Relays in the 48-volt
battery

yes

on

no

48 volts → 12 volts

is closed

no

on

no

48 volts → 12 volts

is closed

no

on

yes

12 volts → 48 volts

is closed

no

off

yes

12 volts → 48 volts

is closed

no

off

no

not functional

open

10-pin connection

The hybrid CAN wires are also integrated into the 10-pin
plug-in connection. If the voltage converter is disconnected
from the communication lines when Terminal 15 is active,
for example, by disconnecting the 10-pin plug-in connector, the voltage converter is deactivated.

The relay in the 48-volt battery stays closed and the 48-volt
electrical system stays active.
The 12-volt system also stays active but the 12-volt battery
is no longer charged.

10-pin connector
12-volt side

664_078

12-volt battery

48-volt battery

Remarks

is charged

is charged

The 48-volt generator runs, charges the 48-volt battery and supplies the voltage converter, which, in turn, converts the voltage and charges the 12-volt battery.

is boosted

is discharged

The 48-volt battery boosts the 12-volt electrical system via the voltage converter in
certain operating states.

is charged

is charged

Charger is detected – 12-volt charger charges 12-volt battery. Voltage converter transforms voltage to 48 volts and charges 48-volt battery.

is not charged

If insufficient charging current is available for the 12-volt battery because an underrated
charger is used, the voltage converter does not supply 48 volts for this Terminal 15 cycle
and, therefore, the 48-volt battery is not charged.

is charged

is charged

Charger is detected – 12-volt charger charges 12-volt battery. Voltage converter transforms voltage to 48 volts and charges 48-volt battery.

is discharged

is disconnected

The 48-volt battery is "disconnected". The 12-volt battery is discharged depending on the
vehicle's static current.

23

Working on the 48-volt electrical system
Aim
The use of 48-volt systems in automative systems is a
potential source of electrical hazards. Service employees
must be able to identify a 48-volt electrical system and
safely perform whatever work is necessary.

All work on the 48-volt electrical system may only be performed by trained employees with a basic knowledge of
electrics and with the system de-energized.

Recognition and identification
Positive wires of the 48-volt electrical system
DC voltage at Terminal 40
Ground wires of the 48-volt electrical system
DC voltage at Terminal 41
Warning label on the 48-volt battery

664_081

Warning label on the protective caps of the 48-volt‑battery and
the 48-volt distributor boxes

664_082

24

Hazards

The following hazards can occur when performing work on
an active 48-volt electrical system in the event of a fault or
incorrect handling:

›› Short circuits.
›› Arcing.
›› Secondary hazards (burns due to molten metal spatter
as well as injuries due to startled reaction).

Taking out of service
Before working on the 48 volt electrical system, it must
first be de-energized. This is done through Guided Functions using the VAS Scan Tool.
Through Guided Functions, Data Bus On Board Diagnostic
Interface J533 opens the relay of the 48 volt battery which
shuts down the system in a defined order. To confirm the
system has been de-energized, you must read the voltage
value of the battery and Voltage Converter, 48V/12V A7.
An inspection report is part of the de-energizing Test Plan
and must be printed out and signed by the Technician. It
should include the vehicle's details as well as the measurement data for the 48-volt system. After that, it must be
attached to the vehicle in an easily visible place.

Test log of 48-volt system, confirming manual voltage de-energization
Use only Guided Fault Finding / functions to de-energize the 48-volt system in the vehicle and check the de-energization status (absence of
power)
VIN:

._._._._._._._._._._._._._._._._._.

Date:

__________________________

Telephone: __________________________
Name:

__________________________

Function test of Test Instruments on 12-volt battery.

Measuring tool check - Reference voltage measurement 12-volt battery
Measured voltage __________ V; Specified value: Current 12-volt battery voltage +/- 10%
Determine the de-energization status on the deactivated 48-volt system using Test Instruments.

Measurement 1 - Voltage measurement on control module Battery, 48 V -A6Measured voltage __________ V; Specified value: < 5 volts
Measurement 2 is only performed when the value of measurement 1 is outside the specified value range.

Measurement 2 - Voltage measurement on control module Voltage converter, 48 V/12 V -A7Measured voltage __________ V; Specified value: < 0.5 volts

In the event of an “Not OK” inspection report/finding, the
48-volt system must be shut down manually.

Example of an inspection report (page 1 of 6)
the display on the diagnostic tester may be slightly different)

Returning to service
The 48-volt system is returned to service after completion
of work/repairs by running the Test Plan in "Guided Functions". If the system is not returned to service, the vehicle
runs without a durable 48-volt/12-volt voltage supply, that
means the 12-volt-battery is not charged. As soon as the
12-volt battery is discharged completely, the vehicle is no
longer roadworthy.

For this reason, special guidelines apply to the storage,
transportation and recycling of lithium-ion batteries.
Special emphasis is placed here on the handling of critical
batteries. These processes are often subject to national
regulations. When handling lithium-ion batteries, all statutory requirements applicable in your country as well as all
instructions given in the service literature and Guided Fault
Finding in ODIS.

Hazards due to lithium-ion technology
Hazards can also occur due to the design of a lithium-ion
battery and electro-chemical processes inside a lithium-ion
battery. The electrolyte in the lithium-ion battery is easily
flammable, potentially explosive, causes chemical burns
and emits toxic vapors.

Note
Always refer to the service literature for information on handling requirements for lithium ion batteries.

Note
For a detailed description of the procedures for de-energizing, repairing and starting up the 48-volt electrical system, please
refer to the current service literature.
25

The 48-volt MHEV (Mild Hybrid Electric Vehicle)
General description
The abbreviation MHEV stands for Mild Hybrid Electric
Vehicle. These vehicles generally have an additional battery
as well as a small e-machine. This means that functions
such as extended recuperation and internal combustion
engine assistance by the e-machine (boost) are possible.

The Audi A8 is designed as a 48-volt MHEV. The 48-volt
battery serves as an auxiliary battery and the starter generator as the e-machine.

The TFSI engine is generally responsible for drive and for
generating electrical energy. All-electric driving is not
possible with the Audi A8 MHEV.

Mild hybrid functions
The 48-volt MHEV concept offers drivers a number of new
functions which enhance both efficiency and ride comfort.
These additional functions also require additional controls
and displays.
The following functions have been implemented in the
2018 Audi A8.
›› Start-stop at speeds of less than 13.6 mph (22 km/h).
›› Internal combustion engine assistance by the starter
generator.

›› Extended recuperation.
›› Intelligent coasting with internal combustion engine
shut-off.
›› More comfortable internal combustion engine start-up
by the starter generator.
›› More comfortable internal combustion engine shut-off
by the starter generator.
›› Immediate restarting of the internal combustion engine
by the starter generator (change of mind).

Extended recuperation
In recuperation mode, the vehicle's kinetic energy is used to
charge the battery during overrun phases. Thanks to their
auxiliary battery, mild-hybrid vehicles have a higher recuperation capacity than previous vehicles. The Audi A8, with
its lithium-ion battery and the 48-volt starter generator
has a recuperation capacity of up to 12 kW.

Medium recuperation

664_133

The so-called "overrun recuperation" cycle is indicated by a
50% display in the instrument cluster.
The "extended recuperation" cycle is initiated by efficient
assist and indicated by the 100% display in the instrument
cluster.
Maximum recuperation

26

664_134

Improvements by starter generator
The use of the starter generator makes restarting the TFSI
engine while driving much more comfortable than is the
case with conventional pinion-type starters. The fact that
the starter generator is permanently coupled to the engine
via the poly V belt allows it to be restarted even if it has not
yet come to a complete standstill. This situation can occur
if, for example, the engine is shut off by the start-stop
system, but has not come to a complete standstill and the
driver presses the accelerator because he wants to continue
driving (change of mind situation).
A further improvement stems from the fact that the TFSI
engine can be braked precisely by the starter generator
during shut-off, allowing “shut-off judder” to be reduced
significantly. The starter generator can be used as an
e-machine while the engine is running and can assist the
engine in certain load ranges, thus improving fuel economy.
The starter generator in the Audi A8 is not designed to
boost the torque and/or power output of the TFSI engine.

664_069

27

Networking
Some of the control modules shown in the overview are
optional and/or country-specific equipment. For the sake of
clarity, not all control modules fitted in the vehicle can be
shown here.

Refer to the current service literature for details of control
modules installation positions, as well as instructions for
installation and removal.

J387
J428

J1023

R161

A27
J1089

J794
J521

J500

J1122

J792

R242

J234

J245

J775

J764
J898

J1088

J285

J527

J1018

J136

J853

J1121

J519

J623
J931

Key

28

J869
J104

A31

A6
A7
A27
A31

Battery, 48 V
Voltage Converter, 48 V/12 V
Right LED Headlamp Power Output Module 1
Power module 1 for left LED headlight

J104
J136
J187
J234
J245
J285
J386
J387
J392
J393

ABS Control Module
Memory Seat/Steering Column Adjustment Control Module
Differential Lock Control Module
Airbag Control Module
Sunroof Control Module
Instrument Cluster Control Module
Driver Door Control Module
Front Passenger Door Control Module
Rear Sunroof Control Module
Comfort System Central Control Module

J428 Control Module for Adaptive Cruise Control
J500 Power Steering Control Module
J505 Windshield Defogger Control Module
J519 Vehicle Electrical System Control Module
J521 Front Passenger Memory Seat Control Module
J525 Digital Sound System Control Module
J527 Steering Column Electronics Control Module
J533 Data Bus on Board Diagnostic Interface
J605 Rear Lid Control Module
J623 Engine Control Module
J764 Electronic Steering Column Lock Control Module
J769 Lane Change Assistance Control Module
J770 Lane Change Assistance Control Module 2
J772 Rearview Camera System Control Module
J775 Drivetrain Control Module

J386

J927

A7

J769

J505

J392

J533

J1019

J605

J187

A6

J772

J525
J393

J770

J926

661_091

J792
J794
J853
J869
J898
J926
J927
J931
J1018
J1019
J1023
J1088
J1089
J1121
J1122

Active Steering Control Module
Information Electronics Control Module 1
Night Vision System Control Module
Structure Borne Sound Control Module
Windshield Projection Head Up Display Control Module
Driver Side Rear Door Control Module
Passenger Side Rear Door Control Module
Subframe Mount Control Module
Left Light Control Module
Rear Axle Steering Control Module
Right Light Control Module
Control Module for Left Front Object Detection Radar Sensor
Control Module for Right Front Object Detection Radar Sensor
Driver Assistance Systems Control Module
Laser Distance Regulation Control Module

R161
R242

DVD Changer
Driver Assistance Systems Front Camera
29

Topology

J926

J386

J927

J387

J605
Data Link Connector

J136

N209

N475

N210

J521

J764

J453

J245

U13

A6

J392

J367

A7

N477

J876

N479

N480

E660

N481

J877

N482

E661

H12

J505

G65

A31

G1052