Remote start systems offer unparalleled convenience, especially during harsh weather conditions. For older General Motors (GM) trucks, particularly those manufactured before 2008, the integration of aftermarket remote start systems often involves a unique approach utilizing the OBD2 port. This article delves into how “OBD2 plug-in remote start” systems function on these vehicles, offering a comprehensive understanding for both enthusiasts and those seeking to enhance their vehicle’s functionality.
Understanding OBD2 Remote Start for Pre-2008 GM Trucks
In many modern vehicles, installing a remote start can be relatively straightforward, sometimes involving simply plugging into the OBD2 (On-Board Diagnostics II) port. However, for older GM trucks, the OBD2 port plays a more intricate role in the remote start process. Specifically, these systems often leverage the vehicle’s diagnostic capabilities to initiate the remote start function.
On 2005 and earlier GM trucks, and extending to models before 2008, the remote start functionality is frequently achieved by commanding the truck into a “diagnostic start mode.” This is accomplished through the data wire located at the OBD2 port. This diagnostic mode is originally a GM troubleshooting feature designed to bypass the Passlock system for diagnostic testing purposes. Interestingly, this same data wire, often the J1850 wire, is used by diagnostic scanners and programmers to communicate with the vehicle’s computer for data monitoring and modifications.
However, using the data wire for both diagnostic communication and remote start simultaneously can lead to conflicts and unreliable operation. The vehicle’s systems are not designed for this dual functionality to be performed seamlessly.
The Relay-Based Solution: Bypassing Diagnostic Mode
To overcome the limitations of directly utilizing the diagnostic mode for remote start, a more robust and reliable method has been developed involving relays. Instead of forcing the truck into diagnostic mode and relying on the data wire for communication, this approach employs relays to simulate the essential key data that normally occurs when you manually start the vehicle with the key in the ignition.
From the truck’s perspective, when started via this relay-based remote start system, it perceives the start command as if you were physically in the driver’s seat, turning the key. Crucially, it does not enter diagnostic mode. This eliminates the potential conflicts and interference associated with the diagnostic start method.
Furthermore, trucks manufactured before mid-2007 typically utilize a resistive immobilizer system. Unlike newer systems that employ a transponder chip embedded in the key, these older GM trucks rely on a resistor within the ignition cylinder with a specific, known resistance value, referred to as the “R Code.” You can determine this R Code value by following specific diagnostic procedures for your vehicle.
The relay-based remote start system ingeniously incorporates this R-Code system. It uses a trim potentiometer (trim pot) that can be adjusted to match the truck’s internal resistor value. When the remote start is activated, the first wire to receive power is the status (-) wire. This action energizes both relays upon the remote start system powering up the truck’s ignition wire. This process momentarily interrupts the factory R-Code wire and injects the precisely set R-Code value from the trim pot into the Passlock module. This injection of the correct R-Code effectively bypasses the immobilizer, allowing for a normal engine start, just as if the physical key were being used.
Addressing Door Lock and Door Pin Considerations
While the relay-based system offers a superior solution for remote starting older GM trucks, disconnecting the data wire for this setup can have a side effect. The door locks and potentially the door pins on these vehicles are often monitored through this same data wire. Consequently, upon disconnection, their normal functionality can be disrupted.
To rectify this, installers need to address the door lock and door pin wiring separately. Similar to the wiring approach used in older 2001 trucks, it becomes necessary to tap into the analog door pin wires and isolate them using diodes. For the door lock motors, a “5-wire” configuration is typically required, utilizing two relays or a specialized component like a 451M module. This ensures that door lock and pin functionalities are restored and operate independently of the remote start system’s OBD2 connection.
Installation and Components Overview
For installers familiar with automotive electrical systems, implementing this relay-based OBD2 remote start solution involves a manageable set of components and wiring steps. The key components required typically include:
- Relays: A total of four standard relays, or alternatively, two relays combined with a 451M module for door lock integration.
- Diodes: Four diodes, or potentially tapping into the dome light wire as an alternative approach for door pin signal isolation.
- Trim Pot: A trim potentiometer to simulate the vehicle’s R-Code resistance value.
By providing these details to a professional installer, they should be well-equipped to understand the specific wiring requirements and successfully install an OBD2-compatible remote start system on these older GM trucks, ensuring seamless functionality without compromising other vehicle systems.
Conclusion
Implementing an OBD2 plug-in remote start on older GM trucks requires a nuanced understanding of the vehicle’s electrical architecture and immobilizer system. The relay-based method, which bypasses the diagnostic start mode and simulates key data along with R-Code, offers a reliable and effective solution. While it necessitates addressing door lock and pin wiring separately, the end result is a fully functional remote start system that integrates smoothly with the vehicle, enhancing convenience and usability for years to come. This approach ensures that the remote start operates as intended, without interfering with diagnostic functions or other critical vehicle systems.
