Notes on immobilizer
A level 2b drive authorization program (DAS 2b) can be installed in Model 638 vehicles. In this drive authorization system level the immobilizer and radio frequency remote control (RRC) central locking functions are installed in the same control module operationally, but decoupled. The immobilizer is deactivated by means of a transponder integrated into the key. The radio frequency remote control (RRC) operates the central locking system. The immobilizer is available with and without RRC.
The immobilizer serves to ensure that the engine cannot be started by unauthorized persons. This is achieved by intervening in the digital engine management. In vehicles without digital engine management the fuel supply is inhibited by a shut-off valve.
Depending upon the installed engine the drive authorization system consists of an engine control unit (Engine 111) or a drive authorization system shut-off valve (DSV in Engine 601) and a DAS control module (Immobilizer with or without radio frequency remote control for the central locking system).
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The two versions differ through the type of communication realized between the DAS control module and the engine control unit or the drive authorization system shut-off valve (DSV). In the version with electronic fuel injection (Engine 111) communication is set up via the engine control unit and the DAS control module using a PWM signal. In the version with DSV (Engine 601) communication is set up between the DSV control module and the DAS control module via the CAN bus.
The immobilizer is deactivated via a transponder integrated into the mechanical key, which communicates with the DAS control module via a coil installed on the steering wheel lock. The immobilizer is decoupled from the radio frequency remote control (RRC) function. In the event that the transmitter key should malfunction (e.g. flat battery in transmitter key) the vehicle can still be opened by mechanical means and started with the ignition key (ATA may need to be deactivated). The immobilizer is self priming. If, e.g. when refueling, the ignition key is removed (Ignition: OFF), the immobilizer is activated, without any need for the vehicle to have been locked by RRC.
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Operating notes on immobilizer
The immobilizer system with transponder consists of the control electronics in the vehicle, the foldable vehicle key with integrated transmitter for the radio frequency remote control (if installed) and the integrated transponder module. The radio frequency remote control operates the central locking system. If the vehicle is equipped with a standard anti-theft-alarm system (ATA), this is also activated or deactivated by the radio frequency remote control. The immobilizer and radio frequency remote control (RRC) are installed in the same control module as functions, but decoupled.
After activating the drive authorization system the engine control unit shuts-off the continuous fuel injection system or the shut-off valve stops the supply of fuel to the mechanical fuel injection. Activation is done by switching off terminal 15. Deactivation is done by the transponder system. When the vehicle is started (Ignition: ON) the transponder coil and the transponder in the key are inductively energized. The transponder in the key transmits the Code in its memory to the DAS control module. If the key is adjudged to be valid, an engine authorization is initiated.
Start authorization follows via a PWM signal (Engine 111) to the engine control unit or via a CAN connection (Engine 601) to the drive authorization system shut-off valve (DSV).
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In the CAN version the DAS control module and the DSV engine control unit are permanently assigned to each other by means of an identification code.
This identification code cannot be deleted. For this reason, it is not possible to exchange the DAS control module or the engine control unit from another vehicle.
Operating notes on the radio frequency remote control
Folding in the mechanical key with its integrated transmitter sends out a "Lock vehicle" signal. This activates the vehicle's central locking system and, where applicable, the anti-theft alarm system (ATA) is activated.
In contrast to this, folding out the key with its integrated transmitter unlocks the vehicle and, where applicable, deactivates the anti-theft-alarm system.
The messages from the transmitter key are sent encoded. The control module decodes the telegram and compares it with a specified value. The telegram is accepted by the control module as valid, if the telegram and the specified value determined by the control module match.
The telegram's code is altered every time the transmitter key is used. This helps to ensure that previously transmitted telegrams are rendered invalid and any "listening in" to the radio telegram by unauthorized persons prohibited.
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Re-synchronization
If the key is operated more than 255 times successively outside the operating range of the receiver unit (control module), the radio telegram is desynchronized. In such an event the receiver (control module) can be synchronized back to the transmitter key's telegram by actuating the hinged key twice within 7 s.
This is only possible for transmitter keys that are still valid.
Transmitter key
The transmitter key contains the following function controls:
Radio frequency (HF) transmitter
Detection of foldable key position
Coding part
Non-volatile data storage (to ensure that data is retained when a battery is changed)
Light emitting diode to battery state display
2 batteries
Transponder module for immobilizer
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Control module
The control module contains the following operational elements:
HF-receiver
If installed: Interface to central locking system and ATA
Interface to motor electronics (immobilizer function)
Triggering of immobilizer control and malfunction indicator lamp
Input terminal 15
Interface to diagnosis socket
Coding part
EEPROM for non-volatile data storage
Interface to transponder coil
Communication with motor electronics
After switching on the ignition the immobilizer status is electronically encoded and sent to the engine control unit (PMS or DSV). Communication is unidirectional in the PWM version (Engine 111) and bi-directional in the CAN version.
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Communication with the central locking system (CL) or convenience locking system (CLS)
A single-wire line is used to send command telegrams to the central locking / convenience locking system or for receiving status telegrams from the central locking / convenience locking system.
Command telegrams are defined as:
Unlock vehicle,
- lock vehicle.
Status telegrams are defined as:
- Vehicle unlocked,
- vehicle locked.
If the ignition is switched off and a valid telegram is received from a transmitter key, then a command telegram is sent to the central locking / convenience locking system. The change of state (locked/unlocked) must be the opposite of the previous state. The central locking / convenience locking system responds to a command telegram with a status telegram after a max. of 1.5 s (unlock) or 3 s (lock).
A new command telegram is sent only after receiving a status telegram or after a period of 1.5s or 3s has expired.
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A status telegram can be received by the central locking / convenience locking system at any time.
If the above-mentioned timeouts are not adhered to, e.g. through unfolding/folding the key too quickly, then in certain conditions the position of the foldable key may not match the vehicle status (unlocked/locked).
Vehicles installed with anti-theft-alarm system (ATA)
In vehicles installed with an anti-theft-alarm system the ATA control module is connected between the DAS control module and the CL or CLS control module. Command telegrams from the DAS control module are forwarded to the CL/CLS control module via the ATA control module. Conversely, status telegrams are sent from the CL control module to the ATA control module and forwarded from there to the DAS control module.
Deactivating the ATA: Deactivation ensues when a "Vehicle unlocked" command telegram is received from the DAS control module.
Activating the ATA: Activation ensues when a "Vehicle locked" status telegram is received from the CL / CLS control module.
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Immobilizer control and malfunction indicator lamp
The immobilizer control and malfunction indicator lamp is driven when a plus signal (terminal 15) in the DAS control module is connected. If a short circuit exists, the output is deactivated and the ignition is not enabled again until after it is switched on.
PWM version (Engine 111)
After switching on the ignition the malfunction indicator lamp is illuminated for approx. 2 s. The indicator lamp is switched off:
- approx. 2 s after ignition "ON" provided that the immobilizer is deactivated,
- if a short circuit exists at the output,
- if the ignition is switched off.
CAN version (Engine 601)
In view of the fact that the preglow and immobilizer malfunction indicator lamps have been combined to form a single indicator lamp (preglow symbol), the following actuation exists in the default mode (immobilizer correctly deactivated):
The malfunction indicator lamp comes on after terminal 15 is connected and goes out after the end of the preglow period.Vehicle can be started Therefore, the malfunction indicator lamp (Ignition = ON) is triggered by the DAS control module only when drive authorization has not been given. Engine status = "No authorization" (can be checked with HHT in the DSV control module under "Actual value" actual value 72).
The indicator lamp is switched off:
- by switching off the ignition,
- or after reception of the "Enable" engine status.