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 sensor and intake air temperature sensors, the A/C selector switch, A/C refrigerant pressure sensor and the vehicle speed sensor. 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.

Cooling Fan Run-On
The PCM usually powers down 10 to 30 seconds after the ignition is turned OFF. Since the cooling fan is PCM controlled, the PCM may remain powered up for up to 7 minutes after the ignition is turned OFF to provide a ground in order to energize the cooling fan relay.

This is called a fan Run-On and will occur if any of the following combinations of conditions exist when the ignition is turned OFF.

^ Engine coolant temperature greater than 100°C (212°F) and intake air temperature greater than 80°C (176°F).
^ Engine coolant temperature greater than 112°C (234°F) and intake air temperature greater than 70°C (158°F).
^ Engine coolant temperature greater than 116°C (241°F).

The PCM and cooling fan will remain ON until these combinations of conditions no longer exist, up to a maximum of 7 minutes.

If the cooling fan is OFF when the PCM powers down, it will remain OFF regardless of any temperatures until the ignition is turned ON again.

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 gage indicated overheating.
^ Check for the proper amount of coolant in the system.
^ If the gage, or light, indicates overheating, but no boil over is detected, the gage or light circuit should be checked. The gage accuracy can also be checked by comparing the engine coolant temperature sensor reading using a scan tool with the gage reading.
^ If the engine is actually overheating, and the gage 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 the table.
^ If the engine is overheating, and the cooling fan is ON, the cooling system should be checked. Refer to the Cooling and Radiator portion.

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 tan. 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 feed 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 Procedure.
27. If no malfunctions have been found at this point, refer to Diagnostic Aids for additional checks and information.