Part 1 of 2

Chart C-12, Cooling Fan Control Circuit Diagnosis (1 of 2):

Fan Control Wiring:

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

The ECM grounds CKT 335 and/or CKT 473 when engine coolant temperature is over about 106°C (223°F), or when A/C has been requested and the A/C pressure is about 1380 kPa (200 psi). Once the ECM 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 secondary fan).

Also, if DTC 14 or 15 sets or the ECM is in back up, the primary fan will run at all time.

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 pressure is above 300 psi (2069 kPa) or circuit is open, the fan would run whenever A/C is requested.

3. With A/C clutch engaged and the A/C pressure sensor is functioning properly, the fan should come "ON" when pressure exceeds about 1380 kPa (200 psi). This signal should cause the ECM to energize the fan control relay(s).

4. This will determine if the A/C pressure sensor is faulty or if the ECM 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 boilover, a hot light or temp. gage indicated over heating.

If the gage or light indicates overheating, but no boilover 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 (or equivalent) 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.