Chart C-12 Coolant Fan Control Circuit

Chart C-12:

CIRCUIT DESCRIPTION:

Battery voltage to operate the cooling fan motor is supplied to relay terminal "1". Ignition voltage to energize the relay is supplied to relay terminal "2". When the ECM grounds CKT 535, the relay is energized and the cooling fan is turned "ON". When the engine is running, the ECM will turn the cooling fan "ON" if the conditions for a coolant temperature sensor trouble code (14 or 15) are present or if the A/C is requested and and the A/C pressure is above 150 psi (1.78 volts). The cooling fan will also come on if the coolant temperature is greater than 100~C (212~F).

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Fan should be "ON," as using the scan tool to turn on the QDM. This provides a ground path for current to flow through the relay coil.

2. The fan should be running with A/C requested and A/C pressure above 150 psi (1.78 volts).

3. Ensures that the ECM is receiving the A/C request signal.

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 gauge indicated over heating.

^ If the gauge, or light, indicates overheating, but no boilover is detected, the gauge circuit should be checked. The gauge accuracy can, also, be checked by comparing the coolant sensor reading using "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", 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.

^ Check for faulty A/C pressure sensor by comparing scan tool reading to manifold gauge reading connected to A/C system.