Electric Cooling Fan Diagnosis

Diagnostic Chart (Part 1 Of 3):

Diagnostic Chart (Part 2 Of 3):

Diagnostic Chart (Part 3 Of 3):

Schematic:

CIRCUIT DESCRIPTION
The electric cooling fan is controlled by the Powertrain Control Module (PCM) through the fan relay based on inputs from the Engine Coolant Temperature (ECT) sensor and Intake Air Temperature (IAT) sensors, the A/C selector switch, A/C refrigerant pressure sensor and the Vehicle Speed Sensor (VSS). The PCM controls the cooling fan by grounding the cooling fan control circuit which turns ON the cooling fan relay. The fan relay will be commanded ON when the following conditions are met:
- Engine running in or out of gear and no vehicle speed for 10 minutes.
- Engine coolant temperature reaches 106°C (223°F) or more.
- A/C clutch requested.
- Vehicle speed is less than 38 MPH. The cooling fan relay will be commanded ON regardless of vehicle speed when:
- Any DTC is set that causes the Malfunction Indictor Lamp (MIL) to be illuminated.
- Engine coolant temperature is 151°C (304°F) or more.
- A/C refrigerant pressure is high.

The cooling fan may be commanded ON when the engine is not running under fan run-on conditions.

DIAGNOSTIC AIDS
- If the owner complained of an overheating problem, it must be determined if the complaint was due to an actual boil over. Or if the Temp light, or temperature gauge, indicated overheating.
- Check for the proper amount of coolant in the system.
- If the gauge or light indicates overheating, but no boil over is detected, the gauge or light circuit should be checked. The gauge accuracy can also be checked by comparing the engine coolant temperature sensor reading using a scan tool with the gauge reading.
- If the engine is actually overheating, and the gauge indicates overheating but the cooling fan is not coming ON, the Engine Coolant Temperature (ECT) sensor may have shifted out of calibration and should be replaced. Refer to Temperature vs Resistance for a Temperature vs. Resistance table.
- If the engine is overheating, and the cooling fan is ON, the cooling system should be checked. Refer to Engine Overheating (Causes) in Engine Cooling. Cooling System

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
3. Cooling system operation and level should be checked before determining if any electrical conditions are present causing a cooling system failure.
4. With the engine OFF, the cooling fan should not be commanded ON by the PCM.
6. If the cooling fan turns OFF after disconnecting the PCM connectors, the PCM was causing the fan to be ON constantly.
8. A test lamp that illuminates indicates that the cooling fan battery feed circuit (CKT 409) is shorted to battery voltage. The cooling fan will also remain engaged when the cooling fan relay electrical connector is disconnected.
9. Connecting CKTs 542 and 409 together bypasses the PCMs control of the cooling fan. A cooling fan that fails to engage at this point indicates that the malfunction is in CKT 409, the cooling fan or cooling fan ground circuit.
10. Both battery teed circuits are supplied to the cooling fan relay through the same fuse. No voltage on both circuits would indicate that the fuse is open. Locate and repair any shorts that may have caused the fuse to open before replacement.
14. With the engine stopped and the cooling fan relay commanded ON, the PCM should ground the A/C relay circuit. This will also illuminate the test lamp.
15. This step determines if the reason the cooling fan did not come ON was due to an open battery feed circuit to the cooling fan.
26. Reprogram the replacement PCM. Perform the crankshaft position system variation procedure. Refer to the latest Techline procedures for PCM reprogramming and the crankshaft position system variation learn procedure. Crankshaft Position Sensor
27. If no malfunctions have been found at this point, refer to Diagnostic Aids for additional checks and information.