Part 1 of 2

Chart C-12 Cooling Fan Control Diagnosis (Part 1 Of 2):

Cooling Fan Wiring Circuit:

Circuit Description:
The primary and auxiliary electric cooling fan(s) are controlled by the PCM based on inputs from the Engine Coolant Temperature (ECT) sensor, the A/C control switches, vehicle speed and state of the A/C refrigerant pressure sensor. The PCM controls the fan(s) by grounding CKT 335 and CKT 473, which energizes the fan control relay. Battery voltage is then supplied to the fan motor.

The PCM grounds CKT 335 and/or CKT 473, when engine coolant temperature is over about 108°C (226°F), or when A/C has been requested and the A/C pressure is about 1380 kPa (200 psi). Once the PCM turns the relay "ON," it will keep it "ON" for a minimum of 30 seconds or until vehicle speed exceeds 70 mph (40 mph for auxiliary fan).

Also, if DTC is set, or the PCM is in backup, the primary fan will run at all times.

Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.
1. When cooling fans are commanded "ON," the cooling fan control driver(s) will close, which should energize the fan control relay(s).
2. If the A/C pressure is above 300 psi (2069 kPa) or the circuit is open, the fan would run whenever A/C is requested.
3. With A/C clutch engaged and the A/C refrigerant pressure sensor functioning properly, the fan should come "ON" when pressure exceeds about 1380 kPa (200 psi). This signal should cause the PCM to energize the fan control relay(s).
4. This will determine if the A/C refrigerant pressure sensor is faulty or if the PCM or circuitry is faulty.

Diagnostic Aids: If the owner complained of an overheating problem, it must be determined if the complaint was due to an actual boil over, a "temp" light, or temperature gage indicated overheating.

If the gage or light indicates overheating, but no boil over is detected, the gage circuit should be checked. The gage accuracy can also be checked by comparing the engine coolant temperature sensor reading using a Tech 1 scan tool and comparing its reading with the gage reading. If the engine is actually overheating, and the gage indicates overheating but the cooling fan is not coming "ON," the coolant sensor has probably shifted out of calibration and should be replaced.

If the engine is overheating, and the cooling fan is "ON," the cooling system should be checked.