VW PASSAT B5 TDI TURBO CONTROL
VAG VNT TURBO CONTROL
Normal boost conditions
1. The vacuum pump pulls the turbo actuator over 20 mm on start up. (The N75 valve does nothing).
2. Exhaust gases spin the turbocharger.
3. Air from the Air filter is pressurised by the turbocharger, cooled in the Intercooler and passed to the Intake
4. The MAP sensor tells the ECU what the boost pressure is.
If the Vacuum pump fails it will be obvious because you will lose servo effect on the brakes as well as dropping into limp mode.
If the N75 valve, Turbo Actuator, Vacuum pipes or Turbocharger fail, the car will drop into limp mode and flag a fault code as long as the MAP sensor detects too high or too low boost pressure.
If the MAP sensor fails the car will drop into limp mode and flag a fault code only if the reading is too high, too low or highly unlikely. If the reading is not wrong enough, the ECU won’t drop into limp mode or flag a fault code. The car may feel different to drive.
Under boost Code 17964
1. The Turbocharger vanes are stuck in the low pressure position. (Not very likely)
2. The N75 valve is stuck open. (Not very likely)
3. The turbo actuator has broken its connection to the turbocharger. (This does happen)
4. The turbo actuator has failed. (Likely)
5. A Vacuum pipe is holed, split or detached. (very likely)
6. The MAP sensor is faulty (Likely).
Over Boost Code 17965
1. The Turbocharger vanes are stuck in the high pressure position. (Very likely)
2. The N75 valve is stuck shut. (Not very likely)
3. The N75 to Air Filter pipe is blocked. (Not very likely)
4. The MAP sensor is faulty (Likely).