Vehicles that are equipped with electric cooling fans have switches that sense the temperature of the coolant and turn the fan on when the temperature reaches a preset level. If this does not happen, coolant will boil and the engine will overheat.
Testing this switch to find out whether it is the reason for overheating is not a difficult task. However, you will need an ohmmeter.
Here is what to do:
- To find out whether the cause of overheating is failure of the fan to turn on, start the engine (which must be cold), open the hood, and watch the fan as the engine warms up.
- Let the engine run for several minutes, but ask a helper sitting in the vehicle to turn off the engine if the temperature warning light on the dash should glow or if the needle of the temperature gauge approaches the HOT mark.
- Does the fan begin to rotate during this period? If it does, the cause of overheating does not lie with the cooling-fan switch.
- If the fan does not begin to spin, turn off the engine and wait for the engine to again get cold before testing the cooling-fan switch.
- After the waiting period, trace the wire from the fan motor to the electrical connector.
- Pull apart the connector, disconnecting the fan from the electrical system.
- Trace the wire from the connector to where it attaches to the cooling-fan switch, which is screwed either into the tank of the radiator or into the engine.
- Pull the wire off the cooling-fan switch.
- Use a wrench to unscrew the switch from the radiator tank or from the engine.
- To verify that the switch has failed, place the switch in a pan of water, but do not immerse its terminal end. Some switches have one terminal; others have two. If the switch has one terminal, also keep the threads around the terminal out of the water. Place a kitchen thermometer in the pan with the switch and set the pan on a kitchen stove burner.
- Test the switch using an ohmmeter. If the switch has one terminal, touch one of the ohmmeter probes to the terminal and touch the other probe to the threads. If the switch has two terminals, touch each ohmmeter probe to a terminal. The ohmmeter should display an infinity reading.
- Turn on the stove burner. When the thermometer indicates that the water has reached at least 220°F, repeat the ohmmeter test. The ohmmeter needle should swing to 0 or nearly to 0. If it doesn’t, the switch is defective.
If the switch has failed, buy a replacement that is specified for the cooling system of your vehicle from a dealership parts department or from an auto parts store. Coat the threads of the new switch with pipe thread sealer or wrap them with teflon plumbing tape. This will prevent a leak. Screw the switch in place by hand until it is fingertight. Then, use a wrench to tighten the switch.
Fill the cooling system with coolant. Start the engine, let it run a while, and turn it off. Check around the switch to make certain that there is no leak.
