Overboost Relay: Description and Operation

The turbocharger boost control system is designed to provide both mechanical and electronic variable boost control. The boost control system provides regulation of the wastegate allowing turbo boost pressures up to a maximum of 60 kPa (8.7 psi). The system includes the actuator, turbo boost control solenoid valve, knock sensor, knock control unit, and overboost warning system. The electronic boost control part of the system is controlled by the Electronic Control Assembly (ECA). A warning buzzer is provided to warn the driver of an overboost condition. When the turbocharger boost pressure (calculated by intake air amount and engine speed by the ECA) reaches a predetermined level, the warning buzzer will sound, indicating to the driver that an overboost condition exists. At the same time, to prevent damage to engine components, the ECA will execute the fuel cut-off operation and stop fuel injection. The warning buzzer indicates a possible fault of either the wastegate, actuator, or turbo boost control solenoid valve. These components must be checked if the buzzer sounds.


MEDIUM LOAD

Under medium load conditions the amount of exhaust gas increases and intake air is pressurized. The wastegate remains closed at this time because intake air pressure is not great enough to overcome the actuator spring constant of 5.4 N-m (48 lb. in.).

Boost Control System:

HIGH SPEED/HEAVY LOAD

During high speed/heavy load conditions the amount of exhaust gas increases and the intake air pressure also increases. When intake air pressure at the compressor housing reaches 60 kPa (8.7 psi), the pressure will act on the actuator diaphragm and will overcome the force of the spring within the actuator, and the wastegate will open the bypass passage. As a result, the flow of exhaust gas supplied to the turbine wheel housing will drop, and the rpm of the turbine wheel will decrease. It is in this way that the boost pressure is regulated to the maximum pressure.