Ford Repair Questions? Ask a Mechanic for Answers ASAP
The cooling fan is controled by the signel from the ECT and the PCM turns the fan on usually when the engine reaches about 225 degrees.
This system is designed to maintain engine temperature by using the low speed cooling fan only. The high speed fan circuit is incorporated as a fail safe to be used only when engine temperature exceeds 230 degrees. There are 2 fuses involved in this system. The first is in the engine compartment fuse box. This fuse is labeled cooling fan. It is a 40 amp, hot all the time fuse that supplies the fan through a combination of relays. The second is in the interior fuse box. This is a 15 amp, key powered fuse, labeled engine. It is used to energize the first relay. If the engine runs, this fuse will be good. Test the 40 amp fuse. If defective, replace and retest the fan operation. If OK, go to step 2.
To test the Cooling Fan Power Relay, which may also be called the Ignition Relay, remove the relay from the engine compartment fuse box and using a 12 volt testlight, check for 1 terminal to be hot all the time (coming from the 40 amp fuse), 1 to be hot with the key on (coming from the 15 amp fuse), and 1 to be a good ground. To test the output of the relay, gain access to the cooling fan high speed relay. It is mounted with the cooling fan low speed relay and is located on the front of the left front strut tower near the bottom where it meets the frame. The high speed relay will have a blue connector and 5 terminals. Using a 12 volt testlight, turn the key on and check for voltage at the Light Green/Black wire at the relay. This wire comes from the cooling fan power relay in the engine compartment fuse box. If there is no voltage, replace the Cooling Fan (Ignition) Power Relay and retest the system operation. If voltage is present, go to step 3.
To test, locate and backprobe the Light Green wire at the High Speed Cooling Fan Relay with a 12 volt testlight with the key on. If dim or no light, replace the relay. If the light illuminates, go to step 4.
To test, gain access to the Low Speed Cooling Fan Relay. This relay is identified by the black connector with 4 cavities. Using a 12 volt testlight and key on, backprobe the Light Green wire to test for voltage. If no voltage, repair the open circuit from the high speed cooling fan relay. If this test OK, test the Black/White wire for voltage. If none present, repair the open circuit from the 15 amp engine fuse. If this tests OK, check the voltage on the Yellow/White wire using a digital voltmeter. It will read under 1 volt if the engine temperature is under 220 degrees. This wire is held to ground by the Powertrain Control Module (PCM) until engine temperature reaches 215 to 220 degrees. At that time it releases the ground to energize the relay and turn on the low speed fan. To test the relay operation, backprobe the Yellow wire with a 12 volt testlight. Remove the Yellow/ White wire from the connector and turn the key on. The relay should click, the Yellow wire should now be 12 volts and the fan should be running. If there is no voltage on the Yellow wire, replace the Low Speed Cooling Fan Relay and retest system operation. If voltage is present and the fan is not running, go to step 5. If voltage is present and the fan is running, go to step 6.
To test, verify the key is on and the Yellow/White wire is removed from the low speed cooling fan relay connector. Check the Black wire at the cooling fan connector for a good ground. If it tests OK, test for voltage at the Yellow wire at the cooling fan connector. If no voltage is present, repair the open circuit between the low speed cooling fan relay and the cooling fan connector. If this tests OK, replace the cooling fan. Retest the system operation.
To test, gain access to the powertrain control module (PCM) and unhook the connector. Locate the Yellow/White wire at pin 35 and test for a short to ground between the PCM and the low speed fan relay. If it is shorted to ground, repair the wire and retest the system operation. If this is OK, reconnect the PCM connector. Locate pin 7, Dark Blue/White wire, and backprobe it with a digital voltmeter. Pin 7 is the input to the PCM from the Engine Coolant Temperature (ECT) Sensor. Start the engine and monitor the voltage as the engine warms up. If possible, also monitor the engine temperature with a thermometer or temperature gun to compare the ECT sensor voltage with actual engine temperature. The voltage decreases proportional to engine temperature. A reading of 0.6 volts equals 195 degrees. The cooling fan will be turned on by the PCM when voltage drops to between 0.55 and 0.45 volts or 215 to 220 degrees of temperature. The failsafe or high speed fan would come on when voltage reaches approximately 0.41 to 0.39 volts indicating 230 degrees has been reached. If engine temperature gets hot and the ECT sensor voltage does not drop to inform the PCM of actual temperature, replace the ECT sensor and retest the system operation.