Cooling Fan Functional Check

Refer to Engine Controls Schematics, Cooling Fans for complete circuit details.

CIRCUIT DESCRIPTION
The cooling fan system in this vehicle provides for two separate modes of operation. These are the low speed fan and the high speed fan operating modes. The low speed mode operates both fans at half speed by using a series circuit configuration. The high speed mode operates both fans at full speed by using a parallel circuit configuration. These two operating modes are accomplished using 3 fan control relays.

This system which uses three relays to control two fan motors appears complicated. To aid in better understanding this system it is suggested to follow the circuits in the schematic as you read the description. This will aid in understanding the two distinct modes of operating the relays and coolant fans.

Low speed fan operation is established when the PCM commands Fan Relay #1 ON by grounding circuit 335 with an internal driver (relays #2 and #3 are left OFF). This closes the relay switch and allows current to flow from the battery, through the switch contacts on circuit 409 to the Left Hand (LH) cooling fan. Current flow continues out of the fan motor on circuit 532 to relay #3. The unenergized relay #3 switch contacts allow current to flow to circuit 504. This provides power to the Right Hand (RH) cooling fan motor. The low side of the RH cooling fan completes the circuit to ground on circuit 150. This circuit is a series circuit which allows half speed operation for each fan since each fan motor will drop half of the supplied battery power.

High speed operation is established when all three relays are energized. Relays #2 and #3 share the same control circuit to the PCM. This is circuit 473. As in the low speed mode, Relay #1 supplies current flow to the LH cooling fan on circuit 409. Current flow continues through the motor on circuit 532 to relay #3 which is now energized. The switch has now provided a path for current to flow through circuit 150 directly to ground. This circuit now only has the LH fan motor to power and allows it to run at full speed. Relay #2 is also energized and provides current flow on circuit 504 to the RH cooling fan. This fan now has its own dedicated circuit and will operate at full speed. This mode is operating as a parallel circuit.

DIAGNOSTIC AIDS
If the owner complained of an overheating problem, determine if the complaint was due to an actual boil over, or the warning indicator light illuminated, or the engine coolant temperature gauge indicated overheating. The gauge accuracy can also be checked by comparing the Engine Coolant Temperature (ECT) sensor reading using a scan tool and comparing its reading with the gauge reading. If the engine is actually overheating and the gauge indicated overheating, but the cooling fan is not coming ON, the Engine Coolant Temperature (ECT) sensor has probably shifted out of calibration and should be replaced.

If the engine is overheating and the cooling fans are ON, the cooling system should be checked, refer to Engine Overheating in Engine Cooling. Symptom Related Diagnostic Procedures

The PCM will command low speed fans ON at 108°C (226°F) and OFF at 103°C (217°F) and, high speed fans ON at 113°C (235°F) and OFF at 108°C (226°F).

TEST DESCRIPTION

Diagnostic Chart:

The numbers below refer to numbers on the diagnostic table.
2. The PCM will enable the engine cooling fans when certain Diagnostic Trouble Codes are set. Go to applicable DTC Table first before proceeding with this table.
3. The engine coolant temperature must be below 100°C (212°F) before proceeding with diagnosis.