Cooling Fan Relay – What Does It Do – Failure Symptoms – Basic Testing

Cooling Fan Relay - What Does It Do - Failure Symptoms - Basic Testing
Cooling Fan Relay - What Does It Do - Failure Symptoms - Basic Testing

Circuits that have a high current draw, usually use a cooling fan relay.

Most cooling fans use electric motors, that have a moderate to high current draw.

As a result, using a cooling fan relay, solves this problem.

How Does A Relay Actually Work

A relay uses a relatively small electric current, that can turn on or off, a much larger electric current. A relay is basically an electromagnet; (a coil of wire that becomes a temporary magnet when electricity flows through it). You can think of a relay as a kind of electric lever. Switch it on with a tiny current and it; switches on (“leverages”) another item using a much bigger current.

Relay Open
Relay Open

Relay Closed
Relay Closed

Why is that useful? As the name suggests, many sensors are incredibly sensitive pieces of electronic equipment. And, produce only small electric currents. But, often we need them to drive bigger pieces of apparatus, that use bigger currents. Relays bridge the gap, making it possible for small currents, to activate larger ones.

Common Failure Symptoms:

Engine running hotter than normal

One of the first symptoms commonly associated with a bad or failing cooling fan relay is; an engine that runs hot or overheats.

Cooling fans not coming on

Non-functional cooling fans are another common symptom of, a potential problem with the cooling fan relay. If the relay fails, it will be unable to supply power to the fans. As a result, they will not operate.

Cooling fans will not turn off

If the cooling fans are staying on all the time, this is another (less common) symptom.

If You Think The Problem Is The Cooling Fan Relay, Test It !

The quickest way to tell whether or not the electric fan(s) are working is to start the engine. Then, let it reach normal operating temperature and then turn the A/C on. The cooling fan in the engine compartment, should turn on to pull air through the radiator and A/C condenser.

Basic testing for any relay here

On many vehicles, there may be two fans:

  • A main cooling fan, for the radiator.
  • A second fan, for the condenser.

Both fans should come on when the A/C is on.

Electric Cooling Fan
Electric Cooling Fan

If one or both fans fail to come on; the lack of additional cooling provided by the fan, may cause poor A/C cooling performance. Also, it may cause the A/C compressor to overheat and fail. The engine may also run hot and overheat, too.

Problems In The Cooling Fan Circuit

The typical electric cooling fan circuit includes, a temperature sensor, a relay, a control module and the fan motor. The relay is the component that fails most often. So, be sure to test the relay, as well as its power supply and ground connections.

Cooling Fan Relay
Cooling Fan Relay

A good relay coil will typically read 40 to 80 ohms. If resistance is high, the coil may still be working, but it is failing; or it may not work when electrical loads are high. If the coil has no resistance, it is open and has failed. Replace the relay.
Another simple relay test is, to shake it. If you hear something rattling inside, the relay armature is probably broken.

There Are Three Basic Types Of Relays:

Normally open

The armature closes, when the coil is energized to route power to the fan motor.

Normally closed

The armature is normally closed. And, is pulled open when energized.

Dual relay

This type conducts current, when open and when closed, in two different circuits.


The cooling fan relay essentially functions as, the switch for the engine’s cooling fans. And, is therefore an important electrical component, of the vehicle’s cooling system. Finally, if you suspect that your cooling fan relay, may possibly be having issues, have it checked.

Thank You !