Auto Service World
Feature   January 1, 2002   by Pat Sugar-Top Gun Technician

Under Lock and Key

GM pass key systems quick tests

Since the 1986 model year, GM has used the Pass Key system as an anti-theft device. It started using this system on the 1986 Corvette, and in 1988 added this system to the Camaro and Firebird models. Since that time, GM has added this system to many of its models.

This system has two functions: first, it has a starter interrupt feature; second, it can disable the fuel injectors electronically if the ECM does not receive the correct signal from the Pass Key system.

The Pass Key system consists of four components, they are:

1. The Ignition Key: This is a special key with a resistor pellet on it. There are 15 different key blanks of this type, each with a different resistance value (refer to Table #1).

2. The Ignition Lock Cylinder: This cylinder looks and functions like a conventional lock cylinder, except for a set of contacts for the resistor pellet, and a pair of leads which connect to the vehicle’s wiring harness.

3. The Pass Key Decoder Module: This solid state component checks for the correct key resistance value so that 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 starter solenoid when the Decoder Module energizes it.

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, here are a few tips to help diagnose this system. First, look for a flashing or illuminated “Security” light.

This is an indication that the Pass Key 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. First, make sure that the ignition key is not damaged, or missing its pellet. Second, measure the resistance across the ends of the key pellet. You should have a resistance value close to one of the values listed on Table #1. 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 in the resistance reading during key rotation will cause a no start situation. If you have the 48 wire connector (refer to diagram #1), the test is performed in the same manner. This connector has 5 rows of terminals marked “A” through “E”. Locate row “E”, terminals #12 and #13, which happen to be the last two terminals in row “E”. As before, if the ohmmeter indicates a change in the resistance reading, the lock cylinder must be replaced.

Another method in diagnosing this system is by checking the voltage from the Decoder Module to the lock cylinder. Normally the Decoder 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.

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