Overboost Relay: Description and Operation

BOOST PRESSURE CONTROL
The boost pressure control system consists of a wastegate valve and a wastegate actuator. The actuator, which is controlled by turbo boost pressure, controls the wastegate valve, which opens and closes the exhaust gas bypass passage.
The amount of turbocharger boost is limited to a maximum of 8.1 psi by the wastegate and actuator. Under normal to moderate loads, the wastegate valve is closed and the intake air pressure changes in accordance with the engine rpm and the amount of exhaust gas. Under heavy loads, when the intake air pressure in the air inlet duct reaches 8.1 psi the pressure acts on the diaphragm and overcomes the force of the spring within the actuator, and the wastegate valve opens the bypass passage. As a result, the flow of exhaust gas applied to the turbine wheel drops, the rpm of the turbine wheel drops, and the boost pressure drops accordingly.

OVERBOOST PROTECTION
To prevent engine damage the fuel injection will be cut off when the pressure in the intake manifold reaches 11.0 psi (77 kPa), and the calculated intake manifold presure reaches a predetermined level. The calculated pressure is determined by the ECA from the engine speed and the amount of intake air.