For 1990-2009 cars

### Vehicle Theft Deterrent (VTD) -- Immobilizer

The vehicle theft deterrent (VTD) system functions are provided by the body control module (BCM). When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the BCM supplies battery voltage to the theft deterrent exciter module. The transponder embedded in the head of the key is energized by the theft deterrent exciter module which is surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains its unique value, which is received by the theft deterrent exciter module. The BCM monitors the theft deterrent exciter module for the transponder value via the security system sensor signal circuit. The BCM then compares this value to a value stored in memory, learned key code. If the value is correct the BCM sends the fuel continue password via the serial data circuit to the powertrain control module (PCM). If the transponders value is incorrect the BCM will send the fuel disable password to the PCM via the serial data circuit.

The components of the VTD system are as follows:

• | Theft deterrent module (TDM)

• | Body control module (BCM)

• | Ignition key (Transponder)

• | Security indicator

#### Theft Deterrent Module (TDM) Immobilizer

Vehicles with steering column mounted ignition switches have the exciter integral with the theft deterrent module (TDM), which is located within the steering column. The TDM can learn up to 10 keys (transponder values).

The TDM uses the following inputs: battery voltage, ignition switched voltage and ground circuit. The theft deterrent control module uses the following outputs:

Password exchange and challenge/response with the engine control module (ECM).

When an ignition key is inserted into the ignition lock cylinder and the ignition is switched ON, the transponder embedded in the head of the key is energized by the exciter coils surrounding the ignition lock cylinder. The energized transponder transmits a signal that contains its unique value, which is received by the theft deterrent control module. The theft deterrent control module then compares this value to the learned key code stored in memory. The theft deterrent control module then performs one of the following functions:

• | If the transponder value matches the values stored in the TDM memory, the TDM will send the fuel enable message to the ECM via the serial data circuit.

• | If the transponders unique value does not match the value stored in the TDM, the TDM will send the fuel disable message to the ECM via the serial data circuit.

• | If the transponders unique value does not match the value stored in the TDM, the TDM will send the fuel disable message to the ECM via the serial data circuit.

• | If the TDM is unable to measure the ignition key transponder value, the TDM will not send any messages to the ECM.

#### Engine Control Module (ECM)

When the engine control module (ECM) receives the theft deterrent module (TDM) fuel enable password, the ECM will challenge the password. The ECM sends this challenge back to the TDM via the serial data circuit. Both the ECM and TDM perform a calculation on this challenge. If the calculated response from the TDM equals the calculation performed by the ECM, the ECM will allow vehicle starting.

The ECM will disable vehicle starting if any of the following conditions occur:

• | The fuel enable password is invalid.

• | The fuel disable password is sent by the TDM.

• | No passwords are received--There is no communication with the TDM.

• | The TDM calculated response to the challenge does not equal the calculation performed by the ECM.

#### The Ignition Key (Transponder)

The ignition key for Passkey III+ (PK3+) equipped vehicles is a standard ignition key with a transponder located in the plastic head of the key. The transponder value is fixed and unable to be changed. The vehicle theft deterrent (VTD) system uses the ignition key transponder value to determine if a valid ignition key is being used to start the vehicle. There are approximately 3 trillion possible transponder values.

Fleets keys allow full access to the vehicle just as a master key would. However, unlike a master key which may only learn 10 keys to a particular vehicle, an unlimited number of fleet keys may be learned to the vehicle. Fleet keys are only used in vehicles configured for police fleet use.

• | Start the vehicle.

• | Lock/unlock all of the door locks and rear compartment.

• | Lock/unlock all of the storage compartments.

#### Security Indicator

The theft deterrent module (TDM) can command the clock to illuminate the security indicator only when the ignition key is in the ON position. The TDM will command the security indicator be illuminated any time a fault is noted in the VTD system and when engine starting is disabled.