Remote start systems offer unparalleled convenience, especially during harsh weather conditions. For older General Motors (GM) trucks manufactured before 2008, a common method for aftermarket remote start installation involves utilizing the OBD2 port. However, this approach can introduce unique challenges due to the vehicle’s communication system and security features. This article delves into the intricacies of remote starting older GM trucks via the OBD2 port, highlighting potential issues and providing effective solutions for a seamless integration.
Understanding OBD2 Remote Start on Pre-2008 GM Trucks
On many vehicles, including older GM trucks, the OBD2 (On-Board Diagnostics II) port serves as a communication gateway to the vehicle’s computer system. For remote start systems, installers sometimes tap into this port to send commands that initiate the engine start process.
In the case of 2005 and earlier GM trucks, and some models up to 2008, the remote start activation via OBD2 often leverages the vehicle’s diagnostic start mode. This mode is originally designed for technicians to troubleshoot and diagnose vehicle issues. By sending specific signals through the OBD2 data wire (specifically the J1850 wire), a remote start system can command the truck to enter this diagnostic start mode, effectively bypassing the standard ignition sequence and starting the engine.
The Diagnostic Start Mode and J1850 Data Wire
The J1850 data wire within the OBD2 port is crucial for communication between diagnostic tools, programmers, and the vehicle’s computer. This same wire is often used by OBD2-based remote start systems to send the “start” command by triggering the diagnostic start mode.
However, this dual usage of the J1850 wire can lead to conflicts and limitations. The vehicle’s computer system is not designed to handle simultaneous communication for both diagnostic purposes and remote start commands through the same data wire efficiently.
Passlock and the Resistive Immobilizer System
Vehicles before the mid-2007 model year (and some later models) often employed a resistive immobilizer system known as Passlock. Unlike modern systems with chipped keys, Passlock relies on a resistor embedded within the ignition cylinder. This resistor has a specific value (R-Code) that the vehicle’s security system recognizes as authorization to start.
When using the diagnostic start mode for remote start, the Passlock system needs to be bypassed. In standard key-based starting, the correct R-Code is automatically provided when the key is turned. However, when initiating a Remote Start Through Obd2 diagnostic mode, this normal key-turn sequence is bypassed, potentially triggering the immobilizer and preventing the engine from starting.
The Problem with Data Wire Interference
The core issue with relying solely on the OBD2 data wire for remote start in these older GM trucks is interference and potential conflicts. The J1850 wire is not designed for continuous or simultaneous dual-purpose communication. This can lead to:
- Unreliable Remote Starts: The remote start may fail to activate consistently due to communication glitches or the vehicle not reliably entering diagnostic start mode.
- Potential Diagnostic Issues: Interference on the data wire could theoretically impact the functionality of diagnostic tools when connected to the OBD2 port for troubleshooting.
The Relay-Based Solution: Bypassing the OBD2 Data Wire for Start Command
To overcome these limitations and ensure a robust and reliable remote start system, a superior method involves bypassing the OBD2 data wire for the engine start command. This is achieved by using relays to simulate the key data that would normally occur when physically turning the ignition key.
Instead of forcing the truck into diagnostic mode, this relay-based system mimics the regular key-start sequence. As far as the truck’s computer is concerned, it is being started by someone turning the key in the ignition, not through a diagnostic command. This eliminates the data wire conflict and bypasses the potential issues associated with diagnostic start mode.
How the Relay System Works: Simulating Key Data and Bypassing Passlock
This refined approach utilizes relays to manage two key functions:
- R-Code Injection: A trim potentiometer (trim pot) is used to replicate the precise resistance value (R-Code) of the vehicle’s ignition resistor. This trim pot is wired into the relay system. When the remote start is activated, the relays interrupt the factory R-Code wire and inject the R-Code value from the trim pot directly into the Passlock module. This action fools the Passlock system into thinking a valid key is present, allowing the engine to start.
- Key-Turn Simulation: Relays are configured to mimic the sequence of electrical signals that occur when the ignition key is turned to the “start” position. This includes powering up the necessary circuits in the correct order, just as if the key were being physically turned.
By simulating the physical key-turn and injecting the correct R-Code, the relay system completely bypasses the need to use the OBD2 data wire for the start command and circumvents the diagnostic start mode altogether.
Addressing Door Lock and Door Pin Monitoring Issues
A potential side effect of disconnecting the factory R-Code wire for the relay-based Passlock bypass is the disruption of door lock and door pin monitoring, which are sometimes routed through the same wire. Once this wire is interrupted for the relay system, these functions may cease to operate through the factory wiring.
To restore door lock and door pin functionality, installers need to:
- Door Pin Wiring: Tap into the analog door pin wires directly. Diodes are often used in this wiring to isolate circuits and prevent feedback, similar to how door pins were wired in older (e.g., 2001) trucks.
- Door Lock Motor Wiring: Implement a 5-wire door lock system using relays (typically two relays) or a specialized module like a 451M. This method provides independent control of the door lock motors, bypassing the factory wiring that may be affected by the Passlock bypass.
Components Required for the Relay-Based Solution
Implementing this robust remote start solution typically requires:
- Relays: A total of four standard relays (or two relays and a 451M module for door locks).
- Diodes: Four diodes for door pin wiring (or potentially tapping into the dome light wire as an alternative for door pin status).
- Trim Pot: One trim potentiometer to mimic the vehicle’s R-Code resistance value.
Conclusion: A Superior Approach to Remote Start on Older GM Trucks
While OBD2-based remote start installations might seem straightforward on older GM trucks, they can introduce reliability issues and potential conflicts due to data wire interference and reliance on the diagnostic start mode. A relay-based system that simulates the key-turn sequence and directly bypasses the Passlock system offers a more robust, reliable, and cleaner integration.
By understanding the nuances of the Passlock system, the limitations of OBD2 diagnostic start mode, and implementing the relay-based solution with proper door lock and pin wiring, installers can ensure a seamless and dependable remote start experience for owners of pre-2008 GM trucks. This approach not only enhances the functionality of the remote start but also minimizes the risk of interference with the vehicle’s diagnostic system and other electronic functions.
