For owners of older General Motors (GM) trucks, particularly those manufactured before 2008, integrating a remote start system might involve a connection to the vehicle’s OBD2 port. This approach leverages the diagnostic capabilities of the On-Board Diagnostics II (OBD2) system to initiate remote starting. However, it’s crucial to understand how this system works and potential drawbacks compared to more conventional methods.
How OBD2 Port Remote Start Works in Pre-2008 GM Trucks
In these earlier GM models, remote starters sometimes utilize a specific function within the OBD2 system – the diagnostic start mode. This mode is originally designed for technicians to troubleshoot and test the vehicle by bypassing the standard Passlock or immobilizer system. By sending commands through the data wire within the OBD2 port (specifically the J1850 wire), the remote starter can trigger this diagnostic start mode, effectively starting the engine remotely.
This method is attractive because it seems straightforward. However, the J1850 data wire is also used by other devices, such as diagnostic scanners and performance programmers, to communicate with the truck’s computer. Attempting to use this single wire for both remote start commands and diagnostic communication simultaneously can lead to conflicts and unreliable operation. The system isn’t designed for “dual duty” on this critical data line.
The Relay-Based Alternative: Simulating Key Start for Reliable Remote Start
A more robust and recommended approach for remote starting these older GM trucks is to bypass the OBD2 diagnostic start method altogether. Instead, a relay-based system can be implemented to simulate the sequence of events that occur when you manually start the truck with the key.
This method involves using relays to mimic the key data signals that the truck normally receives during a regular key start. When the remote start is activated, these relays intelligently send the correct signals, making the truck’s computer believe it’s being started with the physical key. Crucially, this method does not rely on the diagnostic start mode or the OBD2 data wire for engine starting.
For trucks equipped with a resistive immobilizer system (common in pre-mid-2007 models), the relay system also addresses the Passlock security. These systems utilize a resistor within the ignition cylinder, identified by an “R Code.” The relay setup includes a trim pot that can be adjusted to match the specific R Code value of the vehicle. Upon remote start activation, the system first powers up, activating relays that interrupt the factory R-Code wire. It then injects the pre-set R-Code value from the trim pot into the Passlock module. This effectively bypasses the immobilizer, allowing the engine to start as if the correct key were being used.
Potential Downsides and Solutions with Relay-Based Systems
While the relay-based system offers a more reliable remote start solution, disconnecting the factory R-Code wire can impact other vehicle functions that might be monitored through this wire, such as door locks and potentially door pin status.
To address this, installers need to reconnect these functionalities using analog wiring techniques, similar to those used in even older vehicles (like 2001 models). This typically involves:
- Door Pins: Tapping into the analog door pin wires, often using diodes to isolate signals and prevent backfeed. Alternatively, the dome light wire circuit can sometimes be utilized as it often reflects door pin status.
- Door Locks: Implementing a 5-wire door lock motor connection using either two relays or a specialized module like a DEI 451M. This ensures proper control of the door locks independently of the bypassed factory wiring.
Ensuring a Trouble-Free Remote Start Installation
For anyone installing or having a remote start installed on a pre-2008 GM truck, understanding these nuances is vital. By opting for a relay-based system that simulates key start and addresses the Passlock and door lock/pin considerations, you can achieve a far more reliable and less problematic remote start experience compared to relying solely on the OBD2 diagnostic start mode. Communicating this information to your installer can ensure they choose the optimal and most robust approach for your vehicle, avoiding potential data wire conflicts and maintaining the functionality of other vehicle systems.
