Starting Systems
Starting Systems
This domain tests starter-motor construction, control circuits, cranking voltage drops, solenoid operation and no-crank diagnosis.
Core rules
- A starter is usually a series-wound DC motor: high starting torque, heavy current, short duty.
- The solenoid has pull-in and hold-in windings. Pull-in moves the pinion into mesh and closes the main contacts; hold-in keeps it engaged with lower current.
- The pinion drives the flywheel ring gear through a one-way clutch; gear ratio multiplies torque.
- Control circuit: ignition switch → starter relay or neutral/park switch → solenoid. The main cable carries 150–300 A; control current is only a few amps.
- Maximum acceptable voltage drop in the cranking circuit is typically about 0.5 V total, often 0.2 V per cable or connection, measured under load.
- Cranking speed and current depend on battery condition, cable resistance, earth straps, starter condition and engine friction.
Worked example
A 12 V system draws 200 A while cranking. Starter input power = 12 × 200 = 2,400 W = 2.4 kW. If battery voltage falls to 10.5 V at 200 A, available power is 2.1 kW; the 1.5 V difference is lost in battery internal resistance and cables. Measuring 12.4 V at the battery but 9.6 V at the starter shows a 2.8 V drop, far above 0.5 V, so test the main cable, terminals and earth strap before condemning the starter.
Traps
Replacing a starter when the fault is a flat battery or bad earth; measuring voltage drop without the circuit loaded; confusing a solenoid click with a spin-but-no-crank fault; leaving the starter engaged due to a stuck solenoid; ignoring the neutral/park switch and immobiliser.