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The solenoid unit has two electromagnets with a membrane in the middle.
The ECU pulses the current at 90Hz (or 70 Hz above 2500 RPM), alternating
between the coils and pulling the membrane to one side or another. This
allows for precise control of boost pressure.
The boost pressure control valve (BPC), also known as the APC
solenoid valve, is used on most Saab engines to control the turbo
charging pressure. The valves on Trionic5 models are often prone to
failure. In some cases it just needs a good cleaning, but sometimes it
becomes permanently stuck in one position, causing the car to either stay
on base boost, or go into overboost until it hits the fuel cut. This
article provides an insight on what could be casing these permanent faliures.