Charging Systems and Starting Systems
What the exam tests
Alternator, regulator, starter, cranking circuit and voltage-drop diagnosis.
Core
- Alternator: rotor field, three-phase stator, six-diode rectifier, integrated regulator. Output about 13.8–14.8 V; too low causes no-charge, too high boils the battery.
- Ripple test: set meter to AC volts across the battery with loads on. More than about 0.5 V AC suggests a failed diode.
- The charge warning lamp may be excited through the dash lamp or by the regulator; a blown bulb can prevent charging on some older systems.
- Check the B+ cable, fusible link, sense wire and belt condition before condemning the alternator.
- Starter: solenoid engages the pinion and closes the high-current contacts. Cranking voltage should stay above about 9.6 V at the battery.
- Voltage-drop test each cable under load: battery positive to starter terminal, then engine block to battery negative. More than about 0.3 V per cable is excessive.
Worked example
An alternator charges at 13.9 V at idle but the battery goes flat. Under load, measure from alternator B+ to battery positive: 0.9 V. The cable or connection is dropping almost a volt, so the battery never sees the regulator's target. Clean and repair the cable; do not fit an alternator. The voltage drop test localises the fault without guessing.
Traps
Replacing an alternator when the battery is faulty; measuring charging voltage at the battery while surface charge is present; forgetting the sense wire; not checking voltage drop on the ground side; assuming a clicking starter means a bad starter motor.