Boat Diesel – Starter motor and relay

The starting system comprises two components, the motor itself and its solenoid-operated electrical relay.

The relay is necessary because the heavy electrical currents delivered to the starter motor by the battery are too large for the key-type starter switch to carry. When the key is moved to its spring-loaded start position, the relay solenoid closes the large contacts, connecting the batteries directly to the motor.

Simple electrical system
1 Battery. 2 Alternator. 3 Regulator (usually integral to alternator). 4 Starter motor. 5 Starter relay. 6 Preheater/starter switch. 7 Fuse. 8 Fast fuse to protect alternator. 9 Warning lamp. 10 Ammeter. 11 Starting aid. 12 Voltmeter.

As the motor starts to turn, the drive pinion on the output shaft is thrown outwards towards the ring gear on the engine flywheel. The pinion and ring gear have shaped teeth which allow the pinion to engage easily. The output shaft of the starter motor picks up speed and turns the engine.

As soon as the engine fires, the start switch is allowed to return to the run position. The ring gear accelerates, the pinion is thrown out of contact with the ring gear by a spring and the starter motor stops.

If the engine is particularly quiet, you may not be aware it is running, so watch the instrumentation. Never engage the starter while the engine is running as the gears may be damaged beyond repair.

Another type of starter motor has the pinion engaging with the ring gear before the electrical connection is made. Again it is thrown out of contact when the engine picks up and when the starter switch is returned to the run position.

The electrical resistance of starter motors is extremely small and it is impossible to check accurately with a multimeter if the resistance is correct or not. If there is an electrical problem, the motor needs to be taken to a specialist.

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