Charging and Starting Systems
Charging and Starting Systems tests alternator operation, rectification and regulation, starter motor and solenoid operation, and the voltage-drop checks used to find high-resistance cables and grounds. You must be able to explain a charging warning lamp and a no-crank complaint.
Core rules
- The alternator produces AC, rectified by six diodes; the regulator controls field current to hold about 13.8–14.4 V on a 12 V system, or about 27–28 V on a 24 V system.
- The warning lamp provides initial excitation; once charging, equal voltages extinguish it. A lamp that stays on can mean no excitation, an open field, a faulty regulator or an open diode.
- A pre-engaged starter solenoid has pull-in and hold-in windings; it moves the pinion and closes the main contacts.
- The starter circuit needs low resistance: check voltage drop on the positive cable, ground cable and engine earth.
- Never crank continuously; respect the starter duty cycle.
Worked example
During cranking, battery voltage falls to 10.8 V and the starter terminal reads 10.0 V. Drop on the positive cable = 0.8 V, which is excessive; a typical target is under 0.5 V total and often under 0.2 V per cable. The 0.8 V drop means high resistance, a poor terminal, an undersized cable or a bad ground. Correct the cable or connection; do not fit a bigger battery to mask it.
Traps
Testing alternator output only at idle with no electrical load. Replacing the starter when the fault is a bad earth strap. Forgetting that a seized engine or faulty ignition switch can cause no-crank. Over-tightening the alternator belt.