Remote starting your vehicle offers convenience, especially in extreme weather conditions. For owners of older General Motors (GM) trucks, particularly those manufactured before 2008, understanding how remote start systems interact with the On-Board Diagnostics II (OBD2) port is crucial. This article will delve into the common method of remote starting these trucks via the OBD2 port and explore a more effective and less intrusive alternative using relays.
Initially, many remote start systems for these older GM models utilized the OBD2 port to initiate the engine start. This approach leverages a specific function within the GM vehicle’s computer system: the diagnostic start mode.
The OBD2 Diagnostic Start Method: How It Works and Its Drawbacks
Older GM trucks (roughly pre-2008) can be commanded into a diagnostic start mode through the data wire at the OBD2 port. This mode is originally designed for troubleshooting purposes in workshops, allowing technicians to bypass the Passlock immobilizer system for easier diagnostics. Remote start systems using this method essentially exploit this diagnostic mode to start the vehicle remotely.
The system sends a command via the J1850 data wire (a communication protocol used in these vehicles) at the OBD2 port, mimicking a diagnostic request. While seemingly straightforward, this method has significant drawbacks. The J1850 wire is also used by diagnostic scanners and performance programmers to monitor vehicle data. Attempting to use it for both remote start commands and diagnostic communication simultaneously can lead to conflicts and unreliable operation. The data wire isn’t designed for “dual duty” in this manner.
The Superior Relay-Based Remote Start Solution
A more robust and recommended method for remote starting older GM trucks involves using relays to simulate the physical key turning action. Instead of relying on the OBD2 port’s diagnostic mode, this relay system directly interacts with the vehicle’s ignition system, mimicking the signals that occur when you manually start the truck with the key.
How the Relay System Bypasses Passlock and Starts Your Truck
This relay-based approach circumvents the Passlock system and starts the engine as if you were physically in the driver’s seat turning the key. Here’s a breakdown of how it works:
- Passlock System and R-Code: Trucks from this era often use a resistive immobilizer system. Unlike modern systems with chip-embedded keys, these older models use a resistor within the ignition cylinder. This resistor has a specific value, known as the “R-Code.” The Passlock system recognizes this R-Code to authorize starting.
- Relay and R-Code Simulation: The relay system interrupts the factory R-Code wire and injects a simulated R-Code value. This value is typically set using a trim potentiometer (trim pot) that is adjusted to match the truck’s specific resistor value. When the remote start activates, it first powers a status (-) wire, which in turn activates relays. These relays then simulate the key data, including the correct R-Code, effectively bypassing the immobilizer.
Advantages of the Relay Method
By simulating the key start sequence through relays, this method offers several advantages over the OBD2 diagnostic start:
- Avoids Data Wire Conflict: It completely bypasses the J1850 data wire at the OBD2 port for starting, preventing conflicts with diagnostic tools or programmers.
- More Reliable Operation: It provides a more consistent and reliable remote start experience as it doesn’t depend on potentially glitchy diagnostic mode exploitation.
- Proper System Integration: It integrates more seamlessly with the vehicle’s electrical system by mimicking the standard key start process.
Wiring Considerations and Potential Issues
While the relay method is superior, installation requires careful wiring. One potential downside mentioned in the original forum post is related to door locks and door pins. The original post notes that:
The down side to this is that your door locks, and possibly door pins are monitored through that wire as well, so once disconnected they will not function. This means your installer will need to tie into the analog door pin wires with diodes like on an 01 truck, and needs to 5 wire the door lock motors with 2 relays, or 1 451M.
This means installers might need to address the door lock and pin monitoring system separately due to the way the relay system interacts with the vehicle’s wiring. This typically involves tapping into analog door pin wires using diodes and potentially utilizing relays for the door lock motors.
Conclusion
For remote starting older GM trucks, especially those pre-2008, opting for a relay-based system over the OBD2 diagnostic start method is highly recommended. While the OBD2 method might seem simpler initially, the relay system provides a more reliable, less intrusive, and ultimately more effective solution by properly simulating the key start process and avoiding conflicts with the vehicle’s diagnostic communication systems. If you are considering remote start for your older GM truck, discussing the relay-based approach with a professional installer is advisable to ensure a seamless and trouble-free integration.
