The APC Solenoid Valve
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The solenoid valve regulates the charge air pressure. It is connected in the line from the turbo to the waste gate. The inlet port of the solenoid valve contains a fixed diameter restriction. The waste gate actuator is connected down stream of this restriction. The diameter of the outlet port of the solenoid is governed by the frequency and duty cycle that the valve is oscillating at.   When energized, the solenoid valve oscillates between the open and closed positions at a fixed frequency of 12hz (12 times per second).  If the boost pressure rises above 6 psi (.4bar), the pulse width of the valve will change.  The pulse width is the relationship between the amount of time that the solenoid is open and the amount of time that the solenoid is closed within that 1/12 of a second cycle.

When the valve is fully open (energized), pressure is evacuated through the "R" port, to the inlet side of the compressor. The restriction in port "C" is so narrow in comparison with the valve opening that there is a total pressure drop in the small quantity of air flowing through the restriction, thus preventing the waste gate from actuating.

When the solenoid valve is fully closed (de-energized), the waste gate actuator is connected to the full pressure present from the compressed charge air. Since the R port is closed off, the pressure to the waste gate is no longer allowed to dump to the inlet side of the turbo compressor. Therefore, the waste gate is allowed to open and the boost is reduced to the basic setting. The "Basic boost setting" is that which the waste gate would normally open at without any intervention from the APC.
The routing of the solenoid hoses is quite simple. As you're looking at the solenoid with the ports facing you, The port on the left goes to the intake upstream of the turbo and downstream of the fuel metering device (air mass meter, or the air flow plate). On my 740, there is a very expensive formed rubber hose that goes from the air mass meter to the intake of the turbo. There was no good way to tap into this hose, but I was able to build a T fitting (See Photos) in the line from the crank case ventilation to this hose. The center port with the restriction in it goes to the fitting on the side of the turbine housing of the turbo. On my turbo, there was a 90 degree elbow on it which I rotated to the necessary position for hookup to the solenoid. The port on the right goes to the waste gate actuator.

This is a picture of the solenoid valve mounted on the inner fender in front of the coolant reservoir, and behind the air filter housing..

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