Theory Of Operation
GENERAL OPERATION: The electronically controlled Vacuum Pump system is installed to provide supplemental vacuum to the Brake Booster when the engine vacuum supply is low. An Electronic Vacuum Pump (EVP) is especially important on vehicle applications where the engine is not running the entire time, such as for hybrid or Stop Start equipped vehicles. This system maintains a consistent brake pedal feel under all driving conditions. The Vacuum Pump is connected to the engine and the Brake Booster through a series of hoses and one-way flow check valves. A Vacuum Pressure Sensor mounted in the Brake Booster is hard wired to the Anti-Lock Brake (ABS)/Electronic Stability Program (ESP) Module. The Vacuum Pressure Sensor signal information is then sent to the Powertrain Control Module (PCM) over the CAN Bus. The PCM energizes the Vacuum Pump Relay with a Low Side Driver (LSD) to control the Vacuum Pump operation and maintain the Brake Booster vacuum.
DIAGNOSTICS: The PCM performs diagnostics on the Vacuum Pump Relay Control circuit and will set the Vacuum Pump circuit fault if an open or shorted condition is detected. The PCM monitors the Vacuum Pressure Sensor signal input when the relay is energized to make a determination if the Vacuum Pump is functional. If the PCM determines that the Vacuum Pump is not functioning, it will set the Vacuum Pump performance fault.
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 scenario 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 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.