Electrical and Electronic System Servicing (Charging, Starting, Lighting)
Domain 5 tests charging, starting and lighting, plus the battery and circuit testing that support them.
Core rules
- Battery: a healthy rested battery reads about 12.6 V open-circuit (2.1 V per cell) and electrolyte SG around 1.265; below 12.4 V it is part-discharged. Remove surface charge before testing.
- Charging: expect roughly 13.8–14.5 V at the battery with the engine running and loads on; AC ripple above about 0.5 V suggests a failed diode.
- Voltage drop is the fastest diagnostic: measure under load from battery positive to starter terminal and from battery negative to the block; more than about 0.5 V on either side means high resistance in cable or terminal.
- Starter current draw is compared with the maker's figure; a slow crank with normal voltage drop points to the starter itself.
- Parasitic draw: after modules sleep (allow 20–60 minutes), typically under 50 mA.
- Lighting: match bulb type and wattage, check aim on a beam setter, and remember HID ballasts hold high voltage - never probe them.
Worked example
Complaint: slow crank, no start. Battery open-circuit 12.4 V; while cranking, the battery posts read 11.6 V but the starter B+ terminal reads 10.8 V. Reasoning: the 0.8 V difference is dropped across the positive cable and terminals, well above the 0.5 V limit - the battery is not the fault, the connection is. Clean and tighten the terminal, re-measure, and only then decide whether the starter needs replacement. Voltage-drop testing prevents parts-swapping.
Traps
- Replacing an alternator when the belt slips, the sense wire is open or the battery is sulphated.
- Measuring resistance on a live circuit or on the wrong range.
- Reverse-connecting jumper leads, which can destroy control modules.
- Probing airbag or HID circuits with an ordinary test light.