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

In low speed operation, the ECM applies ground to the coil side of the auxiliary fan relay. This energizes the coil and applies voltage directly to the auxiliary cooling fan through the switch side of the relay. The auxiliary fan is connected in series to the main cooling fan through the de-energized series/parallel control fan relay. The series circuit operates both fans at low speed.

In high speed operation, the ECM applies a ground to the coil side of the auxiliary fan relay, the control fan relay, and the main fan relay. When energized, the main fan relay applies voltage directly to the main cooling fan through the switch side of the relay. Simultaneously, the auxiliary fan relay and the control fan relay provide ignition voltage and a direct path to ground for the auxiliary cooling fan. With both fans connected in a parallel circuit, each fan receives full voltage and operates at high speed.

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Cooling Fan Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the DTC Information with a scan tool. Verify there are no DTCs set.

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

2. Command the appropriate fan relay ON and OFF with a scan tool. Verify that the fans turn ON and OFF when changing between the commanded states.

1. Ignition OFF, disconnect the cooling fan relays.
2. Connect a 30A fused jumper between the relay switch controlled output circuit terminal 30 and the relay switch ground circuit terminal 87 at the control fan relay connector. This will complete the auxiliary side fan ground circuit. Leave this jumper in place for the remainder of this procedure.

Important:  The following test must be performed on both the main fan relay circuit and the auxiliary fan relay circuit.

3. 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.

4. Connect a 30A fused jumper between the relay switch B+ circuit terminal 30 and the relay controlled output circuit terminal 87. Verify the appropriate fan turns ON.

⇒	If the appropriate fan does not turn ON, test the relay switch 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.

5. Install the control fan relay. Verify both fans turn ON at low speed.

⇒	If both fans do not turn ON, test the relay switch controlled output circuit terminal 87A for a short to ground or an open/high resistance.

6. 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. Test for less than 2 ohms of resistance between terminals 30 and 87A.

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

5. 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.