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 Cool/Fan 1 Relay 28. 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 the cool SER/PAR relay 29, so that both 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 the Cool/Fan 2 Relay 30. 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 volt. 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

Cooling System Connector End Views

Description and Operation

Cooling Fan Description and Operation

Electrical Information Reference'

1. If DTCs P0480 or P0481 are set, then perform those diagnostics first.
2. Ignition ON, verify with a scan tool that the ECM is not commanding fan activation.
3. Ignition ON, observe that the fan is not activated.

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

1. Ignition OFF, leave the Series/Parallel relay in place while disconnecting the other relays.
2. Ignition ON, observe that the fan is not activated.

⇒	If the fan is not activated, then replace the faulty relay.

⇒	If the fan is activated, then test the fan voltage supply circuits for a short to voltage.

1. Ignition ON, verify with a scan tool that the ECM is not commanding fan activation.
2. Ignition ON, observe that the fan is not activated.