Cooling Fan Inoperative w /V8

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 fan low relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the fan low relay, which is fed by the fan 1 fuse. The right fan is connected in series to the left fan through the de-energized series/parallel relay so that both operate at low speed.

In high speed operation, the ECM applies a ground to the coil side of the fan low relay, the series/parallel relay , and the fan high relay. On the fan high relay, the energized coil closes the switch side of the relay and applies voltage directly to the right cooling fan through the switch side of the relay, which is fed by the fan 2 fuse. At the same time, the ECM energizes fan control relay pulling the switch side over, providing a direct path to ground for the left cooling fan, which has voltage applied through the energized fan low relay. In high speed mode, the fans are operated as a parallel circuit with full voltage applied to each.

The ECM has the capability of providing command to the fan relays even while a scan tool output control is being used. Always refer to the fan control command parameters on the scan tool to know which fans are being commanded ON by the ECM.

Schematic Reference

Engine Cooling Schematics

Description and Operation

Cooling Fan Description and Operation

Connector End View Reference

Electrical Information Reference

Scan Tool Reference

1. Verify that the following DTCs are not set: P0480 or P0481.

⇒	If either of the DTCs is set, repair the DTC first.

2. Ignition ON, command high and low speed fans ON and OFF with a scan tool. Observe to verify that the fans turn ON and OFF when changing between the commanded states.

Important: You must perform the Circuit/System Verification before proceeding with Circuit/System Testing.

1. Ignition OFF, disconnect the series/parallel fan relay.
2. Ignition ON, activate the cooling fan relay 1 with a scan tool. The fan should be OFF.

⇒	If the fan is activated, test the right fan ground circuit for a short to ground.

3. Ignition OFF, disconnect the remaining fan relays.
4. Connect a 30A fused jumper between the normally-open switch contact terminals R15 and R16 at the series/parallel relay connector in order to complete the left fan ground circuit. Leave this jumper in place for the remainder of this procedure.
5. One at a time, connect a 30A fused jumper between the positive terminal at the battery and the fan voltage supply circuit terminal at each relay connection, R15 and R16, and verify the appropriate fan activation.

⇒	If the appropriate fan does not activate, test the fan voltage supply circuit for a short to ground or open/high resistance. If the circuit tests normal, test the fan ground circuit for open/high resistance. If the circuits test normal, replace the fan.

6. Ignition ON, connect a 30A fused jumper between the relay switch voltage supply circuit terminals R40 and R20 and the fan voltage supply circuit terminals R41 and R21 at the low speed and then the high speed relay connections and verify the appropriate fan activation.

⇒	If the appropriate fan does not activate, test the relay switch voltage supply circuit for a short to ground or high resistance.

7. If the circuits and fans test normal, replace the relay.

Ignition ON, command the fan relay ON and OFF with a scan tool. Observe to verify that the appropriate fan turns ON and OFF with each command.

Cooling Fan Inoperative w /V6

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 relay. This energizes the coil and applies voltage directly to the right cooling fan through the switch side of the low relay. The right fan is connected in series to the left cooling fan through the de-energized series/parallel (S/P) 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 low speed fan relay, the S/P fan relay, and the high speed fan relay. When energized, the high speed fan relay applies voltage directly to the left cooling fan through the switch side of the relay. Simultaneously, the low speed fan relay and the S/P fan relay provide ignition voltage and a direct path to ground for the right 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 circuit terminal R15 and the ground circuit terminal R16 at the S/P fan relay connector. This will complete the right 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 high speed fan relay circuit and the low speed fan relay circuit.

3. Ignition ON, verify that a test lamp illuminates between ground and the relay switch ignition circuit terminal listed below:

•	High speed fan relay terminal R20

•	Low speed fan relay terminal R40

⇒

If the test lamp does not illuminate, test the appropriate relay switch ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay switch ignition 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 high speed fan relay circuit and next on the low speed fan relay circuit. Leave the jumper connected to the low speed fan relay circuit for the next test.

4. Connect a 30A fused jumper between the relay switch ignition circuit and the relay controlled output circuit terminals listed below. Verify the appropriate fan is activated

•	Low speed fan relay terminal R40 and terminal R41

•	High speed fan relay terminal R20 and terminal R21

⇒	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 S/P fan relay. Verify both fans activate at low speed.

⇒	If both fans do not activate, test the relay controlled output circuit terminal R17 of the S/P fan 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.