For owners of older GM trucks, integrating aftermarket remote start systems can sometimes present unique challenges, particularly concerning the OBD2 port and vehicle security systems like Passlock. This article delves into how some remote starters interact with the OBD2 system in these vehicles and provides a solution to potential conflicts, focusing on ensuring seamless operation and avoiding diagnostic issues.
The OBD2 Diagnostic Start Method in Pre-2008 GM Trucks
In many GM trucks manufactured before 2008, remote start systems, including some that might be associated with brands like Viper (though this method isn’t Viper-specific, but rather a general approach for these vehicles), utilize a specific feature of the OBD2 port for activation. These systems often initiate remote start by sending a command through the OBD2 data wire, effectively putting the truck into a “diagnostic start mode.”
This diagnostic start mode is originally intended for GM technicians to troubleshoot vehicle issues, allowing them to bypass the standard Passlock immobilizer system for testing purposes. The same data wire (specifically the J1850 wire) used for this diagnostic start is also the communication line for OBD2 scanners and programmers to access vehicle data. This dual usage can lead to conflicts when both a remote starter using this method and another OBD2 device are trying to communicate simultaneously or interfere with each other’s function.
Implementing a Relay-Based Solution to Bypass OBD2 Conflicts
To circumvent the potential conflicts arising from using the OBD2 data wire for remote starting, a more robust and recommended approach involves using relays to simulate a traditional key start. Instead of commanding the truck into diagnostic mode via the OBD2 port, this relay-based system mimics the electrical signals that occur when you physically turn the ignition key.
This method ensures that, as far as the truck’s computer is concerned, it is being started by a key in the ignition, completely bypassing the diagnostic start mode and eliminating any potential interference with OBD2 scanners or other devices.
For GM trucks prior to the mid-2007 model year change, which typically employ a resistive immobilizer system (Passlock system using an “R-Code”), this relay method becomes particularly effective. These systems don’t rely on a chip in the key but instead use a resistor within the ignition cylinder with a specific value (R-Code).
By using a trim potentiometer (trim pot) set to the same resistance value as the truck’s R-Code, the relay system can effectively bypass the Passlock system during a remote start. When the remote start is activated, the system first powers a status wire (-) which, in turn, energizes relays upon receiving the truck’s ignition wire signal. This process momentarily interrupts the factory R-Code wire and injects the pre-set R-Code value from the trim pot into the Passlock module. This sequence allows for a normal engine start, just as if the key were being used directly in the ignition.
Potential Downsides and Considerations for Installation
While the relay-based solution effectively resolves OBD2 conflict issues, it’s important to be aware of a potential side effect. The door locks and possibly door pin monitoring systems in these older GM trucks are sometimes routed through the same wire that is disconnected for this relay setup.
Disconnecting this wire can lead to the door locks and door pin functions ceasing to operate as they normally would. To address this, installers will need to tap into the analog door pin wires, often using diodes similar to the wiring techniques used on older (e.g., 2001) trucks. Additionally, the door lock motors may require a “5-wire” configuration using two relays or a specialized module like a 451M to restore full functionality.
It is highly recommended to share this information with your remote starter installer. An experienced installer familiar with older GM vehicle electronics should readily understand these instructions and be able to implement the relay-based solution effectively.
Components Required for Relay-Based Installation
Implementing this OBD2 bypass and key-simulating remote start solution typically requires the following components:
- Four standard automotive relays (or two relays and one 451M module for door locks).
- Four diodes (or potentially tapping into the dome light wire as an alternative for door pin sensing in some installations).
- One trim potentiometer (trim pot) for setting the R-Code resistance value.
By utilizing these components and following the relay-based wiring method, you can ensure a reliable remote start system in your older GM truck that operates independently of the OBD2 diagnostic data wire, resolving potential conflicts and maintaining the functionality of your vehicle’s security and door lock systems. After this modification, any remaining issues are highly unlikely to be related to the remote start system’s interaction with the vehicle’s data network.
