Limp Home Function

LIMP HOME MODE (FOR 2.7 ENG)
Limp home mode allows the driver to start a car when the immobilizer system has failed but BCM is still able to communicate with the EMS. Limp home mode is activated by "entering" the Limp Home Password of the system via activations of the ignition key. The timing for entering the password are shown blow in Figure 2.

At the dealer, the new owner is asked to choose a PASSWORD which is 4 digit (2 byte) in length.

The PASSWORD may also be placed directly in RAM using the scanner.

If the password is to be changed (for example if the user forgets the password), the dealer shall use the change Password menu on the scanner. The scanner will prompt the dealer to enter the 6 digit PIN code as obtained from the HMC database for the vehicle in question.

NOTE:
- The PASSWORD and the PIN* code are not the same.
- The PASSWORD is a number chosen by the owner for the purposes of limp starting.

Limp home mode is not active when the BCM is in virgin state.
*PIN code : P Identification Number (Vehicle Secret Code)

1. Limp Home lockout timer
If the limp home password is incorrectly entered with the scanner, and the IGN is cycled from IGN OFF to IGN ON, a counter is incremented in the BCM. If this counter reaches 3, the BCM enters lock out mode.

During this time period, the BCM will not respond in Limp home mode if any password is entered via the scanner. If the power to the BCM is removed, when it is restored, the one hour timer will restart. Manual limp home mode (using the ignition key) is still possible though.

Limp home code entry is enabled after the key has been in the ignition position for greater than 5 seconds. If the ignition is turned on for longer than five seconds during the sequence, the sequence is then restarted and 21 timers cleared. If the ignition is off for longer than three seconds during the entry, the code number for that digit of the Limp Home Password is determined. If the ignition is turned off for longer than 10 seconds at any time before during or after code entry, all timers are cleared and limp home mode is deactivated. The number "0" is represented by 10 cycles of the ignition key.

NOTE: For limp home mode, IGN OFF is defined as Ignition OFF position or ACC position.

2. Lock out mode
The BCM will enter lock out mode if the 9 byte seed in EEPROM does not match the 9 byte seed in RAM for three attempts of the following commands.
- Teach Transponder Keys to BCM
- Change imp home password
- Scantool entry of Limp home Password.

Upon activation, this mode locks out access to these commands for a period of 1 hour. This period cannot be reduced in any way? even if diagnostics mode is terminated and then re-entered. If power is disconnected to the BCM, the 1 hour timer will be restarted upon restoration. This ensures that a potential car thief cannot easily crack the 9 byte security code, and hence steal the vehicle.




3. Things to remember before repairing