Component Diagnostic Charts

Chart C-12, Part 1 Of 2:

Chart C-12, Part 2 Of 2:

Wiring Diagram, Chart C-12:

CHART C-12 COOLING FAN CONTROL CIRCUIT


Circuit Description:

The cooling fan is totally controlled by the ECM based on inputs from the coolant sensor and fan control switch. The fan should run if coolant temperature is greater than about 106 degrees C (223 degrees F). Battery voltage is supplied to the fan relay on terminal "E" and ignition voltage to terminal "D". Grounding CKT 335 (relay terminal "F") will energize the relay and supply battery voltage to the fan motor. Once the fan relay is energized, by the ECM, it will remain "ON" for a minimum of 15 seconds. The ECM will remove the ground to CKT 335 if vehicle speed is over 40 mph unless the engine is overheating. The fan control switch, mounted in the A/C high pressure line, will open when head pressure exceeds 233 psi (1600 kPa) and this input causes the ECM to ground CKT 335. If a Code 14 or 15 sets or the ECM is operating in the fuel back-up mode the ECM will turn "ON" the cooling fan. Optional auxiliary cooling fans may be factory installed, but are not ECM controlled. Refer to wiring diagrams for diagnosis of these auxiliary systems.


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 gauge indicated overheating.

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 us "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.