1. Make sure you have your radio code (from the dealer) and a record of your preset radio stations. Reset all of the residuals by removing the battery negative terminal, pumping the brakes to discharge the electrical system and reconnecting the battery negative terminal. You will need to reenter your radio code and preset stations when you are done with these diagnostics.
2. The engine fan has two fan speeds. The speeds are energized by signals from the ECM to the fan relay. The fan relay is designed to make sure that only ONE of the fan speeds can be energized.
3. It is important to note that if your cooling system were filled with pure water and was unpressurized, the water would boil over before the ECM would think it was hot enough to turn on the fan. IT IS VITAL THAT YOUR COOLANT SYSTEM BE A 50/50 MIX AND THAT IT RETAINS PRESSURE IN ORDER FOR THE COOLANT TO REACH A TEMPERATURE THAT WILL CAUSE THE ECM TO TURN ON THE FAN(S).
4. Due to the four layer sandwich design of the condenser/intercooler/radiator/fan assembly, the ECM is supposed to turn on the fan when the A/C compressor is running. So you might want to verify that condition by turning on the A/C or Defroster.
5. Bisect the diagnostics by checking whether the fans are functional since that is the easiest test.
6. With the car OFF, check if the fan blades are easily rotated.
7. With the car OFF, locate the plastic cover above the radiator.
8. Dislodge the plastic cover over the radiator by it pushing slightly toward the engine and lifting up (no screws or clips). This will give you easy access to the fan relay attached to it (slides off) under its left side.
9. There are multiple wires running to the fan relay in three connectors. You are interested in the middle connector. The middle connector has a yellow wire and a yellow/white wire.
10. Remove the middle connector from the relay. Inspect the connector for corrosion and clean if necessary.
11. Use a jumper wire to connect ONE of the exposed pins in the relay to ground (e.g. the engine right in front of you). You should hear the fan start spinning.
12. Use the same jumper wire to connect the other exposed pin in the relay to ground. You should hear the fan start spinning, but at a different speed than the first time.
13. If the fan doesn’t spin (and especially if it only spins on one jumped connection), then check the fuses to the left of the relay. Remove the little black covers and check them both. Alternately, you can use a VOM instead of your jumper wire on the exposed pins from before to measure whether voltage is present (which would mean that the corresponding fuse was good).
14. Reconnect the relay connector and reinstall the plastic cover.
15. If the fan does run at both speeds, then the ECM is not telling it to spin. The most fundamental but rare situation would be that both the wires (yellow, yellow/white) are broken/corroded. Leave that to the dealer since you need a special tool to remove the ECM. The ECM can also sense whether the yellow and yellow/white wires have an open circuit (sets codes).
16. The ECM turns on the fan when the Engine Coolant Temperature (ECT) sensor (a thermistor) delivers a very low resistance reading (e.g. 150 ohms). It will also turn on the fan when the ECT sensor indicates ultra-low or ultra-high temperatures (sets code) because it cannot trust the ECT sensor and this fail-safe mode helps ensure safe coolant temperatures.
17. The ECT sensor is located on the thermostat housing below the upper radiator hose where it meets the engine. It has relatively generous connecting wires to a simple connector that you can pull forward to gain access to that connector.
18. It is important to disconnect and inspect the contacts of this ECT sensor connector. It is possible that corrosion is slightly increasing the resistance which the ECM interprets to be a colder engine (so it never turns on the fan). When making any reconnection of this connector, make sure you can hear it snap into place.
19. If you want to confirm that the ECT sensor is functioning properly. Then get a candy thermometer and put it into the expansion tank with the cap off. Run the engine until it reaches 200 degrees F. Then turn off the engine, disconnect the ECT sensor connector and measure the resistance between the two contacts in the sensor’s pigtail. It should read about 200 ohms (which is NOT a low enough resistance to turn on the fans). Normal boiling point of water (212/100) will register about 180 ohms. The ECM is waiting for the ECT sensor to indicate resistance levels below about 160 ohms.
20. For further confirmation of ECM operation but possibly a bad ECT sensor, do the following.
a. Warm the engine to operating temperature and then turn it completely OFF. We are doing this to make sure the engine starts easily using a lean fuel mixture.
b. Reset the residuals (as above)
c. Locate or acquire two INSULATED alligator clip jumpers and a 150 Ohm resistor from an electronics hobby store.
d. Disconnect the ECT sensor connector.
e. On the connector end going to the ECM, put the two jumpers on the pins making sure they don’t contact each other.
f. Put the other ends of the jumpers on each side of the 150 Ohm resistor.
g. The resistor will now be pretending to be an ECT sensor thermistor that indicates a hot engine.
h. Make sure that the 150 Ohm resistor is not touching anything metal and will remain so when the engine is started.
i. Start the engine. The fan relay should energize and spin a fan motor. The engine temperature gauge should be reading hotter than the 3 o’clock position.
21. You can experiment with other resistances (never below 100 Ohms) to verify that the high fan speed kicks in, but this is usually unwarranted.
22. If the ECM operates the fans when you bypass the ECT sensor with a resistor as above, then replace the ECT sensor. Make sure to keep the ECT sensor connector protected from the coolant loss you may experience when you remove the upper radiator hose from the thermostat housing (in order to get easy access to the ECT sensor).
23. When you restart the car, make sure to top off the coolant (50/50 mix, not water). Remember that the system must be hot and pressurized in order for the ECM to turn on the fan.