Theory Of Operation
The Radiator Fan Module is a smart device which controls the Pulse-Width Modulated (PWM) Radiator Fan speed. The Radiator Fan Module receives a continuous wake-up signal from the Radiator Fan Relay output. The Powertrain Control Module (PCM) uses a Low Side Driver (LSD) to control the Radiator Fan Relay to close the relay and turn on the wake-up signal. The PCM communicates with the Radiator Fan Module through a PWM signal. The PCM communicates the desired fan speed to the Radiator Fan Module using the PWM circuit. If a fault is detected in the voltage supply, ground or internal fan circuitry, the Radiator Fan Module sends a fault message to the PCM using the same PWM circuit. During normal operation, the Radiator Fan can be operated between 10% duty cycle (Low speed) and 92% duty cycle (High speed).
If the wake-up signal between the Radiator Fan Relay and the Radiator Fan Module is lost, no fault is set but the Radiator Fan defaults to low speed operation.
Typical Low Side Driver Operation and Fault Detection: This type of driver circuit is generally used for relay control, controlling a solenoid or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. 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 case the V-Ref will be slightly below Battery voltage.
- 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 the V-Ref, a fault is detected. In this case V-Ref will 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.