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
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