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 low speed fan 1 relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the low relay. The left fan is connected in series to the right cooling fan through the de-energized series/parallel fan 2 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 low speed fan 1 relay, the series/parallel fan 2 relay, and the high speed fan 3 relay. When energized, the high speed fan 3 relay applies voltage directly to the right cooling fan through the switch side of the relay. Simultaneously, the low speed fan 1 relay and the series/parallel fan 2 relay provide ignition voltage and a direct path to ground for the left 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

Cooling System Connector End Views

Description and Operation

Cooling System Description and Operation

Electrical Information Reference

Scan Tool Reference

1. Verify that DTCs P0480, P0481, P0691, P0692, P0693 and P0694 are not set.

⇒	If any cooling system DTCs are set, repair the DTC first. Refer to DTC P0480, P0481, P0691, P0692, P0693, or P0694 .

2. Ignition ON, 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 B+ circuit and the ground circuit at the series/parallel fan 2 relay connector. This will complete the left side or main fan ground circuit. Leave this jumper in place for the remainder of this procedure.

Important:  The following test must be performed on both the fan 3 relay circuit and the fan 1 relay circuit.

3. Ignition ON, verify that a test lamp illuminates between the relay switch B+ circuit 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.

Important:  The following test must be performed first on the fan 3 relay circuit and next on the fan 1 relay circuit. Leave the jumper connected to the fan 1 relay circuit for the next test.

4. Connect a 30A fused jumper between the relay switch B+ circuit and the relay controlled output circuit. Verify the appropriate fan is activated

⇒	If the appropriate fan does not activate, 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.

5. Install the series/parallel fan 2 relay. Verify both fans activate at low speed.

⇒	If both fans do not activate, test the relay controlled output circuit of the fan 2 relay 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.