Continental Automotive 17A920790 User Manual

User manual

VAG Immobilizer System

Type 17A 920 790

Keller, I ID S3 AD AE 1 Page 1 23.10.2017

Table of contents

1 System overview………………………………………………………………………...3
2 Short description of the module………………………………………………………..3

3 Transponder……………………………………………………………………………...4
4 Power supply……………………………………………………………………………..5
5 Technical data……………………………………………………………………………5
6 Label………………………………………………………………………………………6

Keller, I ID S3 AD AE 1 Page 2 23.10.2017

1 System overview

The module described within this document is used in the following system environment:
 VAG Immobilizer System WFS 5a

The VAG Immobilizer System WFS 5a is, among other things, an integral part of all MQB instrument
clusters.

An immobilizer distinguishes between authorized and unauthorized users and thus it prevents the engine
from running unless the correct key (transponder) is present.

The microcircuit inside the key is activated by a small electromagnetic field which induces current to flow
inside the key body, which in turn broadcasts a unique binary code which is read by the instrument cluster
that includes the immobilizer function. When the cluster determines that the coded key is both current and
valid, the ECU activates the fuel-injection sequence.

The immobilizer is an inductive application (Short Range Device), for this reason a radio approval
(homologation) is required.

2 Short description of the module

This document describes a car immobilizer circuit based on base station IC LRES B2, which is distributed
by Delphi Megamos.

This module performs the following functions:

 Antenna driving with carrier frequency
 100% AM modulation of the field for writeable transponders (write mode)
 AM/FM demodulation of the antenna signal modulation induced by the transponder (read mode)
 Communication with microcontroller via two wire interface

The immobilizer module is the interface between the transponder inside the key and the microcontroller.
The microcontroller controls all functions of the instrument cluster including the transponder
communication.

The reader and the transponder are working as a wireless, magnetic coupled communication system, each
with a resonance circuit tuned to the system frequency as close as possible. Both circuits have to be tuned
on the resonance frequency (125 ± 5 kHz). The reader circuit is working in series resonance, the
transponder with a parallel resonance circuit.

The reader provides energy to the transponder by an electromagnetic field. By modulating this field, the
reader can transmit (write) data to the transponder (uplink). The transponder will power up and return its
on-chip data to the reader (downlink).

Keller, I ID S3 AD AE 1 Page 3 23.10.2017