Design of the exhaust gas recirculation (EGR) cooler
Transversely mounted engines

Longitudinally mounted engines

EGR
throttle valve

Coolant return line
EGR
throttle valve
EGR Motor V338
EGR Motor V338

Coolant feed line

Cooled exhaust
gases to turbocharger

Cooled exhaust
gases to turbocharger

Exhaust intake from
diesel particulate filter

Coolant feed line

Coolant return line

Exhaust intake from
diesel particulate
filter

59

Divider plate
Condensate outlet

608_040

Exhaust gas treatment module and SCR system
The substrate of the close-coupled exhaust gas treatment
module is made of metal which allows it to reach its
optimum operating temperature more quickly. The metal
body is coated with a substrate of metal oxides including
aluminum oxide with additional layers of platinum and
palladium. These precious metals act as catalysts for hydrocarbons and carbon monoxide.
Integrating the SCR coating into the particulate filter using
copper ziolite enables the system to be positioned close to
the engine. After a cold start, the operating temperature of
the SCR catalyst is reached more rapidly and maintained for
a longer period during low-load vehicle operation.

Reducing Agent Injector N474 is mounted directly downstream of the oxidizing catalytic converter above a transition
funnel to the diesel particulate filter. This allows the entire
volume of the injector to be available for carburetion.
Because of its location, N474 cannot be sufficiently cooled
by air alone. It is has a cooling jacket and is integrated with
the the low temperature circuit of the engine cooling
system.

Design

Exhaust Gas Temperature Sensor 2
G448

Reducing Agent Injector N474
(water cooled)

Exhaust Gas Temperature Sensor 3
G495

Heated Oxygen
Sensor G39

NOx Sensor
G295

Turbocharger

Mixer

Oxidizing catalytic converter
Exhaust Gas Temperatue Sensor 1
G235
Differential Pressure Sensor
G505

Connection for Exhaust Gas Pressure Sensor 1
G450

SCR-coated diesel particulate filter
622_022
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Ammonia blocking catalyst
An ammonia blocking catalytic converter with a combined
SCR and oxidizing catalyst coating is located downstream
of the SCR-coated diesel particulate filter and performs two
tasks:
Its first task is to oxidize the carbon monoxide (CO) produced during soot regeneration to carbon dioxide (CO2)
through reaction with the precious metal-containing
coating.
Its second task is to ensure that no NH3 leaves the exhaust
system. During this process, NH3 is oxidized to N2 and H2O.

Exhaust gas treatment
module

Reducing Agent Injector N474
(water cooled)
Turbocharger

Exhaust Gas Temperature
Sensor 3 G495

Oxidizing catalytic
converter

Ammonia blocking
catalyst

Exhaust gas recirculation
valve

SCR-coated diesel
particulate filter
Exhaust Door Control Unit
J883

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625_022

Exhaust Door Control Unit J883
Exhaust Door Control Unit J883 is a throttle valve driven by
an electric motor. It is mounted in the exhaust system in
the direction of flow after the diesel particulate filter. J883
allows exhaust gas recirculation to be regulated. It is
actuated by Engine Control Module J623 via a PWM signal.

Oxidizing catalytic converter

Diesel particulate filter

Exhaust Door Control Unit J883

Flexible tube

Exhaust gas
recirculation
cooler

S514_062

Function

Effects of failure

The task of this module is to generate a slight back pressure downstream of the diesel particulate filter. This produces an excess pressure of approximately 30 – 40 mbar
downstream of the particulate filter relative to the exhaust
pressure downstream of the exhaust flap.

If Exhaust Door Control Unit J883 fails, the valve is moved
to the open position by its return spring. Under these
conditions, no exhaust gas recirculation takes place.

This excess pressure results in a positive purging rate in the
EGR cooler and in the downstream EGR valve. The flow of
re-circulated exhaust gas is controlled (mapped) by the EGR
valve. This back pressure is measured by Exhaust Gas
Sensor 1 G450.
The 73° operating range of the exhaust flap door is defined by:
•
•
•

the exhaust pressure downstream of the exhaust flap
the nominal exhaust pressure upstream of the exhaust flap
the mass flow through the exhaust flap

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Exhaust gas module mounting
The exhaust gas treatment module is attached to the
engine block and cylinder head at four places. Compensation elements are used because of the normal production
tolerances between different engine configurations. They
ensure the module can be installed without subjecting it to
stress.

s514_054

Function

1

The external thread of the compensation element is a left
hand thread. When the securing bolt is screwed into the
respective attaching point, the bolt turns the compensation
element with it as it turns due to the friction on the
element tabs.

Securing Bolt

Compensation
Element

Because of the left hand thread, the compensation element
turns in the opposite direction to the securing bolt when it
is being inserted into the bracket.
The compensation element moves in the opposite direction
to the bolt head and compensates the play between the
exhaust gas treatment module and the engine.

Tabs
S514_055a

2
Securing Bolt

Compensation
of Play
Compensation
Element

Tabs

s514_101

Reference

The exhaust gas treatment module must be removed and installed in a defined sequence. The compensation
elements must be returned to their optimal position before re-installation and the securing bolts must always
be replaced. Always refer to the latest repair information when performing these procedures.
63

SCR-System
(2015 A3 model shown)
The SCR system has a special reducing agent tank with a
capacity of approximately 17 liters. It is made from high
grade plastic and adapted to the underbody contours of
the particular vehicle.
The tank has its own filler neck and vented filler cap. The
in-tank module of the system has a heater, various sensors,

a pump and a filter. A special anti-sloshing pan and baffle
mat dampen the motion of the reducing agent that occurs
under normal driving conditions.
The in-tank module is firmly attached to the tank. Only the
pump can be replaced if servicing is necessary. All functions
of the SCR system are controlled by the Engine Control
Module.

Overview
Reducing agent filler neck
In-tank module with:
−− Heater
−− Sensors
−− Delivery unit
−− Filter

Service vent

Reducing agent filler tube

Reducing agent tank
Reducing Agent Heater
Control Module J891

Heated metering line to
Reducing Agent Injector
N474

Baffle mat

Reducing Agent Quality Sensor
G849 (on underside of tank)

Diesel
fuel tank

625_020

Reducing Agent Quality Sensor G849
OBD regulations require the installation of a reducing agent
quality sensor in SCR systems. The tank of this sensor is to
detect insufficient reducing agent quality and manipulation.

The sensors are based on ultrasound technology which is
used to measure urea concentration. This measurement
also makes it possible to identify other fluids due to their
different sound velocity characteristics.

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Ultrasonic channel

Reducing Agent Pump V437 and Reducing
Agent Return Flow Pump V561

In-tank module
In-tank module houses the following components::
• Reducing Agent Tank Heater Z102
• Reducing Agent Reservoir Sensor G697
• Reducing Agent Pump V437
• Reducing Agent Return Flow Pump V561
• Reducing Agent Temperature Sensor G685

Reducing Agent Tank Heater Z102
Because the reducing agent (AdBlue) freezes at approximately 12 °F (-11 °C) the SCR system comes equipped with
a PTC heater. Temperature settings are controlled by
Reducing Agent Heater Control Module J891 in conjunction
with Engine Control Unit J623. In addition, the line to the
externally mounted Reducing Agent Injector N474 is
heated.

625_108

Reducing Agent Temperature
Sensor G685

Reducing agent, AdBlue®

Reducing Agent Tank Heater Z102
(heating circuit 1)

Boundary layer

Air

Reducing Agent Reservoir Sensor G697
G697 is an ultrasound sensor. Ultrasonic waves are guided
through the reducing agent by a special channel to prevent
signal scatter and stray reflection. The waves are reflected
by the boundary layer between the reducing agent and air.
The level is determined by the time difference between the
outgoing and incoming impulses taking into account the
sonic velocity of the reducing agent level.

Reducing Agent Reservoir Sensor G697

In-tank module
625_084

Pulses sent

Pulses reflected

Reducing Agent Pump V437 and Reducing Agent Return
Flow Pump V561 (delivery module)
The delivery module has the following components:
• Twin linear solenoid diaphragm pumps
• A reducing agent supply line to N474
• A line for recirculating the reducing agent after shutting
off the engine

In-tank module

The linear solenoid serves here as a pump drive. The delivery module is mounted separately from the in-tank module
and can be replaced.
Reducing
Agent Pump
V437

To N474

Reducing Agent
Return Flow Pump
V561
625_113

65

Solenoid diaphragm pump
The SCR system on the A3 TDI uses a solenoid driven diaphragm pump to deliver the fluid to the injector instead of
a motor driven pump as seen in the other TDI models. (Q7,
A6, A7, A8, Q5).
The stroke volume is virtually constant and depends only to
a minimal extent on the prevailing pressure. The urea
solution is injected via the reducing agent injector on
demand and depending on the nitrous oxides produced
during the combustion cycle. The quantity of urea solution
injected is then delivered by the reducing agent pump in a
controlled manner. Due to the close tolerances of the
reducing agent pump and reducing agent injector, pressure
equilibrium occurs at approx. 6.5 bar (± 2 bar). This process
of constant volume delivery is also known as volumetric
delivery. There is no need for a pressure sensor, including
the required pressure sensor heater, pressure sensor
housing and control system.

Reducing Agent Recirculating Pump
V561

Tank filter

Linear solenoid
Heater metering line

Pulsation
damper

Screen
filter

Linear solenoid

Reducing Agent Pump
V437

Reducing Agent
injector N474

Engine Control
Module J623
625_085

The pressure in the system can be determined by measuring the time when the pump is first energized to when the
armature in the solenoid moves, as well as by measuring
the amount of electrical current required to move the
solenoid.
During delivery, V437 is energized by ECM J623 using pins
3 and 4. The recirculating pump V561 is energized by J623
using pins 1 & 2. Reducing Agent Recirculating Pump V561
is energized by the ECM using pins 1 and 2.
Large pressure deviations from the normal system pressure
indicate faults in the system; for example, jamming of the
pump, a defective pump diaphragm, a leak in the pressure
line a clogged metering valve, or pump intake problems.

Delivery

Immediately after shutting off the engine, N474 closes and
some of the reducing agent is drawn back out of the metering line by the return flow pump. This is done with N474
closed so no hot exhaust gases are taken in. A short time
later, N474 is opened and additional reducing agent is
drawn back. This process prevents reducing agent from
freezing and creating damage.

Return

Reducing Agent Pump
V437
Linear solenoid

Linear solenoid
Reducing Agent
Recirculating Pump V561

To Reducing Agent Injector
N474

From Reducing Agent
Injector N474

Term. 87 (+)
From tank filter

Ground from J623

To tank filter
Ground fromJ623
Term. 87 (+)

625_138

625_137
66

Special tools and workshop equipment
T10172 with T10172/11 Adapter

T10489 Puller

Adapter for the counterholder of the camshaft
sprocket.

Puller to disassemble
the hub of the highpressure pump.

608_071

608_072

608_064

T10490 Crankshaft stop

T10491 Socket SW22

Crankshaft stop for
holding the crankshaft
when adjusting the
valve timing.

Socket to remove and
install the Oxygen
sensor.

608_068

608_066

T10492 Locking pin

T10493 Assembly tool

Locking pin to lock the
pulley of the highpressure pump.

Tools for installing oil
seals for the camshaft.

608_069

67

608_070

T10497 Engine bracket

T10501 Socket XZN 10

Engine bracket for
removing and installing
the engine in conjunction
with V.A.G. 1383 A

Socket to remove and
install the intake manifold.

S514_070

S514_077

T10511 Assembly aid

T10512 Calibration tool

Assembly aid to remove
and install the exhaust
treatment module.

Calibration tool to adjust
the compensation elements when installing
the exhaust treatment
module.

S514_078

S514_069

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Knowledge Assessment
An On-Line Knowledge Assessment (exam) is Available for this eSelf-Study Program.
The Knowledge Assessment is required for Certification.
You can find this Knowledge Assessment at:
www.accessaudi.com
From the accessaudi.com Homepage:

•
•
•

Click on the “ACADEMY” tab
Click on the “Academy site” link
Click on the Course Catalog Search and select “920143 - The Audi 2.0L Third Generation TDI Engine”
Please submit any questions or inquiries via the Academy CRC Online Support Form
which is located under the “Support” tab or the “Contact Us” tab of the Academy CRC.

Thank you for reading this eSelf-Study Program and taking the assessment.

69

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920143
All rights reserved.
Technical specifications are
subject to change without notice.
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2200 Ferdinand Porsche Drive
Herndon, VA 20171
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