For owners of older General Motors trucks, particularly those manufactured before 2008, integrating a remote car starter might seem like a straightforward upgrade. However, the technology used in these vehicles, especially concerning their On-Board Diagnostics II (OBD2) systems, requires a specific approach to ensure seamless and reliable remote start functionality. This article delves into how OBD2 remote starters operate in these classic GM trucks, highlighting the best practices for installation and troubleshooting.
The Diagnostic Start Mode and OBD2 Data Wire
In 2005 and earlier GM trucks (and models up to 2008), remote starting systems often utilize a unique method that leverages the vehicle’s diagnostic capabilities. These systems are designed to command the truck into a “diagnostic start mode” via the data wire at the OBD2 port. This mode is originally intended for GM technicians to troubleshoot and bypass the Passlock security system for testing purposes.
The same J1850 data wire used for diagnostic start mode is also crucial for scanners and programmers to communicate with the truck’s computer for data monitoring and reprogramming. Unfortunately, this data wire is not designed for simultaneous, dual-purpose operation. Directly utilizing the data wire for remote start and diagnostics can lead to conflicts and unreliable performance.
The Relay Solution: Simulating Key Data
To circumvent the limitations of the OBD2 data wire and avoid putting the truck into diagnostic mode for remote starting, a more robust and recommended approach involves using relays. Instead of directly interfacing with the data wire, this method employs relays to simulate the essential key data that normally occurs when you manually start the vehicle with the key.
From the truck’s perspective, when started via relays, it perceives the process as a standard key-initiated start. This eliminates the reliance on diagnostic mode and any potential interference with data communication, ensuring a cleaner and more reliable remote start operation.
R-Code Immobilizer and Passlock System Bypass
Trucks manufactured before mid-2007 typically employ a resistive immobilizer system. Unlike newer systems with chip-embedded keys, these older models use a resistor within the ignition cylinder with a specific known value, referred to as the “R Code.” This R Code is integral to the Passlock system, which verifies the correct resistance before allowing the engine to start.
When installing a remote starter on these vehicles, the relay system is ingeniously designed to manage the R-Code requirement. Upon remote start activation, the system first powers the status (-) wire, which, in turn, energizes relays when the remote start system powers the truck’s ignition wire. This process momentarily interrupts the factory R Code wire and injects the pre-set R Code value from a trim pot into the Passlock module. This effectively mimics the presence of the correct key, enabling a regular start as if the key were physically turned in the ignition.
Addressing Potential Downsides: Door Locks and Analog Wiring
While the relay method offers a superior solution for remote starting older GM trucks, it’s important to be aware of potential side effects. One notable downside is that disconnecting the factory R Code wire can impact the functionality of door locks and possibly door pin monitoring, as these systems might be routed through the same wire.
To rectify this, installers need to tap into the analog door pin wires, similar to the wiring approach used in older 01 trucks, often utilizing diodes. Furthermore, managing door lock motors may require a 5-wire configuration with two relays or a single 451M module to ensure proper operation after the remote start installation.
Conclusion: Reliable Remote Starts Without Data Wire Conflicts
By understanding the nuances of OBD2 systems in older GM trucks and employing the relay-based remote start method, installers can ensure a reliable and conflict-free upgrade. This approach avoids the pitfalls of relying directly on the diagnostic data wire, which can lead to complications. With the correct implementation of relays, diodes, and a trim pot for R-Code simulation, owners of these classic trucks can enjoy the convenience of remote start without compromising their vehicle’s diagnostic system or security features. Should any issues persist after a relay-based installation, they are unlikely to be related to the remote start system itself, as this method effectively isolates the remote start from the truck’s data communication network.
