Controlling turbocharger (TC) boost pressure
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When the Engine Control Module (ECM) determines that a higher boost pressure is permissible, the turbocharger (TC) control valve opens further and a proportion of the pressure acting on the Boost Pressure Control (BPC) valve pressure servos is allowed through to the turbocharger (TC) inlets. In this way the control pressure is reduced, the Boost Pressure Control (BPC) valves open later and turbocharger (TC) pressure can increase.
The Engine Control Module (ECM) affects the turbocharger (TC) control valve by grounding one of the terminals with a fixed frequency, where the signals duty cycle determines how much the valve should open and therefore how much the boost pressure can increase.