Remote start systems have become a popular aftermarket addition for vehicle owners seeking convenience and comfort. Among the various types available, remote starters that plug into the OBD2 (On-Board Diagnostics II) port are often considered for their seemingly simple installation. However, for certain vehicles, particularly older General Motors (GM) trucks manufactured before 2008, the functionality and implications of an OBD2-connected remote start differ significantly. This article delves into how these systems operate on these older models and why a relay bypass method is often a more effective and trouble-free solution.
How OBD2 Remote Start Functions on Pre-2008 GM Trucks
On many modern vehicles, remote start systems can indeed interface through the OBD2 port to initiate the engine start sequence. However, for 2005 and earlier GM trucks, and many models up to 2008, the approach is unique. These systems exploit a diagnostic start mode built into the vehicle’s computer. Instead of directly commanding the engine to start as a standard remote start would, these OBD2 systems send a signal via the data wire at the OBD2 port – specifically the J1850 wire – to put the truck into this diagnostic start mode.
This diagnostic mode is originally intended for troubleshooting by mechanics, allowing them to bypass the standard Passlock security system for testing purposes. By sending commands through the J1850 data wire, the remote start system effectively tricks the truck into starting without the need for physical key interaction in the ignition.
Limitations and Drawbacks of OBD2 Remote Start on Older GM Models
While seemingly straightforward, relying on the diagnostic start mode via the OBD2 port presents several potential issues:
- Data Wire Interference: The J1850 data wire is also the communication line used by diagnostic scanners and performance programmers to monitor vehicle data. These systems were not designed for simultaneous use. Attempting to use both an OBD2 remote start and a scanner or programmer can lead to conflicts and unreliable operation.
- Passlock System Bypass Concerns: While the diagnostic mode is designed to bypass the Passlock system for testing, continuously relying on this bypass for remote starting might introduce unforeseen vulnerabilities or conflicts with the vehicle’s security protocols over time.
- Potential Door Lock and Door Pin Issues: In some installations, utilizing the OBD2 data wire for remote start can interfere with other vehicle functions that are also monitored through this wire, such as door locks and door pin status. This interference can result in the door locks ceasing to function correctly once the OBD2 connection is made for the remote start.
The Relay Bypass: A More Robust Solution
To circumvent these issues, a more reliable method for remote starting older GM trucks involves a relay bypass system. This approach avoids using the OBD2 data wire for the primary starting command and instead simulates the key data that the truck would normally receive when started with the physical key.
Here’s how the relay bypass works:
- Key Data Simulation: Instead of putting the truck into diagnostic mode, relays are used to mimic the electrical signals and data that occur when the key is turned in the ignition. To the truck’s computer, it appears as if the vehicle is being started normally with the key.
- Passlock Immobilizer Handling: Trucks before the mid-2007 model year often utilize a resistive immobilizer system. These systems, unlike those with chipped keys, use a resistor within the ignition cylinder with a specific value (R Code). The relay bypass system incorporates a trim pot that can be set to match this R Code value. When the remote start is activated, it uses relays to interrupt the factory R Code wire and inject the correct R Code value from the trim pot into the Passlock module. This allows for regular starting as if the key were being used, without relying on the diagnostic mode.
Addressing Door Lock and Door Pin Functionality
As the original OBD2 method can disconnect or interfere with door lock and door pin monitoring, the relay bypass installation often requires additional steps to maintain these functionalities. Installers may need to:
- Tie into Analog Door Pin Wires: Similar to older ’01 trucks, installers may need to connect to the analog door pin wires using diodes to ensure proper door pin signal detection for alarm and security features.
- Implement 5-Wire Door Lock Motor Control: To maintain full control over door locks, installers may need to use relays (or a specialized module like a 451M) to implement a 5-wire door lock motor configuration. This ensures that door locks function correctly and are integrated with the remote start system.
Conclusion: Choosing the Right Approach for Older GM Trucks
For owners of pre-2008 GM trucks considering a remote start system, understanding the nuances of OBD2-based systems versus relay bypass methods is crucial. While OBD2 “plug-in” systems may seem appealing for their apparent simplicity, they can introduce complications and potential conflicts, especially on these older models.
The relay bypass method, while requiring a slightly more involved installation, provides a more robust and reliable solution. By simulating a standard key start and properly addressing immobilizer and door lock system integration, relay bypass systems offer a cleaner and more dependable remote start experience without the drawbacks associated with relying on the OBD2 diagnostic start mode. Consulting with a knowledgeable installer experienced with older GM vehicles is recommended to ensure the best and most appropriate remote start solution is implemented.
