Bosch Motronic - Component Datasheet

Wastegate Frequency Valve (N75)

Arguably the least understood component on Audi's turbo engines... This innocuous looking fellow is also one of the most important devices in the engine bay as it is largely responsible for boost pressure regulation. The function of the wastegate frequency valve is to control the rate at which boost pressure is developed. This is done by opening the wastegate and letting exhaust gas bypass the turbocharger - thus reducing boost. It is located near the turbocharger with tubes connecting to the wastegate, the unboosted airbox and the output tract of the turbo itself as per the diagram below. At full load the 3B permits a maximum system pressure of 1.83bar (0.83 bar boost) just below 2000rpm. No mean feat...

The figure above shows the the wastegate firmly shut. In this instance, the exhaust gas is all forced through the turbine side of the turbo and boost pressure is rapidly created by the connected compressor side of the turbo.  The wastegate frequency valve (N75) is pulsed on and off by a DC duty cycle that is computed by the ECU. You can think of N75 as a kind of switch-valve that decides when boosted or unboosted air pressure should be presented to the wastegate. When excessive boost pressure is permitted to enter the wastegate, it acts against the spring loaded valve to allow exhaust gas to bypass the turbo.

In the event of failure (or disconnection of N75), then responsibility for boost pressure regulation is delegated to a mechanical safety feature of the wastegate which operates at lower boost pressure than N75 permits.

It is quite easy to see how the many chip vendors for the S2 can happily get these 20V turbo engines to generate higher boost and power by simply messing with the duty cycle on N75 - letting the wastegate stay closed for longer times so that the turbo can contine to spin and generate higher boost levels.

When N75 is in the OFF state (or the ECU connector is disconnected) the pneumatic connection is such that boost pressure is connected to N75's output (the wastegate control input). When N75 is in the ON state (with 12V across the solenoid) the pneumatic connection is such that UNboosted air is steered to the wastegate control input. It is the 'modulation' or rapid ON-OFF-ON switching of N75 which determines the control of the wastegate valve.