Car Fan Problems

by Lee Sallings

When your engine is running, it generates a great deal of heat is by burning fuel. The cooling system is designed to contol that heat, and part of that system is the cooling fan. Rear-wheel drive vehicles can use the engine's power to drive a mechanical fan, but the front wheel drive requires an electric fan because the engine is mounted laterally in the engine compartment. When the cooling fan fails, it can cause overheating and engine damage.

Mechanical Fan Clutch

The mechanical fan and clutch used on rear-wheel drive vehicles is attached to the engine and driven by the water pump belt. When the fan clutch goes bad, the fan doesn't match the speed of the engine, and the fan is unable to pull air through the radiator. The most common symptom of this is an engine that overheats while at a stop sign or in slow traffic, but runs at normal temperatures when the vehicle is moving at normal speeds and air is forced through the radiator opening as the vehicle moves.

The fan clutch should have resistance when spun while the engine is off. If it turns freely, it needs to be replaced.

Electric Fan Motors

The electric fans used on front-wheel drive vehicles and late-model rear-wheel drive vehicles is a simple electric motor powered by battery voltage supplied through a relay. Those motors are durable, but from time to time they do fail. Those motors switch on when the engine reaches a certain temperature or if the air conditioning is turned on.

A simple test is to turn the air conditioner on; the fan should come on in a few seconds. If not, use a test light to check if there is electricity getting to the fan. Power supplied to the fan on one wire, and ground on the other wire indicates a bad fan motor. No power supplied to the fan indicates faulty electrical controls, such as a bad switch or a damaged wire.

Electronic Controls

Electric fans are powered by a relay that is turned off and on by either a sensor and computer or by a simple thermal switch. Common symptoms of a bad control circuit is a fan that doesn't recieve power from the relay. The on-board computer controls the relay according to the coolant temperature sensor. The thermal switch operates the fan according to engine temperature, and is switched to the ground. A faulty switch, relay, sensor, or on-board computer will result in an inoperative fan.


Troubleshooting is easy. Turn the air conditioner on and check for power and ground at the fan motor. If there is power on one wire and ground on the other wire with the fan unplugged, the faulty part is a fan motor. No power or no ground indicates a control-circuit fault.

Now, unplug the relay, and test for power and ground at the relay port. You should get readings from the two power terminals and the two ground terminals. If so, replace the relay. If a power terminal gets no reading, check and replace the fuse. If a ground gives no reading, replace the coolant temp sensor or switch.


Proper cooling fan operation is key to the performance and long life of your car's engine. Troubleshooting and repairing fan problems is easy and important.

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