Evaporative Emissions Purge Solenoid
| Refer to COMPONENT INDEX . |
Functional Operation: The normally closed Purge Solenoid is energized to open and allows the fuel vapors to be drawn from the Charcoal Canister into the engine. The PCM energizes the Purge Solenoid using a Pulse Width Modulated (PWM) Low Side Driver (HSD) control circuit.
- When vacuum is present in the Intake Manifold the check valve in the outlet port to the Intake Manifold is pulled open. The other check valve in the outlet port to the Ejector Tee is pulled closed by the vacuum in the cavity of the solenoid and the purge vapors are pulled into the Intake Manifold.
- During boost, pressure in the line from the Intake Manifold closes the check valve in the outlet to the manifold. Vacuum created in the venturi of the Ejector Tee creates vacuum on the line connected to the outlet port for the Ejector Tee, opening the check valve and allowing fuel vapors to be drawn into the Air Cleaner box.
Diagnostic Overview: The Purge Solenoid functionality can be tested using a hand vacuum pump:
- With the Purge Solenoid de-energized, connect the hand vacuum pump to the outlet port (engine side) and apply approximately 10-15 in. Hg. The solenoid should hold the vacuum if the Purge Solenoid is closed.
- To verify the Purge Solenoid is not stuck closed, energize the solenoid to open it with vacuum applied at the outlet port. The vacuum should disappear when the solenoid is energized.
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.