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PassKey GM

Diagnose a PassKey system problem

The antitheft system used in many ‘90’s vintage GM vehicles was named PassKey. Passkey is an acronym for Personal Automotive Security System. GM used several variation of PassKey. This article describes the operation and diagnosis of PassKey II.

PassKey II consists of a square head ignition key that contains a resistor.

See the resistor pellet on the shaft of the key to the left of the plastic.

There are 15 possible different resistor values. The ignition lock cylinder contains a set of electrical contacts that connect with the resistor when the key is inserted into the cylinder. The antitheft system’s theft deterrent module (TDM) reads the resistance of the pellet resistor embedded into the key. If it sees the correct value, it enables certain starting features.

The TDM has outputs to the starter enable circuit, fuel

theft deterrent module

Theft deterrent module is located behind instrument panel behind glove box.

enable, and the security lamp circuit

The TDM has inputs from the resistor reading assembly located on the lock cylinder.

The TDM has input from the ignition circuit.

When you turn the key to START, the TDM receives power from the ignition input circuit. It then reads the resistor value on the key. If the expected resistance reading is not received by the TDM, the engine is prevented by starting. This is done by shutting off battery voltage from the ignition switch to the starter by the theft deterrent relay, and fuel to the injectors is stopped by the PCM. The antitheft system will then shut down for approximately 3-mins and the security light will light.

If the reading matches the value stored in the TDM it provides ground to the control coil of the theft deterrent relay. The relay closes and sends power to the starter solenoid. At the same time, the TDM provides signals the PCM to enable fuel pump operation. The PCM then activates the fuel pump.


On 1996 and later vehicles begin your diagnosis by checking for trouble codes stored in the computer.

First check the condition of the key. Look for a resistor pellet that’s cracked or dirty. Next, check the pellet electrical contacts in the lock cylinder. They should be silver in color. These contacts are a known trouble spot. Replace if they are discolored or you do not receive a good signal from a known good key.

Make sure the following fuses are good: Fuses, 10, 11, 12, and 39

Replace a PassKey theft deterrent module

Disconnect the theft deterrent key resistor connector at the base of the steering column. Insert the key into the lock cylinder. Using a DVOM, check the resistance being read from the key to see if it matches the expected value. Wiggle the key to see if the reading changes. If it does, replace the electrical contacts.

Broken wires from the lock cylinder down to the electrical connector are common, check the continuity and condition of those fragile wires.

Check the electrical connector at the theft deterrent module to make sure the connections are clean and secure.

Check for voltage drop on circuit 1829 that may be altering the resistance reading received by the TDM.

The theft deterrent module is self programming. It accepts the resistance value of the key inserted into the lock the very first time a TDM is installed in the vehicle. This can only be done one time.

Finally, check the transmission range switch (park/neutral) switch.

The TDM is located behind the right hand side of the instrument panel above the glove box. The theft deterrent module is located to the right of the TDM

PassKey pellet resistance values and key code


Key code         Resistance value in Ω

1                      402

2                      523

3                      681

4                      887

5                      1130

6                      1470

7                      1870

8                      2370

9                      3010

10                    3740

11                    4750

12                    6040

13                    7500

14                    9530

15                    11800

Programming a new TDM

  1. Install the new, nu-programmed decoder module.
  2. Insert one of key in the ignition lock cylinder and turn it to the “ON” position
  3. The security indicator lamp should light for about five seconds and then go out.
  4. Start the vehicle.

If the wiring or contacts to the Key Resistance Pellet or the key is defective or intermittent and a new module is installed, the Engine will start but the “SECURITY” Indicator will flash at a rate of one flash

per second until the Ignition Switch is turned off. This indicates that the module did not program and that the system components, wiring and contacts should be checked for a fault

©, 2015 Rick Muscoplat


Posted on by Rick Muscoplat

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