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Home News Niche of Time: GM VTD Systems & Relearn Procedures

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As we all know, in the world of automotive technology, things keep changing. In the aftermarket, we tend to feel these changes a little later than the OE dealers. That being the case, it’s no surprise that it has been only recently that many technicians are feeling the sting and frustrations caused by OE anti-theft devices and vehicle theft deterrent systems, also known as VTD.

Many technicians have blamed a newly installed aftermarket replacement PCM for a no-start condition only to find out later that the vehicle in question was equipped with VTD. Lots of time, money and patience have been spent solving a problem that shouldn’t have existed had the technician checked if the vehicle was equipped with VTD. When dealing with these vehicles, driveability repair procedures may require one more step to be taken before handing the keys back to the customer, and that is a VTD/Anti-theft Relearn procedure.

While each manufacturer has its own way of incorporating VTD into the various models, the concept of “password sharing” among computer control modules within the vehicle is pretty common across the board. This prevents an unauthorized start-up of the engine. In other words, if the proper ignition key is not used to start the vehicle, the vehicle will shut down and the engine will become immobilized for a certain duration of time, usually from four to 10 minutes. General Motors started using VTD systems in the mid ’80s beginning with the Corvette. By 1998, only a handful of vehicles made by GM left the factory without VTD. What this all means is that performing Theft Deterrent Relearn procedures will now become common practice when replacing a PCM, BCM, VTD control module, ignition key, ignition lock cylinder or instrument cluster in late-model GM vehicles.

All of the VTD systems used by GM can be categorized into three groups. An easy way to remember the three groups is to think of them as either the “Resistive-Chip Ignition Key” type, “Coded Lock Cylinder” type or the “Transponder Ignition Key” type. The first group, the Resistive-Chip type, includes those vehicles equipped with the VATS system, (see Figure 1) Passkey system or Passkey II (PK2) system. The second group, the Coded Lock Cylinder type, includes those vehicles equipped with the Passlock system (see Figure 2). The third group, the Transponder Key type, includes those vehicles that have the Passkey III systems (PK3 and PK3+) (see Figure 3). Each of these three groups of VTD systems shown in the schematics (Figures 1-3) has a unique way of identifying whether or not an authorized key is used to start the vehicle. The first group identifies the proper resistance code on the ignition key. The second group identifies a security code issued by the lock cylinder, and the third type communicates with a transponder contained within the ignition key.

Figure 1

Figure 2

Figure 3

VATS, Passkey & Passkey II
(Resistive-Chip Ignition Key)

The VATS and early Passkey systems utilize the PCM, resistive-chip ignition key and lock cylinder, and either a BCM or VTD control module to prevent unauthorized start-up. The prominent feature of these VTD systems is the small resistive chip visible on the steel shank of the ignition key.

There are 15 different resister chips that may be used on any given key type. When replacing an ignition key on this type of system, you must make sure that the new ignition key has the same resistance code as the original key. When the proper ignition key is placed in the lock cylinder, the resistance code of the chip is read by either the VTD control module or BCM (whichever the vehicle was equipped with). With the proper resistance code, the VTD module (or BCM) would then send a signal to the PCM enabling the fuel system and allowing the vehicle to run. Vehicles equipped with this type of VTD system require no additional relearn procedure when replacing any part with the exception of those models equipped with a BCM in place of a VTD control module. VTD control modules learn the ignition key resistance code upon initial ignition power-on following installation. Nice and simple. However, if the vehicle is equipped with a BCM in place of the VTD control module, the VTD relearn procedure may be required. If the BCM does not auto-learn, the security indicator lamp on the instrument cluster will illuminate and a DTC will set indicating the need to perform the VTD Relearn procedure.

Passlock (Coded Lock Cylinder)
The Passlock system utilizes the PCM, instrument cluster and a coded ignition switch lock cylinder to prevent unauthorized engine start-up. The Passlock system does not utilize a resistive-chip ignition key. Instead, the ignition switch lock cylinder assembly creates the proper code to start the vehicle. The Passlock lock cylinder contains a Hall effect switch internally that reads a profiled ring of notches. If you picture a crankshaft sensor reading a profiled ring gear on the crank, you get the idea.

When the proper ignition key is inserted into the Passlock lock cylinder and turned, the Hall effect switch sends the proper pulse-train to the instrument cluster for processing. If the instrument cluster determines that the pulse-train sent by the lock cylinder is the correct one, it then sends a fuel-enable command to the PCM allowing it to run the engine. Vehicles equipped with the Passlock system will require a VTD relearn procedure to be completed after installing a new PCM, instrument cluster or lock cylinder.

Passkey III (Transponder Ignition Key)
The Passkey 3 systems (Passkey 3 and 3+) utilize a special ignition key that is equipped with a built-in transponder in the plastic key head. These ignition keys have PK3 stamped into the steel shank. Along with the transponder ignition key, the system uses a VTD control module, transponder key exciter/reader and the PCM to prevent unauthorized engine start-up.

Vehicles that have steering column-mounted ignition switches have the VTD module mounted on the steering column. This type of VTD module contains the transponder exciter/reader within it. On vehicles equipped with dashboard-mounted ignition switches, the transponder exciter/reader is external to the VTD module and connected to it via the wiring harness.

When the ignition key is placed in the ignition switch lock cylinder and rotated, the exciter/reader energizes the transponder within the key and reads the code. There are approximately 3 trillion different codes possible with Passkey 3 systems. Once the code is read by the exciter/reader, it is sent to the VTD control module for processing. If the proper code is received, the VTD module sends a fuel-enable command to the PCM allowing the engine to run. Passkey 3 systems are capable of learning up to 10 different ignition keys. When programming a new key, always begin the programming with an authorized Master ignition key first. The heads of the Master keys are black in color. The valet keys have a gray head. With the Passkey 3 systems, performing the VTD relearn procedure is necessary if any of the VTD system components are replaced.

The VTD Relearn Procedure
The purpose of the VTD Relearn procedure is to allow the VTD components to share and learn the password that is used between the control modules to allow authorized engine start-up. In order for the proper amount of time to elapse for this to happen, the systems use the security indicator lamp on the instrument cluster to indicate whether the modules are communicating or not. When performing the VTD Relearn procedure, it is imperative to note the status of the security indicator lamp on the instrument cluster before proceeding to the next step. If the security indicator lamp is illuminated during a particular step in the relearn procedure (or flashing), the system is indicating that it is still busy communicating with other modules. Once the security indicator lamp turns off, communication is complete and you are free to proceed to the next step in the process. It is extremely important not to interrupt this communication, otherwise the procedure will not work, and the car will not start.

Once all of the components in the vehicle are installed and properly connected, check the battery voltage. The vehicle must have a fully charged battery. If the vehicle in question is equipped with Daytime Running Lamps (DRL), remove the DRL fuse or the headlight fuse to prevent excess battery drain during the procedure. Also make sure the radio and blower fan (climate control) are turned off.

  1. Insert the Master ignition key and turn the ignition on. Do not attempt to start the vehicle.

  2. Clear any DTCs with a scan tool, and then disconnect the scan tool.

  3. Bump the starter over by rotating the ignition key to Crank, but do not start the vehicle. Allow key to return to the Run position.

  4. Note the security indicator lamp. It may flash for a few seconds, then turn on solid. It will remain on for approximately 8 to 15 minutes. This is the first password communication stage.

  5. When the security indicator lamp turns off, turn the ignition key to the Off position. Wait 10 seconds.

  6. Turn the ignition key to the Run position. Note the security indicator lamp. It may flash for a few seconds, then turn on solid. It will remain on for approximately 8 to 15 minutes. This is the second password communication stage.

  7. When the security indicator lamp turns off, turn the ignition key to the Off position. Wait 10 seconds.

  8. Turn the ignition key to the Run position. Note the security indicator lamp. It may flash for a few seconds, then turn on solid. It will remain on for approximately 8 to 15 minutes. This is the third password communication stage.

  9. When the security indicator lamp turns off, turn the ignition key to the Off position. Wait 30 seconds.

  10. Turn the ignition key to the Run position, pause momentarily, then turn the key to the Crank position. If the engine starts and runs normally, the VTD Relearn procedure is complete and you can check for any remaining DTCs using the scan tool. If the vehicle stalls, simply turn the ignition off and try restarting the vehicle. It should restart and run normally.

If the vehicle does not restart and run normally, use the scan tool to pull up any DTCs, including any VTD diagnostic trouble codes. If the security indicator lamp is illuminated, or if there are any DTCs present, you must perform the appropriate diagnostics before trying to do the VTD Relearn again. Sometimes it’s as simple as a connector not being tightened down enough in the harness.

Passkey 3 Ignition Key Relearn Procedure

  1. Insert the proper Master key in the ignition and turn the ignition switch to the Run position. Do not start the engine. Wait 10 seconds.

  2. Have the next key to be learned at the ready. Turn the ignition switch off and remove the Master key (black key head).

  3. Insert the key to be learned within 10 seconds of having removed the Master key and turn the ignition switch to the Run position. The security indicator lamp will illuminate momentarily (it can be so quick, it may barely be noticeable). When it turns off, the key relearn is complete.

  4. If any additional keys need to be relearned, simply repeat the above for up to 10 keys in total.

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Dan Maslic

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