Theory Of Operation
When the throttle plate is closed suddenly, excessive boost pressure can build up between the Turbocharger and the Throttle Body. This can occur because the compressed air created by the Turbocharger is trapped. This results in compressor surge which can cause compressor lag or damage the Turbocharger. The Powertrain Control Module (PCM) detects this pressure increase from feedback provided by the Boost Pressure Sensor. The Boost Pressure Sensor operates like a typical three wire sensor. The PCM provides a 5-Volt Supply and Sensor Return circuit to the Boost Pressure Sensor and receives the feedback voltage on the signal circuit.
The Turbocharger Surge Valve is used to vent the boost pressure trapped between the Turbocharger and Throttle Body when the throttle plate is closed suddenly. The Turbocharger Surge Valve is spring loaded open, but during normal engine operation the Turbocharger Surge Valve is energized by the PCM to hold it in the closed position and allow boost. When a pressure surge is detected, the Turbocharger Surge Valve is de-energized and the spring loaded valve is opened. When open the valve routes the pressure trapped between the Turbocharger and the Throttle Body back to the inlet of the Turbocharger, relieving the pressure and preventing compressor surge. The Turbocharger Surge Valve is controlled by the PCM using a Low Side Driver (LSD) inside the PCM.
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 would 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 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.