Description And Operation-Four Relay Cooling Fan System Controls
When equipped with A/C, the cooling fan system uses four relays to operate the cooling fans in three different speeds. The PCM controls the operation of the relays through internal Low Side Driver (LSD) circuits. The PCM performs diagnostics on the three control circuits. The relays are controlled by the PCM as follows:
- Radiator Fan Relay 1 and Radiator Fan Relay 2 are controlled simultaneously using the Low Speed Fan Relay Control circuit. It is important to understand that in order for a fault to set due to a faulty relay or missing voltage supply to the relay coil, both relays must be faulty or missing the voltage supply to the coils. If either one of the relays is functional, the Cooling Fan 1 Control circuit fault will not set. This could cause a cooling fan to be inoperative at low speed without a DTC present.
- Radiator Fan High Relay is controlled using the High Speed Fan Relay Control circuit.
- Radiator Fan High Relay 2 is controlled using the High Speed Fan Relay 2 Control circuit.
The operation of the three speeds are described below.
LOW SPEED OPERATION: The Low Speed Fan Relay Control circuit is grounded energizing Radiator Fan Relay 1 and Radiator Fan Relay 2 in the PDC. Both Radiator Cooling Fans are powered through the resistors and run on low speed.
MEDIUM SPEED OPERATION: The Radiator Fan 1 High Relay is grounded. Radiator Cooling Fan 1 (left fan) is supplied direct battery voltage through the Radiator Fan 1 High Relay. Radiator Cooling Fan 1 will run on high speed.
HIGH SPEED OPERATION: For high speed operation, all three relays are simultaneously grounded. The Low Speed Fan Relay Control circuit is grounded to energize Radiator Fan Relay 1 and Radiator Fan Relay 2 in the PDC. The output from the Radiator Fan 2 Relay supplies battery power to the Radiator Fan 2 High Relay. Radiator Fan 1 High Relay is powered by a direct battery feed. Both Radiator Fan High Relay control circuits are grounded to energize both High Speed Fan Relays. Both Radiator fans will run on high speed.