General Motors (GM) has used three different theft deterrent systems, but they are all passive, meaning its activation is automatic. It's operated by communication between a control unit and the ignition key through the ignition switch. If the switch is vandalized, if it's operated with the wrong key or if the Class 2 communication link is disrupted, the engine won't start.

Any time the theft-deterrent system causes an unwanted no-start, chances are it's a simple communication breakdown. To find it, you need to understand how the system detects theft and how it prevents engine starting. What you will find here is information about GM's Passkey I and Passkey II theft deterrent systems, along with a few tips provided by our Troubleshooting Editor Roy Ripple.

We've concentrated on these systems because Roy tells us these are the systems that generate the most help requests. He also says the problem is usually just a faulty connection, botched accessory installation or even just a worn out key or ignition lock cylinder. These can be easy to troubleshoot, because all you need is a wiring diagram, a DVOM and knowledge of how the system is supposed to work.

OPERATING STRATEGY

The Vehicle Anti Theft System (VATS) was first installed on the 1985 Corvette. The Personal Anti-theft Security System (PASSkey) replaced VATS in 1988, and while there are some differences, VATS and Passkey I and II all work the same way. There is no radio communication involved; the system merely looks for the ignition key's unique electronic signature. Passkey III and the newer Passlock systems were introduced in 1998, but the earlier systems weren't completely phased out until 2003. The easiest way to tell the difference is by the appearance of the key.

The older Passkey systems all have a pellet imbedded in the ignition key that communicates with the Theft Deterrent Module (TDM). On VATS and Passkey I, the TDM is hidden deep in the dashboard. On Passkey II, the TDM is built into the Body Control Module (BCM). The key's risible pellet is a resistor, and you can read its value by touching meter probes to the contacts on either side of the pellet. With the key in the ignition, the pellet touches contacts in the lock cylinder. These contacts are wired to the TDM, which reads the value of the resistor to identify the key. If the resistance is correct, the TDM sends a pulse-width modulated signal to the PCM, which then enables the fuel injectors. The TDM also operates the Theft Deterrent Relay (TDR) that allows power to reach the starter solenoid when the key is turned to the START position.

If the TDM doesn't recognize the key pellet, it decides a theft attempt is in progress and it will go into anti-theft mode. In this mode, the TDM will:

* Turn on the "Security" warning light on the instrument panel.

* Prevent starter operation by not turning on the Theft Deterrent Relay.

* Send a signal to the PCM telling it to disable the fuel injectors.

* Disable the key recognition circuit for three minutes.

This last point is important to people who own more than one GM vehicle. With multiple GM keys on the same key ring, accidentally inserting the wrong key in the ignition will prevent start-up, and the TDM waits three minutes before it will read the key circuit again. Also notice that the PCM is looking for a specific "go" or "no-go" signal from the TDM before it will decide whether or not to operate the injectors. The absence of a go signal is not a valid no-go signal, and vice versa.

If the TDM decides a theft attempt is in progress when the engine is already running, it will not stop the engine, but it will turn on the "Security" light and enter anti-theft mode at the next key cycle.

If the starter is operated without the key in the lock cylinder, such as with a remote starter switch, the TDM will interpret that as a theft attempt. Likewise, if there is a short to power or ground in the key recognition circuit, it is interpreted as a theft attempt. This can happen if the wiring in the steering column is damaged during theft, repair or installation of electronic accessories.

Either way, if the correct key is not in the ignition switch when the key recognition circuit fails, the TDM will turn on the "Security" light and enter anti-theft mode. If the correct key is in the cylinder and that circuit fails while the engine is running, this is considered a malfunction, not a theft attempt. The "Security" light may turn on to warn of a system malfunction, but even if it doesn't, a key recognition circuit failure with the proper key in the lock will cause the TDM to enter what GM calls a "fail-enable" mode. Essentially, the theft-deterrent system goes offline and the engine will start and run with any key that turns the lock.

KEYS

In the original VATS system, the key's resistor pellet and the TDM were programmed together at the factory. In the Passkey systems, the TDM will memorize the resistance of the first key inserted into the lock. There are 15 different resistance ranges or codes, and all are available as key blanks from GM dealerships. Within each range, the actual resistance may be more than 100 ohms different from the resistor's nominal code value, and even though a TDM will memorize the resistance of the first key inserted into the lock, it will recognize a key pellet in the same resistance range. Whether the TDM is a separate unit or built into the BCM, that key resistance range memory cannot be reprogrammed. The resistance range of that first key is what it will look for every time.