Part 1 of 2

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

Coolant Fan Control Schematic:

CIRCUIT DESCRIPTION
The electric cooling fan is controlled by the Powertrain Control Module (PCM), based on inputs from the Engine Coolant Temperature (ECT) sensor, A/C request, A/C refrigerant fan request switch and vehicle speed. The PCM controls the fan(s) by grounding CKT 335 and CKT 473, which energizes the fan control relay(s). Battery voltage is then supplied to the fan motor(s).
The PCM grounds CKT 335 (puller fan relay), when coolant temperature is approximately 106°C (223°F), or when A/C has been requested, and the A/C refrigerant fan request switch opens with high A/C pressure, about 200 psi (1380 kPa).
The pusher fan relay is grounded by the PCM when coolant temperature is approximately 110°C (217°F) and/or puller fan is "ON" for A/C pressure.

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. With the diagnostic terminal grounded, the cooling fan control driver(s) will close, which should energize the fan control relay(s).
2. If the A/C refrigerant fan request switch or circuit is open, the fan would run whenever A/C is requested.
3. With A/C requested and 45 seconds after request removed, the A/C refrigerant fan request switch should open, when A/C high pressure exceeds about 1380 kPa (200 psi). This signal should cause the PCM to energize the fan control relay(s).
4. This test checks to see if the coolant puller and pusher fan relay(s) (CKTs 335 and 473) are shorted to ground. This condition would cause the fans to be "ON" constantly.

DIAGNOSTIC AIDS
If the owner complained of engine overheating problem, it must be determined if the complaint was due to an actual boil over, or the hot light, or temperature gauge indicated over heating.
If the gauge or light indicates overheating but no boil over is detected, the gauge circuit should be checked. The gauge or sending unit accuracy can also be checked by comparing the ECT sensor reading using a Tech 1 scan tool and comparing its reading with the gauge reading.
If the engine is actually overheating and the gauge indicates overheating but the cooling fan is not coming "ON" and the Tech 1 scan tool indicates normal readings, the ECT 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.