Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

The engine cooling fan is a variable speed fan. The ECM controls the fan speed by sending a pulse width modulated signal to the cooling fan control module. The cooling fan control module varies the voltage drop across the engine cooling fan motor in relation to the pulse width modulated signal. The cooling fan speed is effected by many different conditions and can be adjusted from 10% to 90% duty cycle (PWM), 90% is considered high speed fan. When multiple cooling fan speed requests are received the ECM uses the highest cooling fan speed of all the requests. During normal operation the control circuit voltage will be low and near battery voltage when OFF.

1. Cooling fan duty cycle starts when engine coolant temperature reaches approximately 95°C (204°F) and reaches high speed at temperatures above 113°C (235°F).
2. Cooling fan duty cycle starts when A/C pressure reaches approximately 1100 kPa (160 Psi) and reaches high speed at A/C pressures above 2480 kPa (360 psi)
3. At engine oil temperatures above approximately 150°C (302°F) the cooling fan duty cycle will be commanded to high speed.
4. At transmission oil temperatures above approximately 132°C (270°F) the cooling fan duty cycle will be commanded to high speed.
5. After the vehicle is shut OFF if the engine coolant temperature at key-off is greater than 113°C (235°F) or the A/C pressure is greater than 1720 kPa (249 psi) the cooling fan duty cycle is set to 50%,low speed. If the coolant temperature drops below 110°C (230°F) and the A/C pressure drops below 1660 kPa (241 psi) the fan will shut OFF. The fans will automatically shut OFF after 2 min. regardless of coolant temperature.

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Cooling System Connector End Views

Description and Operation

Cooling System Description and Operation

Electrical Information Reference

Scan Tool Reference

Ignition ON, command the cooling fan ON and OFF with a scan tool. Observe that the cooling fan turns ON and OFF with each command.

1. Ignition OFF, disconnect the harness connector at the Fan control module.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the ground circuit and ground.

⇒	If greater than 1.0 ohm, test the ground circuit for an open/high resistance.

3. Ignition ON, test for B+ between the battery positive voltage circuit and ground.

⇒	If less than B+, test the battery positive voltage circuit for a short to ground or an open/high resistance. If the COOLFAN fuse is open, test battery positive voltage circuit for a short to ground.

4. With a DMM, test for B+ between the B+ circuit terminal 3 and the control circuit terminal 6.

⇒	If less than B+, test the control circuit for and open/high resistance. If the circuit tests normal replace the ECM.

5. Test for 1.5-.5 ohms of resistance between the low reference terminal 1 and supply voltage terminal 4.

⇒	If not within specified value, test and replace the cooling fan.

6. If all circuits test normal, test and replace the cooling fan control module.

Install a 60A fused Jumper wire between the cooling fan supply voltage terminal 4 and B+. Momentarily install a jumper wire between the cooling fan low reference terminal 1 and ground. The cooling fan should turn ON.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.