Chart C-12 Coolant Fan Control Circuit

Chart C-12:

Cooling Fan Circuit:

CIRCUIT DESCRIPTION:
Battery voltage to operate the cooling fan motor is supplied to relay by CKT 2. Ignition voltage to energize the relay is supplied to relay by CKT 650. When the ECM grounds CKT 335, the relay is energized and the cooling fan is turned "ON." When the engine is running, the ECM will turn the cooling fan "ON" if:

^ A/C is "ON."

When the engine is running, the ECM will turn the engine cooling fan "ON" for any of the following reasons:

^ Diagnostic Trouble Code (DTC) 14 is set.
^ Engine coolant temperature is 106°C (223°F) or greater.
^ A/C refrigerant pressure is 51 psi or greater with A/C selected and a Diagnostic Trouble Code (DTC) 66 is not set.

Once the cooling fan is enabled, the ECM will turn the fan "OFF" when:

^ Engine is turned "OFF."
^ Engine coolant temperature drops to 99°C (210°F) or less.
^ A/C refrigerant pressure drops to 41 psi or less with A/C selected.

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 the hot light, or temperature gauge indicated overheating.

if the gauge or light indicates overheating, but no boil over is detected, the gauge circuit should be checked. The gauge accuracy can also be checked by checking the engine coolant temperature display using a scan tool and comparing its display with the instrument cluster gauge.

If the engine is actually overheating and the gauge indicates overheating, but the cooling fan is not coming "ON," the ECT sensor has probably shifted out of calibration and should he replaced.