The GM Passlock is not a transponder system. It uses a previously a normal metal key and recorded voltage measurement through a Hall Effect sensor to determine if the lock cylinder is aligned and in place during vehicle starting. This is the only security affect provided by the system.
The sensor reading, if in about a 5-10% window of a previously recorded value, will allow the BCM (Body Control Module) to send a code to the PCM (Powertrain Control Module) allowing the engine to start/run. Other security is supposed to be provided by having the properly cut key to access the door and turn the ignition cylinder, since removing the ignition cylinder is supposed to deactivate the vehicle (more on this later).
The Passlock II key is a standard double-sided GM key, with no markings on the shaft. The end part of the key has a long oval hole.
The key is a normal double-sided GM key, the only function of which is to engage the tumblers correctly to allow the igntion cylinder to rotate.
The ignition cylinder works like a standard ignition, engaging the rotary switch with the end of the cylinder. Once removed, the ignition switch itself can be positioned by turning a screwdriver. To remove the cylinder, the key is inserted and turned to 'start' position, then a small pin or screwdriver can be inserted through the top hole to depress the spring-loaded retainer, then the cylinder can be pulled straight out.
On the rear of the casing is a key-retention solenoid, which prevents the cylinder from being rotated into 'Off' when in gear to prevent key removal unless the ignition switch is in park or neutral.
The ignition casing screws to the steering column, and contains the Hall Effect sensor and circuit plugs necessary for the ignition system to function. In order to access all the screws holding the casing to the steering column, the steering wheel and associated parts must first be removed.
On top and slightly to the rear of the casing is a 'key-in' normally-open pushbutton switch. This switch rides in a groove in the lock cylinder that closes it at certain points in the cylinder's rotation and opens it in others.
When the cylinder is rotated, the magnet slotted into the side comes into alignment with the Hall sensor. This changes the resistance in the sensor and sends a reference voltage to the BCM down the signal wire. If the pre-programmed reference voltage matches what is recieved within tolerance, then the BCM will send out the proper code to the PCM allowing start/run. Different size magnets will produce different reference voltages, providing the variance between differnt lock cylinders.
As long as the key functions mechanically in the cylinder, the ignition switch connections will always be made. Only the reference voltage from the Hall sensor determines whether the vehicle will start or not.
Allow the Hall sensor can fail, it is not a normal common failure point. Hall effect sensors are very simple in operation. The permanent magnet in the lock cylinder provides a magnetic interferance in the sensor proportionate to the strength of the magnet. This resistance lowers the voltage output to the reference wire by a certain amount, also proportionate to magnet strength.
Since the sensor is unlikely to fail or the magnet strength to change, the variables are mostly mechanical or electrical. If the voltage to the sensor falls, the output reference voltage will also fall. This is why a weak battery may sometimes prevent the vehicle from starting.
Mechanical failures are mostly limited to extreme temperature differences that can change the distance between magnet and sensor due to shrinking or expanding of the metals. This in turn also changes the resistance. Dirt or debris failures are uncommon and unlikely due to the sealed nature of the unit, although possible.
These variances are why there is a 5 to 10% tolerance built into the system. However extreme temperatures or very low battery voltages may fall outside these tolerances and prevent the vehicle from starting.
If the wiring to the system gets frayed or shorted or too worn, resistance in the wires will increase and also may allow the reference voltage to drop out of tolerance. This wear can be fixed with a recalibration or reprogramming sequence to learn the modified resistance, just as if a new cylinder with a different magnet were installed. However if the wiring is broken, the system will no longer function.
If a reprogramming/relearn of a new reference voltage is required, there is only one method without accessing the system with a GM Tech2 tool.
The relearn procedure is required when either the BCM is changed/disconnected, or the battery completely discharged or disconnected, or a different lock cylinder and/or casing installed, or BCM power fuses are changed or removed. There is no backup battery to store the value in the BCM, so any time power is disconnected the value will need re-learned.
Note that FACTORY programmed BCM's will automatically learn the cylinder reference voltage at the first key turn after installation, being pre-programmed up to step 5 before installation. In this way GM did not have to do any programming on the production line, just plug-and-play. BCM's programmed by other than FACTORY TOOLS will require a relearn procedure.
1) Turn the ignition switch to the 'Run' position. Wait several seconds, then turn the switch to 'Start' for several seconds, then release the key back to 'Run'. The security light should be on/blinking.
2) Wait approximately 10 minutes for the light to turn off. If the light does not turn off after more than about 12 minutes, something other than the Passlock system requires maintenance.
3) Turn the ignition to 'Off' and wait 5 seconds.
4) Repeat steps 1-3 two more times for a total of 3 cycles/30 minutes.
5) The BCM will now read and store the new system reference value on the next ignition cycle from 'Off' to 'Start'.
6) Start the vehicle and clear any codes in the system.
Contrary to popular belief, it is extremely easy to permanently bypass the Passlock II sensor system as long as the key for the lock cylinder is present.
The top plastic cover of the steering column must be removed to expose the wiring. The Hall sensor has a plug with three wires coming from it on the top of the lock casing. The sensor wire to the BCM is yellow. The others are battery + and ground.
With the ignition off, cut the yellow wire. At this point there are two options: match existing circuit resistance or just install fresh and do a relearn. If you do want to try to avoid a relearn, measure the resistance between the yellow wire and ground, and match as closely as possible. If not, just pick a resistor (value isn't terribly important, but about 50 Ohms works well).
Solder the chosen resistor to the end of the yellow wire going to the BCM. Solder the other end of the wire to the ground connection coming to the Hall sensor (usually the black or orange/black wire but verify on your wiring diagrams). Tape off or shrink-tube any exposed connections and replace the plastic column cover.
Run through the relearn procedure. The lock cylinder is now permanently bypassed for the Passlock II system, although the relearn procedure will still be necessary anytime power is disconnected from the BCM, such as battery disconnection or draining.
Also contrary to popular belief, it is possible to bypass the Passlock II system without the proper key, although it requires time and effort based on certain skills, and will permanently damage the lock cylinder and probably the casing.
This method depends on mainly defeating the mechanical portion of the switch. Drill out just the lockbar so the cylinder can be rotated freely. If the cylinder can remain and be turned without a key, nothing further needs done as the key itself need not be present for the Passlock system to function.
If the cylinder is destroyed in the removal process, the ignition switch can internally be rotated with a screwdriver, however the Passlock sensor must still be defeated. At this point it can either be bypassed using the first method above using a resistor, or removing the magnet (or that cylinder portion) from the cylinder and gluing it against the hall sensor in the proper position. Either way at this point if a relearn procedure is required the key is no longer necessary and no matter what the vehicle can still be driven.