Antitheft and Alarm Systems: Description and Operation

Resistor sensing contacts are located in the Ignition Key Lock Cylinder. These contact the key resistor pellet on the key when it is inserted. When the lock is rotated, battery voltage is applied through [1][2]Relay Center Fuse 8 to the PASS-Key Decoder Module. The pellet resistance is then compared against the resistance value stored in the Module.
If the key pellet is the proper resistance, terminal A4 is grounded, energizing the Starter Enable Relay. At the same time, a signal is applied at terminal A3 to enable the Powertrain Control Module (PCM). When this signal is received by the PCM, it allows fuel injector pulses to begin.
If the key resistor pellet is the wrong value, the PASS-Key Decoder Module will shut down the PCM and de-energize the Starter Enable Relay for three minutes. The vehicle cannot be started. During this interval there will be no output at terminals A3 or A4. The SECURITY Indicator will remain on for three minutes whether the Ignition Switch is ON or OFF. The PASS-Key Decoder Module will continue this process even if a key with the correct pellet is used to turn the ignition back on.
Once the Timer has completed its three minute cycle, the SECURITY Indicator will go out, and the PASS-Key Decoder Module and timer are reset. A key having the correct resistor pellet can then be used to start the engine.
The SECURITY Indicator is controlled directly by the Pass Key Decoder Module. If the pass key is actively preventing the vehicle from starting, this Indicator will be grounded. When the Ignition Switch is first placed in RUN, BULB TEST or START, the Indicator Lights for about five seconds as a bulb check.

NOTE:
If the Pass Key Decoder Module must be replaced, there is no special procedure necessary to program the new PASS-Key Decoder Module to the resistance value of the existing ignition key. The new PASS-Key Decoder Module will automatically program itself to the resistance of the key used for its first ignition cycle.
If the wiring or contact to the key resistance pellet are defective, or a defective key is used for the first ignition cycle after a new module has been installed, the new PASS-Key Decoder Module will not program a key code. The SECURITY Indicator will flash at a rate of once per second while the Ignition Switch is ON, indicating that the module did not program and the system wiring, contacts, and ignition key should be checked.