Cooling Fan Always On

Cooling Fan Always On

Circuit/System Description

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 1 relay. This energizes the coil and applies voltage directly to the right cooling fan through the switch side of the fan 1 relay, which is fed by the fan 1 fuse. The left fan is connected in series to the right fan through the de-energized fan 2 relay so that both operate at low speed.

In high speed operation, the ECM applies a ground to the coil side of the fan 1 relay, the fan 2 relay , and the fan 3 relay. On the fan 3 relay, the energized coil closes the switch side of the relay and applies voltage directly to the left 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 2 relay pulling the switch side over, providing a direct path to ground for the right cooling fan, which has voltage applied through the energized fan 1 relay. In high speed mode, the fans are operated as a parallel circuit with full voltage applied to each.

Reference Information
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

* Circuit Testing
* Connector Repairs
* Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs

Scan Tool Reference

Scan Tool Data List

Circuit/System Verification

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

Circuit/System Testing

1. Ignition OFF, disconnect the fan high relay.
2. Ignition ON, observe that both fans are now OFF.

^ If both fans are OFF, replace the fan 3 relay. If either fan is still activated, remove the fan 1 relay. If both fans are now OFF, replace the fan 1 relay. If either fan is still activated, test for a short to voltage in the fan voltage supply circuit.

Repair Verification

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