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 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 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 fan 3 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

Engine Cooling Schematics

Connector End View Reference

Engine Control Module Connector End Views

Description and Operation

Electrical Information 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, which is fan 2 relay.
2. Ignition ON, activate the cooling fan 1 relay with a 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 remaining 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 fan 1 and the fan 3 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 an 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 fan 1 and the fan 3 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 an open/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.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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.