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.

The middle outlet (marked "W") is connected to the wastegate, the top ("C") goes to the compressor housing of the turbo, and the bottom ("R") to the intake. The boost is adjusted by alternatively supplying the wastegate actuator with the pressure from the turbo or venting it to the intake, thus allowing the wastegate to stay closed at pressure levels exceeding the base boost.

The valve contains two solenoid coils. Sometimes these coils burn out, so testing them is always a good first step. It's easy to do right on the car, just unplug the connector. The resistance for each coil should be 3 Ohm (+/- 1 Ohm) between the middle and either of the outer pins. The combined resistance of both should be about 6 Ohm between the outer pins.

The valve is glued together, you can still see traces of adhesive in the seam. You can scrape it off if you're patient. I just used a hack saw.

Inside, you will find the solenoid pair with fragile wires leading to the harness connector. The top and bottom receiving outlets are sealed with rubber O-rings.

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.

In this BPC, the membrane cracked from stress. A piece of it was jammed on the pressure side of the valve, keeping it open. As the result, the wastegate actuator was permanently connected to the compressor, so the wastegate always opened when it reached the base boost.

The remaining part of the membrane was still large enough to cover the valve opening, so I decided to give it a try. The BPC was assembled, glued back together and sealed with automotive adhesive.

After the sealant dried up, I installed the valve on the car and found it fully functional! This proves that at least some BPCs can be repaired.

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