Vacuum system and control pneumatics in the TDI (Articles)

The tax regulations concerning pneumatic systems related to VAG-TDI engines.

In TDI engines, the turbocharger control, the AGR, and the intake manifold flap, used to reduce vibrations when the engine is stopped, are typically pneumatically actuated. In newer pump-injector and common-rail TDIs, the idle control valve/throttle valve and the AGR valve are usually electrically actuated. These swirl flaps (which, in engines with four valves, close off an intake channel for exhaust-related reasons) are usually also actuated pneumatically.

The tangle of thin tubes in the engine compartment can be daunting when troubleshooting, although the functionality of the individual systems is always the same for each engine generation.

Detailed hose diagrams for each engine would exceed the scope of this presentation. Therefore, the following images show the general structure of the common hose systems, which is the same for all engines.
Only some newer engines (mainly PD engines) no longer have a hose routing between the air filter and the VTG (Variable Turbine Geometry). AGR (Abgasrückführung) solenoid valves, in which the external pressure tap is located on the respective solenoid valve.

Depending on the engine, not all systems are present (e.g., the shut-off valve), or other combinations can be found (e.g., a wastegate engine with EGR). Nevertheless, when troubleshooting, you can refer to the relevant diagram and error table that describes the specific system in question.

The depiction of the simple hose from the intake manifold to the boost pressure sensor (as a separate part or within the MSG) has been omitted in older engine models.

The regulated wastegate turbocharger (e.g., in the 1Z, AHU, AAT, AEL engines) has a conventional diaphragm system with a spring that opens the wastegate at approximately 0.6 bar of boost pressure, preventing a further increase in boost pressure.
The membrane of the pressure sensor is connected to the compressor outlet of the heat pump via a control line.
The higher boost pressure required for full performance is achieved by variably mixing the boost pressure supplied to the membrane with the external pressure (air pressure) via a solenoid valve in the hose system. This lower control pressure allows the wastegate to open at boost pressures above 0.6 bar, thereby enabling boost pressures greater than 0.6 bar. The "excess" air from the wastegate valve is returned to the intake manifold via a hose.

The monitoring of the boost pressure using an electronic pressure sensor and the control of the solenoid valve for boost pressure correction are performed by the engine control unit (ECU). Active control at a boost pressure below 0.6 bar is not possible.

In case of incorrect charging pressure, the charger inlet can be directly connected to the charger membrane sensor for problem identification.
If the boost pressure is correctly limited to approximately 0.6 bar, the fault lies within the pressure sensor, MSG (Motor Steuergerät - Engine Control Unit), solenoid valve, hoses, or wiring. In the resting state, the wastegate is closed, see the image in the topic

.

The type of turbocharger can be identified by the hose routing: If one of the hoses from the boost pressure solenoid valve leads towards the turbocharger outlet (or the intake manifold between the turbocharger and the engine), then it is a wastegate turbocharger. Führt ein Schlauch stattdessen (über T-Stücke usw.) zur Unterdruckpumpe, so ist ein VTG-Lader verbaut.

The VTG turbocharger system

, which uses a membrane dose controlled by vacuum, is not stable and cannot operate reliably without constant electronic control, unlike a wastegate turbocharger.

Der Ladedruck wird vom MSG ständig überwacht, mit dem Sollwert laut Kennfeld verglichen und durch Ansteuern eines Magnetventils korrigiert.
This mixes
the pressure coming from the vacuum pump with the outside pressure (the connection is usually located near the air filter), which allows for a continuous adjustment of the VTG (Variable Turbine Geometry) mechanism: The stronger the vacuum at the diaphragm, the more boost pressure is generated, or the start of pressure build-up is shifted to lower engine speeds.

Without vacuum at the diaphragm, no boost pressure is generated.
Für eine präzise Ladedruckregelung muß der Unterdruck am Eingang des Magnetventils möglichst konstant sein. Since vacuum pumps generate pulsations, the vacuum is stabilized by means of

a check valve and a vacuum reservoir, which is located on a hose branch behind the check valve, as viewed from the pump.

Die unterdruckbetätigte AGR-Membrandose am Ansaugkrümmer bewegt das AGR-Ventil. Without vacuum, the valve is closed.

The vacuum for the stepless actuation of the AGR (Abgasrückführung) is mixed from intake and outside pressure, similar to the VTG (Variable Turbine Geometry) turbocharger, using a solenoid valve.

The constant monitoring and correction of the AGR rate, achieved by controlling the solenoid valve, is performed by the MSG (Motor Steuergerät) based on the mass airflow sensor (MAF) signal.

In some engines, the EGR pneumatic system is also supplied from the vacuum reservoir instead of directly from the pump.

AHF, ALH, ASV, and similar generation TDIs have a vacuum-operated throttle valve that closes when the engine is turned off. This reduces vibrations during engine shutdown by decreasing compression.

Since the throttle valve only needs to be fully open or fully closed, its diaphragm requires no continuous pressure control. When parking, a simple solenoid valve is activated, which connects the vacuum supply to the diaphragm chamber.
Hose routing for the pneumatic control systems in the various engines

Most engines have separate solenoid valves for VTG and EGR. Da beide Ventile an ihren Eingängen jeweils Unter- und Außendruck benötigen, verzweigen sich die Schlauchführungen von der Unterdruckpumpe bzw. dem Außendruckabgriff über T-Stücke zu den Ventilen.

In the VTG vacuum system, in addition to the above-mentioned, there is also... Check valve and vacuum reservoir.

The branch for the shut-off valve is supplied from the hoses to the vacuum pump.
The hose connections of the solenoid valves are usually marked: for example, ATM must be connected to the external pressure port, VAC to the vacuum pump or vacuum reservoir, and OUT
to the diaphragm of the component to be controlled.

The latter hoses are never branched; if they are, it indicates a mounting error that can lead to all sorts of malfunctions.

Although the basic principle of VTG and AGR control is the same, many engines use different solenoid valves, which can be identified by the part number and sometimes also by the color.

The valves differ, among other things, in their internal flow cross-sections. If the valves are swapped, the systems will remain fundamentally functional, but the response and control characteristics will be altered.

Some (mainly newer) engines have several solenoid valves grouped together in a block, but the functionality of each individual system is always as described above. In some cases, only one external and one internal pressure line connect to the valve block, with the necessary branch connections integrated within the block itself.

The functions of the individual valves can be determined by examining the hoses connected to their outlets: the line from the charge air pressure sensor leads directly to the charge pressure solenoid valve, etc.

In some engines, the boost pressure sensor is located in the intake manifold (identified by having at least 3 electrical connections), while in others, it is located separately in the MSG, or it is designed as a separate component.

In the case of sensors that are located separately, a hose always leads from the sensor to the intake manifold. Intake manifold.
The AGR (Abgasrückführung) and VTG/Wastegate can be tested for functionality using the VCDS automotive diagnostic software from Dieselschrauber Shop.

To do this, use VCDS to access the engine control unit and select the basic settings in measuring block 3 or 11. For newer engines, you can use the selective actuator diagnosis for testing. The actuators are alternately driven to their minimum and maximum positions.
Typical error symptoms in the field of control pneumatics (only applicable to the EGR system in older TDIs)


The following error tables assume that only the specifically mentioned error is present. With multiple simultaneous errors, other overall symptoms may occur.When kinked

, a complete blockage of the relevant component is assumed. A piece of the hose came loose.
The statements regarding the boost pressure refer to full load at approximately 2000 min-1

.

Engines with wastegate turbocharger (AGR faults as described under VTG + AGR)
Component	Fault	Possible error symptoms / consequences
Boost pressure valve	Malfunctions / hoses reversed	too low (min. 0.6 bar) or too much boost pressure
Boost pressure valve	Wiring fault	KER}Boost pressure limited to approximately 0.6 bar
Hose between the boost pressure valve and the turbocharger outlet	is leaking, kinked, or detached	Boost pressure limited to approximately 0.6 bar
Hose between the boost pressure valve and the turbocharger inlet	leaking or detached	Turbocharger and engine damage due to sucked-in dirt
Hose between the boost pressure valve and the turbocharger inlet	kinked	Boost pressure too high or too low
Hose between the boost pressure valve and the turbocharger reservoir	leaking, kinked, or detached	Boost pressure too lowKER}
Charging valve	leaking	Charging pressure too low
Charging valve	internal spring broken	Charging pressure possibly too low
Hose from the intake manifold / suction pipe to the MSG, respectively. Pressure sensor	leaking, bent, or detached	charging pressure too high
Pressure sensor	defective	too low (min. 0.6 bar) or too high boost pressure

leaking, kinked, or detachedleakinginner spring brokenis clogged or installed backwardsfully permeablehave come loose or are leakingis kinkedis leakingbentleaking, bent, or disconnecteddefective

Engines with VTG or EGR
Component	Fault	Possible error symptoms / consequences
Y-hose to the vacuum pump	leaking, kinked, or detached	No or insufficient boost pressure and / or the EGR does not open or not enough
Y-hose to the air filter	leaking or detached	Malfunctions in the charging system or EGR system due to dirt being sucked in
Y-hose to the air filter	bent	Charging pressure too high and/or EGR constantly open
Solenoid valves	Malfunctions / hoses swapped	No, too little, or too much charging pressure, EGR does not open or opens for too long
Solenoid valves	Cable break	No charging pressure, EGR valve does not open
Hose to the EGR valve	leaking, kinked, or detached	EGR valve does not open or opens too little, malfunctions due to sucked-in dirt
EGR valve housing	leaking	EGR valve does not open or opens too little, malfunctions due to sucked-in dirt
EGR valve housing	R}inner spring broken	The AGR valve may open too early or for too long
Hose to the charging valve		no or insufficient charging pressure, malfunctions due to sucked-in dirt
Charging valve		no or insufficient charging pressure, malfunctions due to sucked-in dirt
Charging valve		possibly due to excessive pressure
Check valve		no boost pressure
Check valve		Problems with turbocharger pressure regulation
Hoses between the check valve, solenoid valve, and vacuum reservoir		resulting in insufficient or no boost pressure, malfunctions due to ingested dirt
Hose to the vacuum reservoir		leading to boost pressure regulation problems
Vacuum reservoir		resulting in insufficient or no boost pressure, malfunctions.ngen caused by sucked-in dirt
Hoses between the check valve and the solenoid valve		no boost pressure
Hose from the intake manifold / intake pipe to the MSG or pressure sensor		boost pressure too high
Pressure sensor		too little or too much boost pressureHE}

Thank you to Michael II for his support!

https://community.dieselschrauber.org/en/viewtopic.php?t=15788