Part 1

Diagnostic Chart:

Wiring Diagram for Chart C-12A Coolant Fan Control Circuit:

CIRCUIT DESCRIPTION
The electric coolant fan is controlled by the Electronic Control Module (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 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.

Also, if Code 14 or 15 sets or the ECM is in throttle body backup, the fan will run at all times.

TEST DESCRIPTION
Numbers below refer to circled numbers 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 boilover or the hot light or temperature 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 "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 coolant fan is "ON," the coolant system should be checked.