Cooling Fan Always On

Cooling Fan Always On

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of the diagnostic category.

Circuit/System Description

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 33. 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 3 Relay 32, 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 Fan 2 Relay 31. 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.

Reference Information
Schematic Reference

Engine Cooling Schematics (Diagrams)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Cooling Fan Description and Operation (Cooling Fan Description and Operation)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

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

• If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle).

2. Verify with a scan tool that the ECM is not commanding fan relay activation.
3. Ignition ON, verify with a scan tool that the ECM is not commanding fan activation.
4. Ignition ON, observe that the fans are not activated.

Circuit/System Testing

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


1. Ignition OFF, disconnect the high speed relay, which is FAN 2 relay 31.
2. Ignition ON, observe that both fans are now OFF.

• If both fans are OFF, replace the high speed relay. If either fan is still activated, remove the low speed fan relay, which is FAN 1 relay 33. If both fans are now OFF, replace the low speed relay. If either fan is still activated, test for a short to voltage in the fan voltage supply circuit.

Repair Instructions

* Engine Coolant Fan Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for ECM replacement

Repair Verification

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.