Theory Of Operation
SYSTEM DESCRIPTION: The two speed cooling fan system is available in light duty and heavy duty. Both systems consist of relays and resistors for high and low speed operation. The light duty system has one Radiator Fan Assembly. The heavy duty system uses two Radiator Fan Assemblies. The PCM controls the Radiator Fan speed by commanding on the appropriate relay through internal Low Side Drivers (LSD). The PCM performs circuit fault diagnostics on the relay control circuits.
Light duty cooling system has a single fan. Heavy duty cooling system has dual fans.
- Light duty:
There are two relays in the Power Distribution Center (PDC) used to control low speed and high speed operation. To achieve low speed operation there is an external resistor between the relay and Radiator Fan that is used to reduce the voltage to the Radiator Fan Motor.
- Heavy duty:
There are two relays in the Power Distribution Center (PDC) used to control high speed operation. The Radiator Fan Low Relay is in the front wiring harness. To achieve low speed operation there are two external resistors between the relay and Radiator Fans that is used to reduce the voltage to the Radiator Fan Motors. The Radiator Fan Low Relay shares a fuse with the Radiator Fan 1 High Relay.
Typical Low Side Driver Operation and Fault Detection: This type of driver circuit is generally used for relay control, solenoid control or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The ground could be constant or Pulse Width Modulated (PWM). The PCM also provides fault detection for the device, wiring and internal driver. Fault detection can be done by monitoring voltage on the circuit, current draw, or a combination of both. For diagnostic purposes the PCM uses an internal pull down diagnostic resistor connected in series and a voltage reference (V-Ref) comparator for fault detection:
- Circuit Open and Circuit Low Detection: The PCM monitors for an open circuit and short to ground when the driver is switched off. When switched off, the available voltage passes through the device and the internal pull down resistor connected in series. The voltage at the comparator circuit should be close to Battery voltage since the majority of the voltage drop occurs through the diagnostic resistor. If the available voltage is less than the V-Ref, a fault is set. In this scenario the V-Ref would be slightly below Battery voltage. An alternative method of fault detection for an open or short to ground that is used is to monitor current draw when the internal driver is switched on. If the module does not detect any current draw it determines that the component or circuitry is open. Excessive current draw detected would indicate a short to ground.
- Circuit High Detection: The PCM monitors for a short to voltage when the driver is switched on. When the driver is switched on providing a path to ground through the transistor, the available voltage should be pulled low, near zero volts since the comparator circuit is monitoring the ground side of the device. If the voltage is greater than V-Ref, a fault is detected. In this scenario V-Ref would be slightly above zero volts.
A load that has a resistance that is below manufacturer specification, or a second load device shorted to the low side driver circuit can cause excessive current draw on the internal driver. The driver will be switched off to protect against overheating and damaging the driver. In this instance the Circuit High fault may be detected because the available voltage on the comparator circuit is above V-Ref.
LIGHT DUTY OPERATION:
- Low Speed:
For low speed operation the PCM grounds the Series/Parallel Relay Control circuit energizing the relay. Battery voltage on the Series/Parallel Relay Output circuit passes through the Radiator Fan Resistor, reducing the voltage to the Radiator Fan Motor.
- High Speed:
For high speed operation the PCM grounds the Radiator Fan High Relay Control circuit energizing the relay. Battery voltage on the Radiator Fan High Relay Output circuit connects directly to the Radiator Fan Motor, bypassing the Radiator Fan Resistor.
HEAVY DUTY OPERATION:
- Low Speed:
For low speed operation the PCM grounds the Radiator Fan Low Relay Control circuit energizing the relay. Battery voltage on the Radiator Fan Low Relay Output circuit passes through both the Radiator Fan 1 Resistor and the Radiator Fan 2 Resistor, reducing the voltage to the Radiator Fan Motors.
- High Speed:
For high speed operation the PCM grounds the Radiator Fan Relay 1 and Radiator Fan Relay 2 Control circuits energizing the relays. Battery voltage on the relay output circuits connects directly to the Radiator Fan Motors, bypassing the Radiator Fan Resistors.