Welcome to our database on the GM Passlock II system.

This page covers in detail how the system works, what it does (and does not do), and details about maintenance and bypassing.
 GM Passlock II Basic description
 GM Passlock II System summary
 GM Passlock II Functionality
 GM Passlock II Malfunctions
 GM Passlock II Reprogramming
 GM Passlock II Bypass


Basic description

The GM Passlock II uses a normal key with no chips, resistors, or transponders in the key. The Passlock II system does not disable the starter. If your ignition is having trouble activating the starter the Passlock II system is not at fault (unless too many failed attempts are made in a row in some situations, see note later).

The ONLY part of the vehicle deactivated with the Passlock II system in a security situation is the fuel pump. This happens if the computer does not see a resistance match between the hardware sensor value in the ignition key cylinder and the resistance value stored in the computer as the 'expected' value. This has nothing to do with the key; the key only functions to rotate the cylinder to align the sensor.

Speaking in basic terms to compare this security system to an older style ignition system, what this type of system prevents is a ‘hotwiring’ that directly connects wires to start and run the engine by bypassing the ignition key. Performing this on a standard Passlock system will result in the engine starting, but then immediately dying as the computer cuts off fuel flow; not having seen the LOCK CYLINDER BEING TURNED in the ignition and thus ASSUMING the proper key has not been used.

Passlock II key and cylinder basic function

In the illustration, it can be seen that the sensor and magnet only align during the 'run' ignition position. There is no way for this system to prevent the actual starting of the vehicle; only to stop the engine once the key has returned to the 'run' position after starting. If the key returns to 'run' from 'start' and the computer does not see the resistance it expects to see, the fuel pump is shut off to stop the engine.

Note: depending on the vehicle or computer, there may be a maximum number of cycles where Passlock deactivating the engine too many times in a row will eventually deactivate the starter cicruit for a set time period. This is the only effect the Passlock II system can have on the starter system. This can usually be overcome by pulling relays, fuses, or battery power to reset the timer.

Since the computer judges whether the cylinder has been turned (and thus the proper key used) by the Hall effect sensor and magnet, all we have to do is bypass this circuit with the proper resistance so the computer thinks this has happened. There is really nothing else involved with the system.

What GM hoped to accomplish with this system is not true security, but only making the vehicle more difficult and time-consuming to steal. Obviously the system is easy to overcome for those with some knowledge, but it takes time to find the proper resistance and take the extra wiring apart and so on. So this is what makes the system more effective than standard ignitions: the time required to overcome it, not necessarily the difficulty. Making a vehicle take longer to steal makes it a lot less desirable to thieves, in theory.

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System summary

The GM Passlock II system is not a transponder system. It uses 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 running. This is the only security affect provided by the system.

The sensor reading, if in approximately 5-10% 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 continue running. If it sees an incorrect value (or no value) it deactivates the fuel pump power circuit. 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).

Inserting the properly cut key will align the lock tumblers and retract the cylinder lock bar, allowing the cylinder to rotate inside its casing and, in turn, rotate the electrical ignition switch on the far end of the cylinder. This functionality remains the same between Passlock II and standard ignition cylinders.

The Passlock II key is a standard double-sided GM key, with no markings on the shaft. The end part (key ring slot) of the key has a long oval hole.

Passlock II key and cylinder

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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 inserting and turning a screwdriver inside the cylinder casing. To remove the cylinder from the casing, 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.

Passlock II cylinder and casing

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 an electrical circuit at certain points in the cylinder's rotation and opens an electrical circuit in others. This allows the vehicle to determine if a key is inserted in the ignition.

Passlock II cylinder and casing

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 engine operation. Different size magnets will produce different reference voltages, providing a variance between different lock cylinders so that they are not all directly interchangable without reprogramming the computer for a new expected resistance value.

Passlock II lock cylinder

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 engine will be allowed by the computers to run.

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Although 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, very 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.

Intermittent issues can sometimes be due to severe temperature differentials or changes, and wiring issues. Depending on where the system is bypassed, a bypass may not fix these issues without running over the entire harness to look for anything that might alter the wire resistance such as fraying wiring.

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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 (which also works, but is pretty much self-explanatory if you have the tool).

The relearn procedure might be required when power is removed from the BCM for a length of time; sometimes when the BCM is disconnected/re-connected, sometimes when the battery completely discharges or is disconnected, sometimes when BCM power fuses are changed or removed. This seems to vary by vehicle or situation.

The relearn precedure will always be required when a different lock cylinder and/or casing installed, or when a new BCM is installed.

Note that FACTORY programmed BCM's will automatically learn the cylinder reference voltage at the first key turn after the first 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.

Also note that there is no 'factory default' or 'initial value' for a Passlock II reference voltage. A new or reflashed BCM always requires a relearn.

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.

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Contrary to popular belief, it is extremely easy to permanently bypass the Passlock II sensor system as long as the proper 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 a resistor within a proper range and do a relearn. If you do want to try to avoid a relearn, measure the resistance between the yellow wire and ground with the ignition switch in the RUN position, and match as closely as possible. There is a tolerance, but this can vary with temperature and other conditions, so we recommend as close a match as possible to prevent future issues.
If not worried about matching the existing resistance, choose a resistor, or series of resistors to get a value in the proper range. 500 Ohms to 10,000 Ohms seems to be the best workable range, based off of existing VATS data. The recommended wattage is 1/4 to 1/2 Watt. This is the power rating of the resistor. This circuit is not extremely high-amperage and is only intended as a reference signal, so a high power rating is not necessary.
The final value stored in the BCM is the voltage that results from the drop through the resistor. So if the starting reference voltage is 5v, then ideally a final output roughly between 1 and 4 volts final is a good bracket. If you have a GM Tech2 tool, the existing value can usually be read off the BCM data. Keep in mind this is the resulting final voltage and not the resistance that needs to be placed in the circuit to acheive that value.
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.
Passlock II bypass
Run through the relearn procedure. The lock cylinder is now permanently bypassed for the Passlock II system, although the relearn procedure may still be necessary in the future if the BCM loses the stored value.

Also contrary to popular belief, it is possible to bypass the Passlock II system if you have lost 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 should only be used when all other methods are exhausted.
This method depends on mainly defeating the mechanical portion of the switch by drilling out the lockbar so the cylinder can be rotated freely. We recommend approximately a 1/4" diameter drill bit centered about halfway between the inscribed circle in the black plastic of the cylinder and the key insertion hole as shown in the picture below. Drill at increasing depth, stopping to check if the cylinder yet rotates before drilling deeper.

The cylinder can then remain in place and be rotated without a key; nothing further needs done as the key itself need not be present for the Passlock system to function.
Passlock II bypass
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.

Note that if you want to operate the ignition without ever having to use a key, and you already have a key, merely remove the cylinder, take out the lockbar, and reinsert the cylinder. Without the lock bar no key is required for operation. Although this defeats ALL the anti-theft systems and we do not recommend it.

Because Passlock II only deactivates the fuel pump, another way to completely bypass the Passlock II system is to hotwire the fuel pump to run off a manual switch. This is inadvisable because the pump is too easily left on with the engine not running which can result in the pump being burned out. But it can run the vehicle's engine in an emergency.

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