Remote start systems offer convenience, especially in extreme weather conditions. For owners of older General Motors (GM) trucks, understanding how these systems interact with the vehicle’s On-Board Diagnostics II (OBD2) port is crucial. Specifically for certain models produced before 2008, the OBD2 data wire plays a unique role in enabling remote start functionality.
In these earlier GM trucks, remote start systems often utilize the diagnostic start mode, a feature accessed via the OBD2 data wire. This method essentially commands the truck to start as if it were undergoing a diagnostic test, effectively bypassing the standard Passlock security system for starting purposes. The same J1850 data wire used by diagnostic tools and programmers is also employed for this remote start activation.
However, this dual usage of the data wire can present challenges. The OBD2 port and its data wire are not designed for simultaneous operation of remote start and other diagnostic or programming functions. This can lead to interference or conflicts when attempting to use a scanner or programmer while the remote start system is connected in this manner.
A more robust and interference-free solution involves employing relays to simulate the key-turned starting process. Instead of relying on the OBD2 data wire and diagnostic mode, this relay-based approach mimics the electrical signals that occur when you physically turn the ignition key. From the truck’s perspective, it’s being started normally, without entering diagnostic mode and without utilizing the OBD2 data wire for the start command.
For trucks manufactured before mid-2007, which use a resistive immobilizer system rather than a key chip, the relay method also addresses the Passlock system. These systems use a resistor within the ignition cylinder with a specific resistance value (R-Code). The relay setup can incorporate a trim pot, adjustable to match the truck’s R-Code value. When the remote start initiates, the relays momentarily interrupt the factory R-Code wire and inject the correct resistance value from the trim pot, effectively mimicking the presence of the correct key and allowing for a standard start.
It’s important to note that disconnecting the factory R-Code wire can impact other systems that might rely on it, such as door locks and potentially door pin monitoring. Installers need to address this by tapping into the analog door pin wires using diodes, similar to older ’01 truck models. Furthermore, the door lock motors may require a 5-wire connection with relays or a dedicated module like a 451M to ensure proper functionality after the system modification.
In summary, while utilizing the OBD2 data wire for remote start was a method employed in older GM trucks, a relay-based system offers a more reliable and less intrusive alternative. This approach avoids potential conflicts with diagnostic tools and ensures consistent remote start operation by directly simulating the standard key-start process. For installers familiar with automotive electrical systems, implementing this relay-based solution, along with addressing potential door lock integration, provides a comprehensive and effective remote start upgrade for these older GM models.
