring parasitic drain when diagnosing a flat battery.
- Assuming a scan-tool code names the failed part rather than the circuit to test.
- Fitting a standard battery where an AGM/EFB is specified, or skipping the reset procedure.
- Overlooking network faults that mimic multiple component failures.
Worked example
A vehicle cranks slowly. Measure battery voltage at rest (about 12.6 V charged) and during cranking - below roughly 9.6 V suggests a weak battery. Then measure voltage drop from battery negative to engine block while cranking; more than about 0.2–0.3 V indicates a poor earth. This sequence separates battery, cable and starter faults before any part is replaced.