Auto Care Advice: GM Passlock Anti-Theft Systems
History of the GM Passlock Family of Anti-Theft Systems
In the mid 1990s, the first Passlock systems were introduced and are still used to this day. The Passlock I and Passlock II systems use a sensor in the ignition lock cylinder, which looks for proper rotation of the lock cylinder with the correct key. Upon successful rotation of the ignition lock cylinder, a code will be sent to the Passlock module.
The Passlock III system uses a transponder located under the plastic covering of the key. In most cases, these keys are stamped "PK3" on the metal blade near the plastic covering. An antenna located at the entry point of the ignition lock cylinder reads the transponder data from the key and sends a code to the Passlock control module. This system is very similar to the immobilizer system used by many other manufactures. Some higher end models use the key fob for the Passlock code and the starter will not turn unless a correctly programed key fob is present.
The Passlock III antenna and the Passlock I and Passlock II sensors in the lock cylinder are all commonly referred to as Passlock sensors. Upon receipt of the code from the sensor, the Passlock module will match the code received to previously learned values. If there is a match, the Passlock module will send a signal to the engine control module to allow the engine to start and run. The Passlock module is commonly part of the Body Control Module (BCM) or Instrument Panel Cluster (IPC).
What Does the Warning Light Mean?
Operating states of the warning light on Passlock systems should be:
- On for bulb check
- Off when engine is running
- On steady if a fault occurs
- Flashing in the learn mode
Under normal operation, you should see the warning light on during bulb check and off when the engine is running—no message displayed on the DIC.
When the system detects a fault, the warning light will illuminate or a message will be displayed on the DIC, depending on the model. If a fault occurs while driving, the engine may or may not start after turning the ignition off. If a fault occurs when trying to start the vehicle, it probably won't start.
The Passlock family has two basic relearn procedures—a ten-minute procedure and a thirty-minute procedure—and which one you use depends on the system and what components have been replaced.
The ten-minute procedure involves trying to start the engine. If nothing happens or the engine starts and dies, the security light will be flashing. Wait ten minutes. The flashing light should go off or illuminate without flashing. When it does, turn the ignition off, wait twenty seconds, and then try to start the engine. If it starts and runs, everything is okay. If you get the same result as previously—nothing happens or the engine starts and dies with the security light flashing—wait ten more minutes.
You will now begin the thirty-minute relearn procedure, which is just the ten-minute procedure repeated three times. After turning the ignition off for the third time and waiting for twenty seconds, the engine should start and run. If it doesn't, there may still be something wrong with the system.
After the learn procedure is completed and the engine is running, the security light may stay on steady for a few minutes and then go off—this is normal. Depending on the system, replacement of key components will necessitate that a relearn procedure be completed. These include the ignition lock cylinder, Powertrain Control Module (PCM), Body Control Module (BCM), or Instrument Panel Cluster (IPC).