http://www.youtube.com/watch?v=Gc2awh0O0Bc
http://www.youtube.com/watch?v=QdqMmB3hxTM&feature=related
http://www.youtube.com/watch?v=Qu0UGKS7-JQ&feature=related
http://www.youtube.com/watch?v=CbdY1rymBJg&feature=related
Not sure if you're asking
1: how the electrical signals from the ECU are turned into movement of the turbo vanes, or
2: how moveable vanes in the turbine gas flow control boost.
1) The ECU signal is turned into a variable depression, a variable amount of vacuum, or to put it another way a pneumatic pressure between low single digit PSI and the 14 PSI of atmospheric pressure. The little valves on the firewall (N75, N18 etc) have two air inputs, one at atmospheric pressure from the air cleaner and the other from the vacuum pump, and one variable-pressure output, which is generated by the valve cycling between the two input pressures. So the output is a variable amount of suck. This acutates the vane mechanism by means of a diaphragm attached to the control rod, as shown in the first video.
2) The vanes are aerodynamically shaped so as not to disturb the gas flow, and move so as to change the effective size of the turbine input, between the exhaust manifold and the turbine blades.
At low engine rpm (and thus low exhaust gas volumes), reducing the size of the inlet leads to an increase in the speed of the gas directed against the turbo blades, which spins them faster. Hence at low rpm the turbine is made to spin faster and produce more boost.
As engine rpm increases, exhaust gas volumes increase and the ECU opens up the vanes to keep the turbine producing maximum boost. At max rpm, the vanes are fully open. These turbos do not have a wastegate, all boost regulation is done by the vanes.
So, if the vanes stick as you increase rpm, then the gas speed over the turbine blades increases further, spinning the turbo faster and producing overboost.
The vanes' rest position is fully open, so a completely failed or disconnected turbo produces minimum boost at all rpm below maximum. As the engine is fired up and vacuum produced, the ECU commands the vanes to move to the minimum-gap position, to be as closed-up as they can get. So simply moving the vanes isn't the answer to defeating sticking problems - they move from min to max every time you start.
The non-invasive method of cleaning soot out of the exhaust turbine is to make it get very hot, normally referred to as the "Italian tune-up". The best description for its application to our
problem was given to me as running in 3rd or 4th gear at high revs for 3-5 minutes, uphill if possible. This allows you to keep within the speed limit while giving the turbo maximum load and subjecting it to the largest possible flow of hot exhaust gas. The aim being to burn the soot away.
The exhaust in these circumstances will have as little soot as you can get. Most of the soot is produced by the EGR action and EGR is shut off at full chat.
Hope that helps
