On turbocharged engines, the exhaust gases pass from the exhaust manifold through the turbocharger, turning the turbine blades. The compressor side of the turbocharger also turns, pulling air through the air filter and pushing the air into the intake manifold, pressurizing the intake manifold.
The turbocharger wastegate is normally closed when the engine is running, but it opens to bypass exhaust gas to prevent an overboost condition. The wastegate will open when vacuum is vented from the actuator, and is controlled by the pulse width modulated wastegate solenoid.
The wastegate solenoid is pulsed “ON” and “OFF” by the PCM. Under normal driving conditions and idle, the wastegate solenoid is “ON,” allowing vacuum to close the actuator to build boost pressure. A boost increase will be detected by the boost sensor. The PCM will pulse the wastegate solenoid “OFF,” allowing it to vent and open the actuator to bleed off boost pressure, thereby preventing overboost.
If an overboost condition exists, the PCM will reduce fuel delivery to prevent engine damage.
Wastegate Solenoid Circuit Operation
The wastegate solenoid receives power through the ignition switch on CKT 539. The PCM controls solenoid operation by providing a ground at PCM terminal A3 for CKT 972 (figure 5-46).
DTC 78 — “Wastegate Solenoid Fault”
DTC 78 sets when engine RPM is greater than 1800, fuel rate is greater than 20 cubic millimeters and boost pressure is greater or lesser than the desired amount as determined by the PCM’s internal setting.