Passlock fixes

How to fix PassLock problems

This is one in a series of Passlock fixes articles. READ THIS ARTICLE FIRST

GM made several different types of anti-theft systems. VATS, PassKey, and PassLock. The troubleshooting and reset procedures are different for each. PassLock seems to have the highest failure rate. So if you have a key WITHOUT a chip in it, chances are it’s a PassLock system. This article describes how to troubleshoot and fix a PassLock system. But here are some more articles to help

Click here to see the resetting procedure for PassLock

Click here to see a typical PassLock wiring diagram

 IF you have a PassKEY system, click here to see the reset procedure

Security light on, Security light blinking, car starts and stalls, Passlock, how Passlock works, how to test passlock

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. During that time, the Passlock system checks to see if a sensor inside the lock cylinder case has provided the proper voltage to either the instrument panel cluster (IPS) in older vehicles or the body control module (BCM) in late-model vehicles. If the sensor voltage is correct, the IPC/BCM sends a coded message to the powertrain control module (PCM), informing it that it has received the proper voltage. The PCM maintains fuel and ignition, and you’re set to drive off. If the BCM does not receive the proper voltage, it will not send the coded message to the PCM and the PCM will immediately shut down the fuel and ignition systems.

The lock cylinder itself doesn’t really do much except confirm that a properly cut key has been inserted, and the tumblers then allow rotation. As the lock cylinder rotates, a magnet is moved past a Hall Effect sensor inside the lock cylinder case. The sensor detects the presence of the magnet. As soon as you turn the ignition key to ON, the IPC/BCM feeds battery power to the sensor. The sensor returns a 4.9-volt signal back to the IPC/BCM. Once you turn the key to START, the sensor adds resistance to the circuit, reducing the 5-volt return signal to between .86 and 4.28-colts. Each sensor is built with one of ten different resistance values. Each vehicle is different.

So, if you’re following so far, when you turn the key to ON, the IPC/BCM sends out 12-volts and receives 4.9-volts back. Then, it’s looking for a voltage reduction as soon as you rotate the key to START. If the IPC/BCM doesn’t see the reduction, or the returned voltage isn’t steady, the coded message won’t be sent and the car will start and then stall and the SECURITY light will flash. There’s a second sensing mode in the sensor to detect an attempt to simply pass a hand-held magnet across the sensor. In that case, the sensor will report a different return voltage which the IPC/BCM interprets as a “tamper” signal. If it sees that, it will not communicate a message to the PCM and the vehicle will start and then stall.

There are 4 main components to the system:
1) Passlock lock cylinder
2) Lock cylinder case
3) Powertrain Control Module (PCM)
4) Instrument Panel Cluster (IPC) or Body Control Module (BCM)

What goes wrong:

GM vehicles experience a very high failure rate in the lock cylinder case (with the sensor). Fortunately, they’re fairly inexpensive and easy to replace. However, these systems can also fail due to wiring problems and data line issues.

Testing the sensor:

Click here to see a wiring diagram.

Disconnect the electrical connector to the lock cylinder case sensor. Then using a digital multimeter set to DC volts, connect the red meter lead to the white wire pin in the supply connector and the black meter lead to the black wire pin. Turn the key to ON, but don’t start the engine. You should see 12-volts. If so, you’ve just confirmed that battery power is getting to the connector and that the ground at the connector is good. If you don’t see battery voltage, backprobe the white and black wires at the IPC/BCM. If you get power and ground there, you have an open between the IPC/BCM. Repair that open. If you’re not getting ground, check the ground connection for the BCM. If you’re not getting power, check the fuses.

Turn the key to OFF and reconnect the electrical connector to the sensor. Backprobe the yellow wire with the red meter lead, while connecting the black lead to ground. Then turn the key to on. You should see 4.9-volts. Then turn the key to START and the voltage should drop and hold at that reduced value. If you don’t see 4.9-volts during ON or a reduced voltage during START, suspect the wires going to the sensor or the sensor itself. The lock cylinder case with the built-in sensor is a fairly high failure rate item. After replacing the lock cylinder case, you must perform a relearn procedure.

Click here to see the relearn procedure.

Most people don’t know that the Passlock system is really designed to be diagnosed by a scan tool. Yes, it sets trouble codes. Click here to see the codes and their definitions.

© 2012 Rick Muscoplat

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