1998 AUDI S4 sensor

51
The measuring principle of the return flow
recognition
The sensor element is embedded in the
mounting plate.
The sensor element comprises a diaphragm
with a heating zone and two symmetrically
arranged temperature sensors T
1 and T2.
The heating zone is set to an overtemperature
by means of a heating resistor and
temperature sensor T
2.
If there is an incoming flow, the upstream part
of the diaphragm cools down along with the
temperature sensor T
1.
The temperature of the upstream temperature
sensor T
2 is maintained due to the heated air in
the heating zone.
Temperature sensors T1 and T2 indicate a
temperature difference of DT.
In the case of a return air flow, the temperature
difference occurs at temperature sensor T
1.
The amount and direction of this difference are
therefore dependent on the incoming flow.
• Advantage: the differential signal permits a
direction-dependent characteristic which
enables the Motronic to detect a return air flow.
T
T
1T2
1
0
SSP 198/36
Incoming flow
Temperature profile
without incoming flow
with incoming flow
Mounting plate
Diaphragm
Heating zone
Sensor element
Temperature difference
evaluation: DT = T
2 - T1

52
Sensors
Lambda probes G39 and G108
The planar lambda probe is a further
development of the finger-type lambda probe
and has a transient response at lambda = 1.
There is a single lambda probe in the exhaust
pipe running to each of the primary catalytic
converters.
To ensure that the exhaust gases are treated
efficiently, it is important that the lambda
probe should react quickly. The lambda probe
should therefore reach its operating
temperature within as short a space of time as
possible. Its planar (= flat, elongated) design
makes this possible.
The probe heater is integrated in the sensor
element. It quickly reaches its operating
temperature despite its lower heating capacity.
Note:
At an exhaust gas temperature as low as 150
°C, the probe heater generates the necessary
minimum temperature of 350 °C.
The lambda control is ready to operate approx.
10 seconds after engine start-up.
A porous, ceramic protective layer is sintered
onto the sensor element.
This layer prevents the sensor element being
damaged by residues in the exhaust gas.
It ensures that the sensor element will have a
long service life and meet the tough functional
demands.
Substitute function:
Controlled operation based on a characteristic
curve (cylinder bank-specific).A new generation of probes used in
the biturbo for stereo lambda
control.
Advantages:
• The warm-up period is short, which means
lower emissions during the warm-up phase
• Low heating power consumption
• More stable control characteristic
SSP 198/37
Section
Probe heater
Sensor element

53
Hall senders G40 and G163
To permit cylinder-selective knock control and
sequential injection, cylinder 1 must be
defined precisely.
The signal which Hall sender G40 supplies
together with the signal which engine speed
sender G28 generates (incremental sender for
engine speed and reference mark) enable
ignition TDC of cylinder 1 to be identified
(synchronization of cylinder 1).
After the simultaneous input of both signals,
initial injection and ignition are enabled.
By using Hall senders G163 and G40 as
camshaft sensors, the adjustment of both
camshafts can be monitored closely and
evaluated by the self-diagnosis.
Substitute function and self-diagnosis:
If Hall sender G40 fails, Hall sender G163 takes
on the task of synchronising first cylinder.
If both Hall senders fail, it is possible to start
the engine and the engine runs with substitute
functions.On V-engines with variable valve
timing, a Hall sender acting as a
camshaft sensor is attached to the
left- and right-hand cylinder banks.
SSP 198/35
Hall sender G40
Hall sender G163

54
Sensors
Engine speed sender G28
The engine speed sender is an inductive
sender which records the engine speed and
the exact angular position of the crankshaft
(single-sender system).
Attached to the flywheel is a separate sender
wheel for the G28.
The sender wheel is designed as a segmented
wheel and is subdivided into 60 segments.
If the sender wheel moves past G28 , this
produces an alternating voltage whose
frequency changes as a factor of engine speed.
The frequency is the magnitude of the engine
speed.
To enable it to recognise the crankshaft
position, there is a gap of two segments in the
sender wheel.
The G28 recognises the engine speed.
Together with Hall sender G40, the G28
recognises the exact position of the engine
mechanics, i.e. ignition TDC of cylinder 1. The
injection and ignition timing are determined
using this information.
Substitute function and self-diagnosis:
The signal which G28 generates is checked
together with the signal supplied by the G40
for plausibility.
If the Motronic control unit does not detect any
segment gaps during 8 “phases“ of the G40,
an entry is made in the fault memory.
If the engine speed sender fails, it is not
possible to start or run the engine.
Since the G28 is an inductive sender,
the self-diagnostics are unable to
perform electrical tests (short circuit
to positive or negative or open
circuit).
SSP 198/64
Two mass flywheel
Sender wheel
Engine speed sender
Segment
gap

56
Sensors
Brake light switch F and brake
pedal switch F47The information “brake operated“ is required
for the following functions:
• Function of cruise control system
• Safety interrogation of electronic accelerator
function (idling speed recognition during
emergency running mode of accelerator
position sender)
Brake light switch F and brake pedal switch F47
are combined as a unit. Both serve as
information senders for “brake operated“,
which means they are redundant (for safety
reasons).
Brake light switch F is open in the “off”
position and is supplied with voltage from
terminal 30. It serves as an additional
information input for the Motronic.
Brake pedal switch F47 is closed in the “off”
position closed and is supplied with voltage
from terminal 15. It serves exclusively as an
information input for the Motronic.
Substitute function and self-diagnosis:
The two switches are cross-checked for
plausibility by the self-diagnosis.
Please read the note on the “Safety function“
on page 39.
Clutch pedal switch F36 ...
Wrong settings, electrical malfunctions or maloperation (driver keeps foot on clutch
pedal) may result in load change jolts or engine speed overshoots.
... switches the cruise control system off.
... deactivates the load change functions during the gearshift operation. The load change
function is controlled via ignition angle intervention and throttle valve closing speed.
The clutch pedal switch is closed in the “off” position and is supplied with voltage from terminal
15.
Substitute function and self-diagnosis:
The F36 is not included in the self-diagnosis, which means that no substitute functions are
initiated.
SSP 198/63
Brake light switch F and
brake pedal switch F47
Clutch pedal switch F36

62
Functional diagram
Components:
F Brake light switch
F36 Clutch pedal switch
F47 Brake pedal switch
F96 Altitude sender (integrated in engine
control unit)
G2 Coolant temperature sender
G6 Fuel pump
G28 Engine speed sender
G31 Charge pressure sender
G39 Lambda probe (cylinder bank 1)
G40 Hall sender (cylinder bank 2)
G42 Intake air temperature sender
G 61 Knock sensor (cylinder bank 1)
G62 Coolant temperature sender
G66 Knock sensor (cylinder bank 2)
G70 Air mass meter
G79 Accelerator position sender 1
G108 Lambda probe (cylinder bank 2)
G163 Hall sender (cylinder bank 1)
G185 Accelerator position sender 2
G186 Throttle valve drive (electric throttle
control)
G187 Angle sender 1 for throttle valve drive
G188 Angle sender 2 for throttle valve drive
G235 Sender 1 for exhaust gas temperature
G236 Sender 2 for exhaust gas temperature
J17 Fuel pump relay
J220 Motronic control unit
J338 throttle valve control part
K132 Warning lamp for electric throttle
control
N Ignition coil, cylinder 1
N30 Injector, cylinder 1
N31 Injector, cylinder 2
N32 Injector, cylinder 3
N33 Injector , cylinder 4
N75 Solenoid valve for charge pressure
control
N80 Solenoid valve for activated charcoal
canister
N83 Injector, cylinder 5
N84 Injector, cylinder 6
N122 Output stage (cylinder bank 1)
N128 Ignition coil, cylinder 2
N158 Ignition coil, cylinder 3
N163 Ignition coil, cylinder 4
N164 Ignition coil, cylinder 5N189 Ignition coil, cylinder 6
N192 Output stage (cylinder bank 2)
N205 Camshaft adjustment valve 1 (cylinder
bank 1)
N208 Camshaft adjustment valve 2
(cylinder bank 2)
N249 Divert air valve for turbocharger
Z19 Heater for lambda probe
Z28 Heater for lambda probe 2
I To dash panel insert
II To dash panel insert (warning lamp)
Additional signals
1 CAN-high (automatic gearbox)
2 CAN-low (automatic gearbox)
3 „Set/decelerate“ signal for cruise
control system
4 “Off“ signal without cancellation for
cruise control system
5 “On/Off“ signal with cancellation for
cruise control system
6 Resume/accelerate“ signal for cruise
control system
7 Road speed signal
8 Immobiliser/diagnosis signal
9 Air conditioner compressor “On/Off“
signal
10 Coolant temperature signal
11 Engine speed signal
12 Fuel consumption signal
Colour codes:
Input signal
Output signal
Positive
Earth
Bidirectional

64
SSP 198/39

Self-diagnosis
Vehicle diagnosis, test and
information system VAS 5051
VAS 5051 has the following three operating
modes:
Vehicle self-diagnosis
• Communication via the vehicle’s diagnosis
interface
• Offers the functional capability of currently
available diagnosis testers V.A.G 1551 and
V. A. G 1552
Test instruments
• Measurement of the vehicle’s electrical
parameters (voltage, current, resistance)
and testing of diodes
• DMO (Digital Memory Oscilloscope) for
representing the voltage curves of the
various individual sensors and actuators
Guided fault finding
• Vehicle and control unit identification
• A test plan is prepared on the basis of the
fault messages issued by the the self-
diagnosis, the fault description of customer
complaints or assumptions regarding the
cause of the trouble.
You will find introductory notes and
technical information on this system
in Self-Study Programme 202.
For self-diagnosis, please use the
Workshop Manual in which the
procedure for the various individual
functions is described.
VAS 5051

66
Power Transmission
Self-adjusting clutch
Advantages:
• Constant clutch releasing loads throughout
the service life of the clutch plate.
• Greater wear reserve of the clutch plate. For the biturbo engines, Audi is
using an SAC clutch pressure plate
with a wear compensation feature
for the first time.
„
SAC
“ stands for
S
elf-
A
djusting
C
lutch.
SSP 198/42

Auxiliary spring
Adjusting ringSensor plate spring
Main diaphragm spring
Compression spring
Housing cover