Remote starting your vehicle offers convenience, especially in extreme weather conditions. For older General Motors (GM) trucks manufactured before 2008, a unique method of remote start installation often involves the OBD2 port. This approach leverages the vehicle’s diagnostic capabilities, but it’s essential to understand how it works and its potential implications.
Initially, installers discovered that on these older GM models, remote start systems could be activated by commanding the truck into a diagnostic start mode through the data wire connected to the OBD2 port. This diagnostic mode is a built-in GM troubleshooting feature designed to bypass the Passlock system for testing purposes. The same J1850 data wire used by diagnostic scanners and programmers was being repurposed to initiate the remote start.
However, this “diagnostic mode” method isn’t ideal. The data wire’s capacity for simultaneous tasks is limited. Using it for both remote start commands and regular data communication can lead to conflicts and potential issues.
A more robust and recommended method for installing remote start on these older GM trucks bypasses the diagnostic mode altogether. This superior approach utilizes relays to simulate the key data that the truck normally receives when you physically turn the ignition key. Essentially, the truck perceives a standard key start, rather than being forced into a diagnostic state.
This relay-based system works by interacting with the truck’s Passlock system, an immobilizer designed to prevent theft. Trucks from this era (pre-mid-2007 change) employ a resistive immobilizer. Instead of a chip in the key, there’s a resistor within the ignition cylinder with a specific value known as the R-Code.
When a remote start is triggered using relays, the system first powers the status wire (-), activating relays upon ignition wire engagement. This action momentarily interrupts the factory R-Code wire and injects the R-Code value, often configured via a trim pot, directly into the Passlock module. This fools the system into thinking a valid key is being used, allowing for a normal engine start.
While this relay method is more effective for remote starting and avoids potential OBD2 communication issues, it does present a secondary challenge. The door locks and possibly door pin functionalities are often monitored through the same wire that needs to be manipulated for the relay system. Disconnecting this wire for remote start integration can disable these features.
To overcome this, installers need to tap into the analog door pin wires, typically using diodes, similar to the wiring process on older models like the 2001 trucks. Additionally, the door lock motors require a “5-wire” setup, often achieved with two relays or a specialized module like a 451M.
In summary, a properly implemented relay-based remote start installation for older GM trucks involves:
- Relays: A total of four relays (or two relays and one 451M module) to manage key data simulation and door lock integration.
- Diodes: Four diodes (or potentially tapping into the dome light wire as an alternative) for door pin wire integration.
- Trim Pot: A trim potentiometer to set the correct R-Code value for the vehicle’s Passlock system.
By using this relay method, the remote start system operates independently of the OBD2 port’s data wire after the initial installation. If any issues arise post-installation, they are unlikely to be related to conflicts with the OBD2 data system. For installers familiar with automotive electrical systems and remote start installations, this approach provides a reliable and non-intrusive way to add remote start functionality to these classic GM trucks.
