Chart C-12 Coolant Fan Control Circuit

Chart C-12: Part 1 Of 3:

Chart C-12: Part 2 Of 3:

Chart C-12: Part 3 Of 3:

Cooling Fan Circuit:

CIRCUIT DESCRIPTION
Cooling Fan Control
The cooling fans are controlled by the Engine Control Module (ECM) based on various inputs. The primary cooling fan relay is energized by the cooling fan control relay when the cooling fan control relay is de-energized. The primary fan relay will be energized and supply battery voltage to the primary cooling fan motor. Grounding CKT 473 (relay terminal "2") will energize the secondary cooling fan relay (Fan 2) and supply battery voltage to the secondary fan motor.

Secondary Cooling Fan Enable
The A/C cooling fan switch, mounted in the A/C high pressure line, will be grounded when head pressure exceeds 1800 - 2070 kPa (200 - 300 psi) and enable the secondary cooling fan motor.

Cooling Fan Defaults
If a DTC 14, or 15 sets or the ECM is operating in the fuel back-up mode, the ECM will turn "ON" the cooling fans.

DIAGNOSTIC AIDS
Overheating Problem
If the owner complained of an overheating problem, it must be determined if the complaint was due to an actual boil over, or the warning indicator light, or engine coolant temperature gage indicated overheating.

Checking Gage Accuracy
The gage accuracy can also be checked by comparing the Engine Coolant Temperature (ECT) sensor reading using a Tech 1 scanner and comparing its reading with the gage reading. If the engine is actually overheating and the gage indicated overheating, but the cooling fan is not coming "ON," the ECT sensor has probably shifted out of calibration and should be replaced.

Cooling System Check
If the engine is overheating and the cooling fans are "ON," the cooling system should be checked, refer to Cooling System.

Cooling Fan Enable
The ECM will command Fan 1 "ON" at 108°C (226°F) and "OFF" at 105°C (221°F) and, Fan 2 "ON" at 111°C (231°F) and "OFF" at 107°C (225°F).