GM Passlock–Diagnosing and Fixing GM Anti-theft related no-starts
The GM Passlock system is fairly simple, especially when compared to other makes. However, if yours is goofing up, you won’t think it’s so simple.
Here’s how it works: When you first turn the key, the PCM provides power to the fuel pump, injectors, and ignition system. You can crank and start the engine. However, there’s a magnet built into the lock cylinder. As you rotate the key, the magnet passes over a Hall Effect sensor—a sensor that detects the presence of a magnetic force. The magnet produces a unique voltage when it passes over the sensor. The sensor sends this voltage signal to the Body Control Module where it is compared to a stored value. If the sent voltage doesn’t match the stored value, the PCM shuts down the fuel injectors for a minimum of 10 minutes and, well, you’re pretty screwed at that point. If you try to start the car again, you will see a blinking Security light. That’s your sign that the Passlock system has shut you down.
What goes wrong: GM vehicles experience a very high failure rate in the lock cylinder and lock housing mechanisms. Fortunately, they’re fairly inexpensive and easy to replace.
Testing the Passlock sytem: Remove the plastic cover shrouds from the steering column near the lock cylinder. Inside you will see the Passkey module surrounding the lock cylinder. It will have 3 wires coming out of it. Turn the key to the ON position and use a DIGITAL multimeter (Do NOT use an analog meter) to backprobe the probe the black and white wires. If you see 12 volts on the white wire and 0 volts on the black wire, you can proceed to the test for the Passkey module. If you do not see those voltage values, you have a wiring problem between the connector and the BCM.
Next, connect your voltmeter to the yellow wire. It should read 5 volts. Keep the probe connected and turn the key to the off position and back to the start position. The Passlock Data Voltage has a potential of 10 different values, ranging from .86 to 4.28 volts. The voltage should drop to a voltage level between those values and stay within .02 volts of the initial reading for at least one minute. Repeat the test several times—you must receive the same voltage for each test. If you do not, replace the lock cylinder and the Passkey housing.
To remove the lock cylinder, first disconnect the negative battery cable and allow the airbag to power down for 5-10 minutes. Slide the Passkey module off the cylinder housing. Insert the key and turn the lock cylinder to the start position. Place a 1/16” allen wrench into the hole on top of the lock module to release the steering column lock cylinder set pin. Then turn the lock cylinder to the RUN position and pull the lock cylinder out. Reverse the procedure to install.
After you replace the cylinder and Passkey module, you will have to conduct a relearn procedure for the BCM to learn the new voltage value. Here’s the procedure: Lock doors and unlock. Clear codes. Turn key and bump starter, leave on for 10 minutes. After a few minutes the dash light will go out. At that point, turn the key to the off position for 90 seconds to 3 minutes. Repeat this procedure 3 times for a total of 30 minutes and on the 4th try the car will start.
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© 2007 Rick Muscoplat
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Amazing Article!!! I followed the instructions to reset my passlock parameters and it worked just as it was written. Thanks!!!