Starting and Charging Systems
Starting and Charging Systems - what the exam tests
Starter motor and solenoid operation, the control circuit, alternator construction and regulation, belt drive, the warning-lamp circuit, and voltage-drop testing of both systems.
Starting system
- A series-wound DC motor gives high torque at low speed; cranking draw is typically 150–250 A.
- The solenoid does two jobs: it pulls the pinion into mesh and closes the heavy contacts feeding the motor.
- Control path: ignition switch → clutch or neutral switch → relay → solenoid. A no-crank with a healthy battery usually lies in this path, not in the motor.
Charging system
- Alternator: belt-driven rotor field, three-phase stator, six diodes rectifying to DC; the regulator holds about 13.8–14.8 V, temperature dependent.
- The charge warning lamp often supplies the initial field current, so a failed bulb can mean no charge on older machines.
- Ripple test: more than about 0.5 V AC at the battery with loads on suggests a failed diode.
Worked example
Complaint: the battery goes flat. Engine off, 12.3 V. At 2000 rpm with lights on, 12.2 V at the battery but 14.3 V at the alternator B+ stud. The 2.1 V difference is a voltage drop: inspect the B+ cable, its terminal and any fusible link. Reasoning: the alternator is charging, but the path to the battery is not carrying the current.
Traps that cost marks
- Fitting an alternator before checking the belt, tensioner and voltage drop.
- Judging charging at idle with no electrical load switched on.
- Disconnecting the battery while the engine runs - the load dump can destroy electronics.
- Blaming the solenoid for a single click when the battery is simply flat.
- Measuring charging voltage at the alternator and assuming the battery receives it.