Remote start systems offer convenience, especially in extreme weather conditions. However, installing them in older General Motors (GM) trucks (specifically those pre-2008) can present unique challenges, particularly concerning the OBD2 port and the vehicle’s immobilizer system, Passlock. This article delves into how remote start systems function in these older GM models and addresses potential issues and solutions related to OBD2 integration.
The Diagnostic Start Mode and OBD2 Connection
For 2005 and earlier GM trucks, remote start systems often utilize a diagnostic start mode activated through the data wire at the OBD2 port. This method leverages a GM troubleshooting feature that bypasses the Passlock system for diagnostic purposes. The same J1850 data wire used by scanners and programmers to access vehicle data is also employed for remote starting. However, this dual functionality can lead to conflicts. The data wire may not handle both diagnostic communication and remote start commands simultaneously, potentially causing issues.
Simulating Key Data: The Relay Solution
To circumvent the limitations of using the data wire for both diagnostics and remote starting, a more robust method employs relays to simulate the key data. Instead of putting the truck into diagnostic mode via the OBD2 data wire, this approach uses relays to mimic the signals that occur when you physically turn the ignition key. From the truck’s perspective, it’s being started as if the driver were in the seat with the key, completely bypassing the diagnostic mode and its associated data wire conflicts.
This method is particularly relevant for trucks manufactured before mid-2007, which typically use a resistive immobilizer system. Unlike newer systems with chip-embedded keys, these older trucks rely on a resistor within the ignition cylinder with a specific resistance value known as the R-code.
Passlock Bypass and the R-Code
The resistive immobilizer system in these older GM trucks, known as Passlock, doesn’t use a chip in the key. Instead, it uses a resistor in the ignition cylinder with a known value (R-Code). This R-code is crucial for starting the vehicle.
Alt text: Wiring diagram showing GM Passlock system with resistor R-code and bypass circuit using trim pot for remote start installation on older trucks.
When installing a remote start, the system needs to bypass this Passlock to start the engine. The R-code value can be determined using specific diagnostic procedures. A trim pot (potentiometer) can then be set to match this R-code value. During a remote start sequence, the first wire activated is the status (-) wire, which energizes relays when the remote start powers up the truck’s ignition wire. This action interrupts the factory R-code wire and injects the pre-set R-code value from the trim pot into the Passlock module. This process effectively fools the system, allowing a normal engine start as if the physical key were being used.
Addressing Door Lock and Door Pin Monitoring
A potential drawback of disconnecting the factory R-code wire is that it can also disrupt the functionality of door locks and potentially door pin monitoring, as these systems may also be routed through this wire. Once disconnected for the remote start bypass, these features might cease to operate correctly through the original wiring.
To resolve this, installers need to tap into the analog door pin wires, similar to the wiring approach used in older models like 2001 trucks. Diodes are necessary for proper isolation and function. Furthermore, the door lock motors require a 5-wire configuration, typically achieved using two relays or a single 451M relay module. This ensures that door locks function correctly even after the Passlock bypass and R-code wire modification.
Conclusion: A Robust Solution for Older GM Remote Starts
By employing relays to simulate key data and a trim pot to bypass the resistive Passlock system using the R-code, remote start installations on older GM trucks can be effectively achieved without relying solely on the potentially problematic OBD2 data wire diagnostic start mode. While additional wiring for door locks and pins is necessary, this relay-based method provides a reliable and clean solution. Once properly installed, the remote start system operates independently of the truck’s data wires, eliminating potential conflicts and ensuring seamless functionality. For installers familiar with these older GM systems and wiring techniques, this approach offers a proven path to successful remote start integration.
