Passlock – How it works
The Passlock anti-theft system disables the engine unless you have inserted the proper key. The diagram shown here is for vehicles with a steering column mounted ignition cylinder. Late model GM vehicles may have a dash mounted cylinder, but the system is similar. In late model vehicles, GM move the theft logic to the Body Control Module instead of the instrument panel cluster.
There are 3 main components to the system:
1) Passlock lock cylinder (R)
2) Powertrain Control Module (PCM)
3) Instrument Panel Cluster (IPC) or Body Control Module (BCM)
When you insert a key into the lock cylinder and turn it, the cylinder rotates a magnet past a stationary sensor to create a signal. The signal, referred to as a “Resistance Code” (R-code) is sent to the IPC (BCM in later GM vehicles). The IPC contains a computer, stored R value, logic, and data transmission circuitry. If the IPC determines that the R-code is correct, it sends an encoded password to the PCM. The PCM allows the fuel injectors to operate.
The R-code must be read by the IPC within a preset time period or the Vehicle Theft Deterrent (VTD) system will begin “Short Tamper Mode.” When Short Tamper mode kicks in, the engine will run for about 4 seconds and the “THEFT” light will flash. Then the PCM will cut off the injectors and the engine will quit. The user can attempt to start the vehicle a total of 3 times before the system goes into Long Tamper Mode. During Long Tamper, the PCM will shut off the injectors for 10 minutes and flash the THEFT light. The user may be able to crank the engine, but it will stall due to fuel starvation.
The most common failure points on this system are the IPC and the lock actuator (housing the magnet and sensor).
The IPC or BCM provides 5 volts to the sensor on the white wire and ground on the black wire. Start your diagnostics by checking for power and ground on these wires. Then check the voltage on the yellow wire while turning the key. It should go down as you turn the key. If it does not, replace the sensor. Repeat the key-turn procedure and continue to check that the voltage drops to the SAME value with each key turn. If it does not, replace the sensor.
RELEARN Procedure.
If you replace the IPC or BCM, you must perform a relearn procedure.
1) Try to start the vehicle. If it runs and quits, go to step #2. If it continues
to run, make sure the THEFT light turns off after 10 seconds. If the light stays on, there is another problem with the system.
2) The THEFT light should be flashing. Leave the ignition switch in the RUN
position until the THEFT light glows solid (10 minutes—stick around and use your watch to time this). Turn the key to OFF. Then try to start the vehicle again. It should now operate.
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© 2007 Rick Muscoplat
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