{SYSTEM NAME} Pressure Control System and Pneumatics on TDI (Articles)

The pneumatic control systems around the VAG-TDI-engines

In TDI engines, the turbocharger control, the AGR and the intake valve are primarily pneumatically operated to reduce vibrations when the engine is stopped. In newer pump-nozzle and common-rail TDIs, the shut-off valve/throttle valve and the AGR valve are typically electrically operated. These are the control valves (which, in most 4-valve engines, close an intake channel for exhaust reasons) and are typically pneumatically operated.

The tangled mess of thin hoses in the engine compartment can be intimidating when troubleshooting, even though the functionality of individual systems is always the same for each engine generation.

Detailed hose diagrams for each engine would exceed the scope of the illustration. Therefore, the following images show the basic structure of common hose systems, which is the same for all engines.

Only some new (mainly PD) engines no longer have a hose routing between the air filter and the VTG (Valve Timing Control) AGR (Engine Coolant Return) Magnetic Valves, where the external pressure return port is located on the specific magnetic valve.

Depending on the engine, not all systems are present (e.g., the parking flap) or other combinations (e.g., a Wastegate engine with AGR) can be found. Nevertheless, one can refer to the diagram and the error table, in which the relevant system is described, when troubleshooting.

Instead, the simple hose from the intake to the boost pressure sensor (as a separate part or integrated in the MAP sensor) used in older engines has been omitted.

The regulated wastegate diverter (e.g., in 1Z, AHU, AAT, AEL) has a conventional diaphragm-type unit with a spring, which opens the wastegate at approximately 0.6 bar of boost pressure and prevents further increase in boost pressure.

To this end, the membrane of the pressure vessel is connected to the outlet of the TL compressor via a control line.
The higher boost pressure required for full performance is achieved when, in addition, the boost pressure supplied to the membrane is varied by mixing it with the external pressure (air pressure) via a solenoid valve in the hose system. This lower control pressure allows the wastegate to function at boost pressures above 0.6 bar, and thus enables boost pressures > 0.6 bar. The "excess" air from the "charge pressure regulation valve" is returned to the air intake via a hose.

The charging pressure is monitored using an electronic pressure sensor, and the charging pressure correction is controlled by the engine control unit (ECU). An active regulation at a boost pressure of less than 0.6 bar is not possible.

If the boost pressure is incorrect, the boost inlet can be directly connected to the boost solenoid housing for troubleshooting purposes.
Will the boost pressure then be cleanly limited to around 0.6 bar? Is the error in the range of the pressure sensor / MSG / solenoid valve / hoses / cables? In the idle state, the wastegate is closed, see the image in the topic /viewtopic.php?t=15788

Identifying the type of charger is possible by examining the hose routing: If one of the hoses leads from the charger's solenoid valve towards the loading outlet (or the intake pipe between the charger and the engine), it is a wastegate charger. Instead, a hose leads to the vacuum pump (via T-pieces, etc.), in which case a VTG charger is installed.

The VTG-charger system, which uses a membrane box operating under vacuum, is, unlike a wastegate-charger system, not permanently stable and operable without constant electronic control.

The boost pressure is constantly monitored by the MSG, compared to the target value according to the characteristic curve, and corrected by actuating a solenoid valve.

This mixes the pressure coming from the vacuum pump with the external pressure (the outlet is usually in the range of the air filter), which allows for continuous adjustment of the VTG mechanism: The stronger the vacuum at the membrane housing, the more boost pressure is built up or the initial pressure build-up is shifted to lower engine speeds.
Without vacuum at the membrane housing, no boost pressure is built up.

To achieve precise charging pressure control, the vacuum at the inlet of the solenoid valve must be as constant as possible. Since vacuum pumps generate pulsations, the vacuum is regulated using a check valve and a vacuum storage , which is located behind the check valve on a pipe branch, as seen from the pump.

The vacuum-operated AGR membrane housing on the intake manifold moves the AGR valve.

Without vacuum, the valve is closed.

The underpressure for the stepless operation of the AGR is mixed from under- and overpressure, similar to the VTG loader.
The constant monitoring and correction of the AGR rate through the appropriate control of the solenoid valve is carried out by the MSG based on the LMM signal.

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

TDIs of the AHF, ALH, ASV, etc. generations have a vacuum-operated suction valve that closes when the engine is stopped. The resulting reduction in compression reduces vibrations during engine startup.
Since the suction valve only needs to be fully open or closed, no continuous pressure control is required for its diaphragm. When shutting down, a correspondingly simple solenoid valve is activated, which switches the vacuum to the membrane chamber.

Pneumatic control system hose routing for various engines
Most engines have separate solenoid valves for VTG and AGR.
Since both valves require both a low-pressure and a high-pressure supply at their respective inlets, the tubing runs from the low-pressure pump and the high-pressure outlet branch off via T-pieces to the valves.

In the VTG undercarriage system, in addition to the aforementioned, there is also... Backflow preventer and the vacuum accumulator.

The branch for the stowage flap is supplied with air from the vacuum pump hoses.

The connections of the solenoid valves are usually marked: for example, ATM must be connected to the external pressure pickup, VAC to the vacuum pump or vacuum reservoir, and OUT to the membrane chamber of the part to be controlled.
The latter hoses are never branched - if they are, there is a manufacturing defect 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 color.

The valves differ from each other, among other things, in their internal flow cross-sections. If you swap the valves, the systems will still function in principle, but the responsiveness and control behavior will be changed.

Some (primarily newer) engines have multiple solenoid valves combined into a single block, but the operation of each individual system is always as described above. Sometimes, only one external and one internal line lead into the valve block. The necessary branches are then integrated into the block.

The functions of each valve can be determined by examining the connections at their outlets: the line from the compressor diaphragm valve leads directly to the boost solenoid valve, etc.

In some engines, the boost pressure sensor is located in the intake manifold (with at least 3 electrical connections), while in others it is mounted separately in the MSG, or it is designed as a separate component.
In the case of separate sensors, a hose always leads from the sensor to the intake manifold. Intake manifold.

AGR and VTG/Wastegate can be tested for functionality using the VCDS automotive diagnostic software from the Dieselschrauber Shop. To do this, you use VCDS to access the engine control unit and select the basic settings in block 3 or 11, and for newer engines, you can use the selective actuator diagnostic function to test.

The actuator arms are then alternately driven to minimal and maximal positions.

Typical failure symptoms in the area of the throttle valve (only applicable to older TDIs for the AGR)
The following fault tables assume that only the specified fault exists. If several simultaneous faults occur, other overall symptoms may arise.
If the
valve is completely blocked, this results in a complete closure of the relevant... Couplings have been dispatched.

The statements regarding the charging pressure refer to full load over approximately 2000 minutes^-1.

Inlet Manifold Pressure Valve and Intake Port
internal spring brokenCharger

Engines with Wastegate Turbochargers (AGR issues as described under VTG + AGR)
Component	Fault	Possible Symptoms / Consequences
Boost Pressure Valve	Malfunctions / Switched Hoses	Too little (min. 0.6 bar) or too much boost pressure
Boost Pressure Valvefont>	Fuel Line Disconnection	Fuel Pressure Limited to Approximately 0.6 bar
Hose between Fuel Pressure Valve and Fuel Outlet	leaky, kinked, or detached	Fuel Pressure Limited to Approximately 0.6 bar
Inlet Manifold Pressure Valve and Intake Port Leak or Disconnected	Damage to Intake and Engine due to Dirt
Kinked	High or Low Intake Pressure	Inlet Manifold Pressure Valve and Intake
Leak, Kinked or Disconnected	Low Intake	drig
Charger Charger leaky	Charger low pressure	Charger
low pressure possible	Hose from intake manifold / intake pipe to MSG bzw.
Pressure sensor	leaking, bent or detached	Charging pressure too high
Pressure sensor defective due to too low (min. 0.6 bar) or too high charging pressure

Fault
Backflow Valve
Backflow Valve
Hoses between the backflow valve, solenoid valve, and vacuum accumulatorVacuum accumulator

Variable Valve Timing (VTG) or AGR (Exhaust Gas Recirculation) engines Component
Possible Symptoms / Consequences	Y-hose to the vacuum pump, leaking, kinked or detached	No or too little boost pressure and/or AGR does not open or closes too much
Y-hose to the air filter
ter	leaky or faulty	Functional problems with charging system or AGR due to dirt sucked in
Y-pipe to air filter	kinked	High boost pressure and / or AGR constantly open
Magnetic valves	Functional problems / hoses mixed up	No, too much or too little boost pressure, AGR does not open or stays open for too long
Magnet valves	Cable break	No boost pressure, AGR does not open
Hose to AGR valve	Leaking, bent or detached	AGR does not open or opens too little, malfunctions due to sucked in dirt
AGR valve	Leaking	AGR does not open or opens too little, malfunctions due to sucked in dirt
AGR Dose internal spring broken AGR may open too early or too long
		{d>
Charger	internal spring broken	possibly too high charging pressure
Blocked or installed incorrectly No boost pressure
fully permeable Problems with charging pressure regulation
have fallen off or are leaking		lack of or insufficient boost pressure, malfunctions due to sucked-in dirt
Hose to the vacuum accumulator	Boost pressure regulation problems
	lack of or insufficient boost pressure, malfunctions	Disturbances caused by sucked-in dirt
Hoses between check valve and solenoid valve	kinked	no boost
Hose from intake manifold / intake pipe to MSG or pressure sensor	leaking, kinked or detached	high boost
Pressure sensor	defective	too much or too little boostnt>

Thank you, Michael II, for your support!