P1556

P1556
Charge Pressure Deviation - Below lower limit

P1557
Charge Pressure Deviation - Upper limit exceeded

Function
The charge pressure regulation adjusts the throttle valve angle and the engine speed to a specified value. The Mass Air Flow (MAF) Sensor records the actual value. The signal calculated this way serves as a control value and is compared with the specified value. If a deviation exists it will be compensated.
The adjustment value control is used as a plus range modulated signal to trigger an electro-pneumatic valve (frequency valve).
The frequency valve varies the pressure in the membrane boxes for the waste gates between charge pressure and ambient pressure until the required waste gate position has reached the desired specified value.

Diagnostic conditions
- Throttle valve angle larger than 35°
- Engine speed higher than 2800 1/min
- Engine temperature higher than 126°C
- Boost pressure regulation active





Diagnostic procedure
Check intake air system for leaks.
1. Remove charge air cooler and air filter unit.
2. Remove mass air flow sensor.
3. Remove protective grid on the right.








4. Attach sealing plate 9264/1 to mass air flow sensor using M 4 x 40 screws.

NOTE: Before final tightening of the screws, the position of the mass air flow sensor on the sealing plate must be checked, i.e. no gaps must be detected during a visual inspection.





5. On the rear, right-hand wheel housing, remove the front wheel housing liner half. Seal the hose running from the oil tank to the pressure relief valve using an appropriate plug.
6. Generate an excess pressure of approx. 0.5 bar. Major leaks can be detected by the sound of air being released, whereas minor leaks can be detected using a leak indicator spray.

NOTE: The leak test includes the complete intake air system, the pressure section of the charge air cooler and the oil tank.




Check and adjust waste gates of turbo charger.
1. Remove cover frame of boost air cooler.
2. Remove hose between frequency valve and throttle body from throttle body.
3. Connect special tool 9160/1 to removed hose.








4. Remove hose between frequency valve and throttle body from throttle body.
5. Attach dial gauge and dial gauge support VW 387 to turbo charger deflector plate (e.g. using a screw clamp).
- Place sensor pin on safety bracket.
- Use special tool 9160/1 to generate an excess pressure of 0.5 bar.
- The push rod must be lifted by 4 ± 0.5 mm; otherwise adjust lift by turning adjustment screws.

Turning clockwise lift decreases
Turning anti-clockwise lift increases