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Diagnose Engine Cooling Fan Relay Problem

Engine overheating or poor air conditioning performance can be caused by an engine or A/C condenser cooling fan that fails to come on. In many cases, the underlying fault is a bad cooling fan relay.

The quickest way to tell whether or not the electric fan(s) are working is to start the engine, 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. On many vehicles, there may be two fans: a main cooling fan for the radiator, and a second fan for the condenser. Both fans should come on when the A/C is on.

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, and it may cause the A/C compressor to overheat and fail. The engine may also run hot and overheat, too.

Another problem that can sometime occur with cooling fan relays is that the relay may stick in the ON position, causing the fan to continue running long after the engine has been shut off. The fan may continue to run for a few minutes after shutting off a hot engine (especially during hot weather), but if it keeps running and never shuts off, it will eventually run your battery down after several hours, preventing your car from restarting.

If a defective fan relay is preventing the cooling fan from coming on when it should, or fails to turn the fan off long after the engine has been turned off, the fix is to locate the fan relay in the power center and replace it with a new relay. The location of the fan relay shold be marked inside the power center cover, or in your owners manual. A replacement relay should not cost more than $15 to $20. Just unplug the old relay and install the new one in its place.

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.

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.

Types of Electric Cooling Fan Relays

There are three basic types of relays:

Some Older Vehicles with Cooling Fan Problems

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