The engine cooling fan is a variable speed fan. The engine control module (ECM) controls the fan speed by sending a pulse width modulated signal to the cooling fan speed control module. The cooling fan control module varies the voltage drop across the engine cooling fan motor in relation to the pulse width modulated signal. The cooling fan speed is affected by many different conditions and can be adjusted from 10-90 percent duty cycle. Ninety percent is considered high fan speed. When multiple cooling speed requests are received, the ECM uses the highest cooling fan speed of all the requests. During normal fan ON operation, the control circuit voltage will be low and near battery voltage when the fan is OFF.

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

Engine running, command the cooling fan speed from 0-100 percent with a scan tool. The fan speed should transition with the commanded states.

Important: The Circuit/System Verification must be performed first or misdiagnosis may result.

1. Ignition OFF, disconnect the connector from the cooling fan speed control module.
2. Ignition OFF, install a test lamp between the cooling fan speed control circuit terminal 6 and B+. The lamp should be OFF.

⇒	If the lamp is ON, test the cooling fan speed control circuit for a short to ground. If the circuit tests normal, replace the ECM.

3. Ignition ON, command the cooling fan speed from 0-100 percent with a scan tool. The brightness of the lamp should change with the commanded states.

⇒	If the lamp brightness does not change correctly, test the cooling fan speed control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

4. Ignition OFF, measure between the cooling fan speed control module ground circuit terminal 2 at the harness connector and ground for less than 1 ohm of resistance.

⇒	If greater than the specified range, repair the module ground circuit for open/high resistance.

5. Ignition OFF, verify that a test lamp illuminates between the B+ circuit terminal 3 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

6. Install appropriately fused jumpers between the cooling fan harness connector terminals 1 and 4 power and ground. The cooling fan should activate at full speed.

⇒	If the cooling fan does not activate, replace the cooling fan.

7. If the ECM, fan, and all circuits test normal, replace the cooling fan speed control module.

Ignition ON, command the cooling fan speed from 0-100 percent with a scan tool. The fan speed should transition with the commanded states.