The engine control module (ECM) commands the fans ON in either high speed or low speed, depending on cooling requirements. In low speed, the fan is turned ON at a reduced speed. High speed the fan is turned ON at full speed.

In low speed operation, the ECM applies ground to the coil side of the low speed fan relay. This energizes the coil and applies voltage directly to the cooling fan through the switch side of the low relay. The fan is connected in series to the cooling fan resistor causing the fan to operate at low speed.

In high speed operation, the ECM applies a ground to the coil side of the fan high or fan control relay. When energized, the fan high or control relay applies voltage directly to the cooling fan through the switch side of the relay. Because the low speed fan relay is de-energized the cooling fan receives full voltage and operates at high speed.

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Description and Operation

Cooling Fan Description and Operation

Electrical Information Reference

Scan Tool Reference

1. Verify that DTCs P0480 and P0481 are not set.

⇒	If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

2. Ignition ON, command the fan relays ON and OFF with a scan tool. You should hear or feel each relay click.

⇒	If any fan relay does not click, refer to DTC P0480 or P0481 .

3. Ignition ON, command the appropriate fan relay ON and OFF with a scan tool. Verify that the fan turns ON and OFF when changing between the commanded states.

1. Ignition OFF, disconnect the cooling fan relays.

Important:  The following test must be performed on both fan relay circuits.

2. Ignition ON, verify that a test lamp illuminates between the relay switch B+ circuit terminal 30 and ground.

⇒

If the test lamp does not illuminate, test the relay switch B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay switch B+ circuit fuse is open, test the relay controlled output circuit for a short to ground. If the circuit tests normal, test or replace the relay.

3. Connect a 20A fused jumper between the relay switch B+ circuit terminal 30 and the relay controlled output circuit terminal 87 of the fan low relay. Verify the fan is ON at low speed

⇒	If the fan is ON at high speed, test the cooling fan resistor for a short circuit.

⇒	If the fan is not ON, test the relay controlled output circuit for a short to ground or an open/high resistance. If the circuit tests normal, test the fan ground circuit for an open/high resistance. If the circuit tests normal, replace the fan.

4. Connect a 30A fused jumper between the relay switch B+ circuit terminal 30 and the relay controlled output circuit terminal 87 of the fan high or fan control relay. Verify the fan is ON.

⇒	If the fan is not ON, test the relay controlled output circuit for a short to ground or an open/high resistance.

5. If all circuits test normal, test or replace the relay.

1. Ignition OFF, disconnect the cooling fan relay.
2. Test for 70-110 ohms of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If not the specified range, replace the relay.

4. Install a 20A fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.