Audi Workshop Service and Repair Manuals

In order to limit the boost pressure, a valve flap is opened by a pressure unit at the exhaust gas turbine. This can be used as required to allow some of the exhaust gases to bypass the exhaust gas turbine. This reduces the power output of the turbine and therefore also the delivery of the compressor.

The pressure unit on the turbocharger is actuated via a control line using tapped air from the compressor. The control pressure needs to be reduced in order to increase the boost pressure.

The pressure in the control line is reduced via boost pressure control solenoid valve -N75, which releases pressure from the control line and therefore reduces the control pressure (in order to minimise noise the air is directed into the large intake hose). Due to the low control pressure the exhaust gas valve flap remains closed for longer.

When there is no voltage on the valve, the full control pressure immediately becomes effective at the pressure unit and the boost pressure is governed down to a minimum value.

In order to lower the control pressure (with the aim of increasing the boost pressure) it is necessary for the solenoid valve to open further. This requires a current increase. This change in current is achieved by increasing the duty cycle.