Functional Description - Purge Solenoid
Functional Operation: The normally closed Purge Solenoid is energized to open and allow the fuel vapors to be drawn from the Charcoal Canister into the engine. The solenoid has one inlet port that connects to the canister and two outlet ports. One outlet port connects to the Intake Manifold and the other to the Ejector Tee. Each outlet port has an internal check valve. The check valves allow flow through the line that has vacuum applied to it during purging.
- When vacuum is present in the Intake Manifold, the check valve in the outlet port to the Intake Manifold is pulled open. The Ejector Tee port check valve is pulled closed by the vacuum in the cavity of the solenoid and the pressure on the other side of the check valve. The purge vapors are pulled directly into the Intake Manifold.
- During boost, pressure in the line from the Intake Manifold to the Ejector Tee creates vacuum in the line connected to the outlet port for the Ejector Tee. The Intake Manifold port check valve is pulled closed by the vacuum in the cavity of the solenoid and the boost pressure on the other side of the check valve. This allows fuel vapors to be drawn indirectly into the Intake Manifold through the Air Cleaner box.
The OBD port connects to the OBD Vent Valve. The normally closed OBD Vent Valve is energized to open when it is necessary to vent vacuum trapped in the EVAP system.
Purge Solenoid functional check: The Purge Solenoid functionality can be tested using a hand vacuum pump. To check the purge valve, disconnect the tubes connecting to the Intake Manifold and Ejector Tee ports. Block off the outlet port that is not being connected to the vacuum pump. There must be constant vacuum on one side and pressure on the other side of the check valve for the valve to close. Failure to block the other port will lead to misdiagnosing the purge valve.
- With the Purge Solenoid de-energized, connect a hand vacuum pump to the outlet port for the Intake Manifold and apply approximately 10 - 15 in. Hg. The valve should hold the vacuum if the Purge Solenoid valve is closed.
- With the Purge Solenoid de-energized, connect the hand vacuum pump to the outlet port for the Ejector Tee and apply approximately 10 - 15 in. Hg. The valve should hold the vacuum if the Purge Solenoid valve is closed.
- To verify the Purge Solenoid is not stuck closed, energize the solenoid to open it with vacuum applied at either outlet port. The vacuum should dissipate when the solenoid is energized.
Typical High 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 12 volts to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. In the example below, the PCM uses an internal pull up diagnostic resistor and calibrated voltage reference (V-Ref) as a comparator for fault detection.
- Circuit Open and Circuit High Detection: The PCM monitors for an open circuit and short to voltage when the internal driver is switched off. A milliampere voltage is provided to the device through the internal pull up diagnostic resistor connected in series with the device. The diagnostic voltage is monitored between the pull up resistor and device and compared to V-Ref. If the resistance in the device or circuitry becomes too large (approaches open) the voltage supply will increase on the comparator circuit and become greater than V-Ref, and a fault is detected. A short to Battery voltage will have the same effect.
- Circuit Low Detection:
The PCM monitors for a short to ground
when the driver is turned on. When switched on, the voltage to the device should be close to the 12 volt supply voltage. A short to ground will pull the voltage at the comparator circuit
below V-Ref and a circuit low fault is detected.