Functional Description - OBD Vent Valve
Functional Operation:
The normally closed OBD Vent Valve is used to provide open venting to the atmosphere when energized by the Powertrain Control Module (PCM). The valve is used as part of the Evaporative System diagnostics on Turbocharged engines for two purposes:
- ESIM Switch Stuck Closed Diagnostic: The evaporative system must be vented to atmosphere to relieve the vacuum and open the ESIM switch to perform the small leak diagnostic. Due to the check valves in the Proportional Purge Solenoid (PPS) on turbocharged engines, the OBD Vent Valve is energized by the PCM to vent the vacuum from the system. When the system is being actively vented the ESIM Switch Stuck Closed Diagnostic is used to determine that the ESIM switch is functioning properly.
- Purge Performance Diagnostic: This diagnostic looks for a delta pressure change in the Fuel Tank Pressure (FTP) Sensor signal when the Purge Solenoid is energized. If vacuum is already trapped in the system it may be difficult to generate enough of a delta pressure change to obtain good separation between a passing and failing system. Since both sides of the system are tested during each drive cycle, the OBD Vent Valve is energized to eliminate the excess vacuum before the Purge Flow Monitor Diagnostic begins for each side.
Diagnostic Overview: The PCM performs open circuit and short circuit diagnostics on the control circuitry for the OBD Vent Valve. Detecting a valve that is mechanically stuck open or closed is caught by other EVAP system diagnostics. A stuck open valve would cause a system leak and would fail the small leak or large leak diagnostic. A valve that is stuck closed would cause one or both of the ESIM Switch Stuck Closed or Purge Performance diagnostics to fail.
Typical Low Side Driver Function 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.