GM Security Systems
If you’ve worked on any GM vehicles over the past 20 years, most likely you have encountered one of their security systems.
Passkey, one of GM’s most popular earlier systems, used a pellet in the steel shank of the key with wires routed through the steering column. Those wires often broke at the key cylinder in the steering column. Another popular earlier system, Passlock, used a sensor located in the key cylinder housing and a magnet on the key cylinder. The Passlock sensor was also less than dependable. Both systems had wiring that could be tested. The injector pulse was disabled and depending on model and year, some security systems disabled engine cranking and injector pulse. No security systems disabled spark.
One of GM’s newest systems is the Passkey III, also known as PK3. The difficult part of the PK3 system is that very few wires are used, and system diagnosis is primarily done with the scan tool. A chip is embedded under the rubber cover of the key – this is the transponder. The receiver surrounds the key cylinder and is often referred to as the PK3 module, Theft Deterrent Module (TDM) or Vehicle Theft Deterrent Module (VTD). The Theft Deterrent Module will simply have one or two power supplies, a ground and data wiring for communications with the vehicle.
For troubleshooting, scan for trouble codes in all modules and document. Clear all trouble codes and note what code(s) reappear. The reason for scanning all the modules is there may be a communication code stored in another module indicating a problem with the TDM. Next, scan the TDM. Locate the parameter regarding the key, and then verify a transponder key is present and that a valid key is indicated. If the key is present but invalid, perform a key learn.
If the key is valid, that key information, referred to as a password, has to be sent from the TDM through the vehicle’s data network to the Engine Control Module (ECM). Once the password is accepted by the ECM, it will then send the key confirmation back to the TDM. The parameter in the TDM that confirms the password has made it to the ECM, has been accepted and reported back to the TDM is labeled “PCM Authentication Status.” A successful transmission of the password will indicate “VALID.”
If the key data is good but the authentication status is ‘INVALID,’ program the theft deterrent system and verify there are no communications problems with the vehicle, particularly between the TDM and the ECM.
Now for repair: If a component is replaced, programming is needed. This is done using AC Delco’s TIS2Web programming website. Some models offer manual programming very similar to the Passlock programming procedure.
Even if you perform all the steps correctly, programming may still be unsuccessful. It is my opinion that if you are having trouble programming, you most likely are using a key that the vehicle has seen before. If the vehicle sensed a problem in the past with that key, often it will not allow the vehicle to start with that key. Programming the system with a key the vehicle has not seen before often improves success. Adding additional keys is relatively easy, including the key that was previously used.