GM Passkey, Passkey II and VATS

Many GM vehicles use the Passkey II or VATS (Vehicle Anti-Theft System) system for theft deterrent. These systems all have an ignition key with a resistor pellet at the base of the key (as seen below). Diagnosing problems with this system is fairly simple if you understand how it works. The major components are the key with a resistor pellet, the lock cylinder, the Passkey decoder module and the PCM. On some systems, the decoder module is called TDM (Theft Deterrent Module) and on some Passkey II systems, the body control module (BCM) acts as the theft deterrent module. Newer systems started showing up in 1998, but the Passkey II system was not completely phased out until 2003.

How it works

The Passkey Decoder Module (or BCM with some Passkey II systems) monitors the resistance across the resistor pellet in the ignition key and then gives the PCM a fuel enable signal if the resistance value of the key is correct. There are two small wires that connect to the ignition lock cylinder. The decoder module sends a 5 volt reference voltage down one of the wires and the other wire is grounded inside the module. When the key is inserted, the resistor pellet in the key completes the circuit which causes the 5 volt reference voltage to drop to a certain value which depends on how much resistance the key has. If the decoder module decides the voltage value is correct, it then sends a 50Hz/5-volt square wave fuel enable signal to let the PCM know it is okay to allow the engine to start. With Passkey II systems that house the TDM within the BCM, the fuel enable signal is sent to the PCM via a class 2 serial data circuit. Failures will often involve no-crank, no starter engagement, or the engine may crank but not start,or the engine may start but then stall immediately. Trouble codes P1626 and P1629 are also very common.

Common Problems

One of the most common failures is the wiring to the ignition lock cylinder. The wires are very small and tend to break where they attach to the lock cylinder. Often, this will cause an intermittent no-start situation as the broken wire may have intermittent contact and allow the engine to start. There are two ways to check the wiring in the column without having to remove the lock cylinder.

Under the dash at the base of the steering wheel, look for a two pin connector with a purple/white and white/black wire. The two wire harness will have a bright orange or red sheathing where it routes into the steering column. Connect a voltmeter to the white/black wire at the connector under the steering column or, if it is easier, connect to the white/black wire at the decoder module as seen in the picture below.

Now rock the key back and forth and rotate it to all positions except off. If the vehicle has tilt steering, move the steering wheel up and down while turning the steering wheel back and forth. If the voltage value ever significantly changes, then the wires at the lock cylinder are probably broken.

These next two scope images show a good and a bad lock cylinder. The top picture (below) shows how the voltage remains steady at about 1 volt no matter how much the column is moved or the key is jiggled. The second image shows voltage fluctuate significantly as the ignition lock cylinder is disturbed by moving the column and rotating the key.

The capture below is from a 1994 Cutlass Supreme with an intermittent crank-no-start condition. One of the wires to the ignition switch was hanging on by a few threads. This would cause excessive resistance when rotating the key. Sometimes the resistance would be close enough that the decoder module would send a fuel allowed signal to the PCM, and other times it would not. Notice that there is a large spike as soon as the key is turned on. The decoder module did not like the value it saw at key on, therefore it locked out fuel for that ignition cycle.

Special thanks to Harvey Chan for providing these images.

In this next image, the resistor line voltage does spike a little bit at key on, but not enough to cause the decoder module to deny fuel. There are large voltage spikes when the key is turned from on to start, but this time it does not affect the decoder module because it has already made its decision to allow fuel when the key was first turned on.

NOTE: If the white/black wire measures at 5 volts, then one of the two resistor wires is probably completely broken.