APC Function

Having done a little research I think I now understand the working of the wastegate and APC.

Wastegates allow exhaust gas to bypass the turbo. Without a wastegate opening a positive feedback loop is formed which should be able to destroy an engine.

Wastegates are opened by an actuator. This is connected by some vacuum hose to the compressor side of the turbo. When the compressor reaches a certain pressure the wastegate opens. The wastegates in Saabs are set at about 0.5 bar and this is known as base boost.

Saab has a more sophisticated system using a number of sensors and the APC solenoid valve to allow greater boost pressures than this base boost, and to vary them according to a number of factors such a fuel quality.

The APC valve has 3 pressure connectors, one to the compressor (C), one to the wastegate (W) and one to atmosphere (R). It also has 3 electrical pins connected to the ECU. These operate an induction loop which moves a piston inside the valve.

If there is no electrical signal, air is allowed to flow freely from C to R, effectivly giving base boost performance.

I believe that after some period of normal operation my valve jammed here for a while.

During Friday’s testing, after I disconnected and reconnected the valve it jammed in such a position that the C port is sealed and now air moves freely from W to R. I have tested this by blowing through the various connectors.

This is a dangerous setting, giving me silly levels of boost but at great risk to the car, as I discovered.

I have now ordered a replacement and am returning the failed valve. I also decided that a new Forge dump valve and a 3 gauge din mount would be a good idea…

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