Chart C-12 Coolant Fan Control Circuit

Cooling Fan Circuit:

Circuit Description
The electric cooling fan is controlled by the ECM, based on inputs from the coolant temperature sensor, the A/C fan control switch, and vehicle speed. The ECM controls the fan by grounding CKT 335, which energizes the fan control relay. Battery voltage is then supplied to the fan motor.

The ECM grounds CKT 335, when coolant temperature is over about 106°C (223°F), or when A/C has been requested, and the fan control switch opens with high A/C pressure, about 200 psi (1380 kPa).

Cooling Fan Diagnostic 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 will close, which should energize the fan control relay.

2. If the A/C fan control switch or circuit is open, the fan would run whenever A/C is requested.

3. With A/C clutch engaged, the A/C fan control switch should open, when A/C high pressure exceeds about 1380 kPa (200 psi). This signal should cause the ECM to energize the fan control relay.


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 gage indicated over heating.

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 coolant 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," and the Tech 1 "Scan" tool indicates normal readings, 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.

Cooling Fan Diagnostic Circuit:

Cooling Fan Diagnostic Diagnostic Chart:

Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.

1. 12 volts should be available to both terminals "A" & "D", when the ignition is "ON."

2. This test checks the ability of the ECM to ground CKT 335. The "Service Engine Soon" light should also be flashing at this point. If it isn't flashing, Related Tests, Information and Procedures

3. If the fan does not turn "ON" at this point, CKT 702 or CKT 150 is open, or the cooling fan motor is faulty.