N75 explained

[BigEd]

Perhaps I was the only one on AW not understanding N75's integration into wastegate control and, more importantly, how to interpret group 114/115 logs and see if the wastegate is operating at its optimum. Not anymore. Hope the explanation below helps you too. Here goes:

The N75 is the valve that plays a large part in regulating the wastegate. An unenergized solenoid (0% duty cycle) permits full charge pressure on actuator diaphragm. So the turbocharger naturally regulates itself. As boost builds, the wastegate opens. The ECU can operate the N75 to vent some of this pressure so the wastegate either does not open, or opens less. For example if reading is 62.8% duty cycle it means over a 100 millisecond period (1/10 of a second) the charge pressure in the actuator line was filled for 37.2 milliseconds and vented for 62.8 milliseconds. An orifice in the charge pressure connection restricts the flow rate of charge air into the actuator system. If you are seeing a large difference between the two reading off block 115 (request high, actual low), and you see that duty cycle on the N75 is high (90-100% range), then you are probably leaking boost from a pressure hose (turbo outlet to intake manifold) or the diverter valve (DV). You should then check all clamps and hoses along the whole pressured tract as well as swap DVs.

So, when we look at, let's say, log 115 and see that actual boost is less than requested boost while ECU is yelling "somebody close that f%cking wastegate!" (N75 duty cycle close to 99%), we know there is a problem: either there is a boost leak, or the wastegate is not closing all the way (stuck).

• link