An electric cooling fan that is not coming on when it should can cause an engine to overheat and poor cooling performance from the air conditioner.
The electric cooling fan is typically mounted behind the radiator. On some vehicles with large, wide radiators, there may be two cooling fans, or there may be a separate fan for the air conditioning condenser.
The fan only runs when needed to help cool the engine. The engine coolant sensor or a separate engine temperature switch is used to monitor engine temperature. Extra cooling is not needed when a cold engine is first started, so the fan does not come on until the engine reaches normal operating temperature (195 to 215 degrees). The fan will then cycle on and off as needed to maintain the coolant temperature. So the fan runs mostly at idle or low speed when the engine is at normal temperature.
On some late model vehicles, the cooling fan can change speed to increase or decrease cooling as needed. Some fans may have a low, medium and high speed range while others have additional speed settings.
CAUTION: The fan circuits on many vehicles are wired so the fan can come on at any time, whether the engine is running or not. Be aware of this if you are working in the engine compartment and the engine is hot. Keep fingers and tools away from the fan blades.
You may hear the fan running when you shut off the engine after driving. This is normal. But the fan should shut off after several minutes as the engine cools down.
The fan also comes on when the A/C is turned on to provide additional airflow through the condenser for good cooling performance. This may occur regardless of engine temperature or vehicle speed.
The fan is usually not needed when the vehicle is traveling fast enough for ram airflow through the grille to provide cooling (typically at speeds above 30 mph).
One reason why electric cooling fans are used on many vehicles in place of a belt-driven mechanical fan is to improve fuel economy and reduce fan noise, especially at highway speeds. A belt-driven fan can consume up to 12 or more horsepower depending on engine speed and cooling load.
The fan's temperature-sensing power circuit only runs the fan when extra cooling is needed. On older applications, fan operation is usually controlled by a temperature switch located in the radiator or on the engine. When the temperature of the coolant exceeds the switch's rating (typically 195 to 215 degrees F), the switch closes and energizes a relay in the engine compartment that supplies voltage to the fan. The fan then continues to run until the coolant temperature drops back below the opening point of the switch. A separate circuit turns on the fan when the A/C compressor clutch is engaged.
In newer vehicles with computerized engine controls, fan operation is often regulated by the powertrain control module (PCM) or a fan control module. Input from the engine coolant sensor, ambient air temperature, vehicle speed sensor and other sensors may be used by the PCM to determine when the fan needs to be energized. On applications that have variable fan speeds, the PCM generates an on-off duty signal for the fan motor ("pulse width modulation") that causes the fan to run faster or slower.
A fan failure, or a failure of the fan relay or control circuit is bad news because it can allow the engine to overheat. On applications that have variable fan speed, the engine may also overheat if fan speed fails to increase when additional cooling is needed. The fan may work but it only runs at low speed, which may not be fast enough to prevent overheating.
Six things may prevent an electric cooling fan from coming on:
One way to quick check the fan circuit is to start the engine and turn on the A/C to max. If the fan runs, the fan motor, relay, fuse and wiring are all okay. But this test doesn't tell you if the temperature switch or temperature sensor and PCM are energizing the fan when coolant temperatures are high.
On applications with variable fan speed, the PCM's operating strategy likely takes into account inputs from a variety of sensors to determine how fast the fan should be turning. If any these inputs are faulty due to a bad sensor or wiring fault, the PCM may not run the fan fast enough to keep the engine cool. If the Check Engine light is on and there are one or more sensor trouble codes (especially a coolant sensor code, air temperature sensor code or vehicle speed sensor code), faulty input from the sensor may be affecting the normal operation of the cooling fan. Diagnosing and repairing the sensor problem should restore normal fan operation.
To check the temperature at which the fan comes on, turn the A/C off and keep the engine running until it reaches normal operating temperature. Most fans should come on when the coolant reaches about 200 to 230 degrees. If the fan does not come on, something in the control circuit is defective. Resistance checks should then be made on the temperature sensor or switch, and a voltage check on both sides of the relay (you will probably need a wiring diagram of the fan cooling circuit on your vehicle to identify the relay terminals and wiring connections).
The fan motor itself can be checked by using jumper wires. Unplug the wiring connector on the fan, and use the jumper wires from the battery to route power directly to the fan. If the fan motor is good, the fan should spin at normal speed when supplied with 12 volts. Noisy bearings or a slower than normal speed would indicate a worn motor.