For owners of older GM trucks, particularly models from 2005 to around 2008, installing a remote start system might come with unique considerations, especially when it comes to OBD2 integration. These vehicles often utilize the OBD2 port in a specific way for remote start functionality, which can sometimes lead to conflicts with other OBD2 devices. Understanding how Obd2 Remote Start systems work in these trucks and knowing how to address potential issues is crucial for a seamless installation and reliable performance.
How OBD2 Remote Start Works on 2005-2008 GM Trucks: Diagnostic Start Mode
In many 2005 and older to early 2008 GM trucks, remote start systems are designed to initiate the engine by engaging a specific “diagnostic start mode” through the data wire at the OBD2 port. This method leverages a General Motors troubleshooting feature that temporarily bypasses the vehicle’s passlock security system for diagnostic purposes. By sending commands via the J1850 data wire at the OBDII port, the remote starter essentially tricks the truck into starting as if it were in a diagnostic session.
This approach, while functional for remote starting, uses the same data wire that your OBD2 scanners, programmers, and other diagnostic tools rely on to communicate with the vehicle’s computer. This shared usage can become problematic when you need to use these tools while also having a remote start system connected.
The Problem with Data Wire Conflict
The core issue arises because the OBD2 data wire in these older GM trucks isn’t designed for simultaneous, dual-duty operation. When the remote start system commands the truck into diagnostic mode via the data wire, it may interfere with the ability of other devices to properly communicate or function through the same OBD2 port. This can lead to conflicts, communication errors, or even prevent other OBD2 devices from working correctly when the remote start system is active or recently used.
The Relay Solution: Simulating Key Data
To circumvent this potential conflict and ensure reliable OBD2 remote start functionality without data wire interference, a more robust solution involves using relays to simulate the key data that naturally occurs when you start the vehicle with the key in the ignition. Instead of relying on the diagnostic start mode via the OBD2 data wire, this relay-based method effectively mimics the normal key-start sequence.
With relays in place, the truck perceives the start command as if you were physically turning the key in the ignition. This bypasses the need to put the vehicle into diagnostic mode and eliminates the reliance on the shared OBD2 data wire for remote starting. As far as the truck’s computer is concerned, it’s being started in the conventional manner, thus avoiding any potential conflicts with other OBD2 port usage.
Addressing Resistive Immobilizer Systems
Trucks manufactured prior to mid-2007 often incorporate a resistive immobilizer system. Unlike newer systems with chipped keys, these older models utilize a resistor within the ignition cylinder with a specific, known value, often referred to as the “R Code.” This R Code is crucial for the vehicle to recognize a valid key and allow starting.
The relay-based OBD2 remote start solution effectively addresses this resistive immobilizer. By incorporating a trim pot (potentiometer) into the relay circuit, installers can set it to match the precise resistance value of the truck’s internal resistor (R Code). When the remote start initiates, the system first powers the status (-) wire, activating the relays. This action interrupts the factory R Code wire and seamlessly injects the pre-set R Code value from the trim pot into the passlock module. This process allows for a normal, key-like start, effectively bypassing the immobilizer without triggering any security issues.
Handling Door Locks and Door Pins
A potential side effect of disconnecting the factory OBD2 data wire for the relay-based solution is the potential loss of functionality for door locks and possibly door pin monitoring, as these features can sometimes be routed through that same wire. To restore this functionality, installers will need to tap into the analog door pin wires, similar to how it was done on older models like 2001 trucks. This typically involves using diodes to isolate the signals.
Additionally, controlling the door lock motors may require a “5-wire” configuration, often achieved with two relays or a specialized module like a 451M. This ensures that the remote start system can properly control the door locks even after the OBD2 data wire is re-routed for the relay-based start method.
Conclusion
For older GM trucks within the 2005-2008 range, opting for a relay-based OBD2 remote start installation offers a superior and more reliable solution compared to solely relying on the diagnostic start mode via the OBD2 data wire. By simulating the key-start sequence and addressing resistive immobilizer systems with a trim pot, this method effectively bypasses potential conflicts with other OBD2 devices and maintains crucial vehicle functionalities like door locks and pin monitoring. When properly installed, this relay-based approach ensures that your remote start system operates seamlessly and without interfering with your ability to use diagnostic tools or other OBD2 dependent devices on your GM truck.
