Since the Mid 1980’s, many GM cars have factory fitted Anti-theft systems. Let’s start by looking at the different systems in use and the cars that you might find them in.

VATS - the first system that appeared on the 86 Corvette, uses a resistive pellet in the key attached to a pre-programmed module. It will disable the starter circuit and injector pulse during cranking.

PASSKey I & II - Same as VATS except module can “learn” key pellet on first power up after installation. PassKey II disables injector pulse after startup.

PASSKey III - Uses key and transponder module in column. PCM disables the injectors and the starter enable relay. Module or keys can be “learned” after replacement.

PASSLock - Disables the injectors approximately 1-2 seconds after startup. It has no effect on the start circuit. May be controlled by the IPC (Instrument Panel Control), BCM (Body Controller), EVO (Electronically Controlled Orifice steering assist) and other systems.

The system that most people are liable to come across is the original VATS (Vehicle Anti-Theft System)–also know as PassKey so here is a breakdown of how this system works and the key components it contains.

First introduced in 1986 on the Corvette and subsequently added to the Camaro/Firebird in 1988, the VATS system has two functions: first, it has a starter interrupt feature; second it can disable the fuel injectors if the system does not provide the correct signal. The system is made up of four components:-

1) The Ignition Key: This is a special key with a resistor pellet in it. There are 15 different key blanks of this type, each with a different resistance.

2) The Ignition Lock Cylinder: This looks and functions exactly like a conventional lock cylinder, except for a pair of contacts that connect to the resistor pellet in the key and a pair of leads that connect to the vehicle wiring.

3) The VATS Decoder Module: This solid-state component checks to see that the correct key has been inserted, by measuring the resistance value of the circuit. If correct, the vehicle can be started.

4) The Starter Enable Relay: This relay is connected between the ignition switch and the starter solenoid. It allows battery power to the solenoid when the Decoder Module energizes it. Chances are, if the vehicle’s starter does not turn, this is because the decoder is not sending the “OK to start” signal to the vehicles Electronic Control Module (ECM).

If a vehicle is experiencing a “No Crank” condition because the Pass Key decoder module is not sending the “OK to start” signal to the ECM, first, look for a flashing or illuminated “Security” light. This is an indication that the system is disabling the starter. Remember, even if you supply power to the starter solenoid, the vehicle will not start, because the ECM will shut down the fuel injectors until it gets the proper signal from the Decoder Module. The most likely component to fail is the ignition lock cylinder. Make sure that the ignition key is not damaged, or missing its pellet. Second, measure the resistance across the ends of the key pellet.

Next, locate either a 2 wire connector at the base of the steering column, or a 48 wire connector under the dash. If you have a 2 pin connector, which is usually orange (don’t confuse the yellow air bag connector for the lock cylinder connector), disconnect it. Now, insert the ignition key and turn the key to the “Start” position while measuring the resistance across the two wire connector leading up the steering column. The resistance reading should match the key resistance throughout the entire range of lock cylinder rotation. Any change will cause a no start situation. If you have the 48 wire connector , the test is performed in  the same manner . This connector has 5 rows of terminals marked “A” through E”. Locate the last two terminals in row “E”. As before, if the ohmmeter indicates a change in the resistance reading, the lock cylinder is faulty.

Another method is by checking the voltage from the Decoder Module to the lock cylinder. Normally the module output is approximately 5 volts. The resistance of the key will drop this voltage to a certain value. If you see a constant 5 volts while back probing this circuit when the key is turned, or any large change in the voltage reading, this indicates that the cylinder needs to be replaced. Any of the other components in the Pass Key system can fail, however, the lock cylinder is usually the cause of most problems.

If the decoder cannot “see “ the key, then the system will not allow the vehicle to be started. It’s very unusual for the decoder module itself to become faulty – except perhaps if it has been subjected to a “voltage spike” or other problem that has occurred within the vehicle’s wiring. However, it’s not unknown for the key to require replacement due to loss or damage. Provided an original is available, there’s a simple way to find a replacement by measuring the resistance of the embedded pellet.

There are, in fact, only fifteen different replacement keys available. These cover a wide range of resistances and for each key there is a degree of tolerance. Any modern digital multi-meter should be quite capable of providing an accurate figure and therefore the correct replacement blank.