Fan & thermostat

How it works

The radiator fan takes over from airflow when the bike is moving slowly or stopped. It is switched by a sensor or thermo-switch that turns it on above a set temperature, directly or via the ECU and a relay. The thermostat is a valve in the circuit: it stays closed when cold so the engine warms up quickly, then opens to let coolant through the radiator. A thermostat stuck closed makes the engine overheat fast while the radiator stays cold; stuck open, the engine struggles to reach temperature. A fan that doesn't start makes the bike overheat in traffic while it stays at the right temperature on the move. Common causes are a fuse, a relay, a corroded connector, a faulty sensor or the fan motor itself. The simple test is to let the bike idle at a standstill while watching the temperature and when the fan cuts in.

Maintenance tips

• ✓Test the fan: bike stopped, engine idling, the fan should start before the temperature reaches the red zone.
• ✓If it doesn't start, check the fuse, relay and connectors before replacing the fan.
• ✓If the engine heats up fast but the radiator stays cold, suspect a thermostat stuck closed.
• ✓Switch off as soon as the temperature nears the red zone; don't keep riding.
• ✓Never open the cooling system on a hot engine; wait for it to cool completely.