Cooling Fan Inoperative w /V8

The engine control module (ECM) commands the fans ON in high speed or low speed. In low speed, the fans are configured in series. In high speed, the fans are configured in parallel.

In low speed operation, the ECM applies ground to the coil side of the low speed relay, which is R1 cool/fan 1 relay. Voltage is applied directly to the left cooling fan through the switch side of the low speed relay. The right fan is connected in series to the left fan through the de-energized series/parallel relay, which is R7 cool/fan 2 relay, so that both fans operate at low speed.

In high speed operation, the ECM applies a ground to the coil side of the low speed relay, the series/parallel relay, and the high speed relay, which is relay R2 cool/fan relay. Voltage is applied directly to the right cooling fan through the switch side of the high speed relay. The left cooling fan obtains voltage through the low speed relay and ground through the series/parallel relay.

When the ECM is commanding a fan relay ON, the voltage of the control circuit should be low, near 0 volts. When the ECM is commanding a fan relay OFF, the voltage of the control circuit should be high, near battery voltage. If the ECM detects that the actual voltage of the relay control circuit is different than the expected voltage, DTC P0480 or P0481 will set and the affected ECM driver will be disabled.

Schematic Reference

Connector End View Reference

Description and Operation

Cooling Fan Description and Operation

Electrical Information Reference

Scan Tool Reference

1. If DTCs P0480 or P0481 are set, then perform those diagnostics first.
2. Ignition ON, command each relay ON and OFF with a scan tool. Feel or listen to verify that each relay turns ON and OFF with each command.

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

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

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

3. Ignition OFF, disconnect the fan relays.
4. Connect a 30A fused jumper between the normally-open switch contact terminals 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 the cool/fan 1 and the cool/fan relay connections and verify 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 an 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 terminal and the fan voltage supply circuit terminal at the cool/fan 1 and the cool/fan relay connections and verify fan activation.

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

⇒	If the circuit tests normal and its fuse is open, test all connected components and replace as necessary.

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

Ignition ON, command the relay ON and OFF with a scan tool. Observe to verify that the 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 cooling 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 normally closed contacts of the 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 cooling fan 1 relay, the fan S/P relay, and the fan 2 relay. When energized, the fan 2 relay applies voltage directly to the right cooling fan through the switch side of the relay. Simultaneously, the fan 1 relay and the fan S/P 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 circuit terminal R75 and the ground circuit terminal R79 at the fan S/P relay connector. This will complete the left 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 2 relay circuit and the low speed fan 1 relay circuit.

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

•	High speed fan 2 relay terminal R80

•	Low speed fan 1 relay terminal R67

⇒

If the test lamp does not illuminate, test the appropriate 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 high speed fan 2 relay circuit and next on the low speed fan 1 relay circuit. Leave the jumper connected to the low speed 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 terminals listed below. Verify the appropriate fan is activated

•	High speed fan 2 relay terminal R80 and terminal R83

•	Low speed fan 1 relay terminal R67 and terminal R70

⇒	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 R77 of the fan S/P 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.