As a seasoned auto repair expert at obd-de.com, I often encounter questions about remote start systems, particularly concerning older vehicles. A common query revolves around installing Obd2 Remote Start on pre-2008 General Motors (GM) trucks. This article will delve into how these systems function and address potential challenges, offering a comprehensive understanding for both DIY enthusiasts and those seeking professional installation.
Understanding OBD2 Remote Start Challenges in Older GM Trucks
For 2005-2008 GM trucks, remote start systems frequently utilize the OBD2 port to initiate the engine. Specifically, these systems are designed to command the truck into a “diagnostic start mode” via the data wire at the OBDII connector. This diagnostic mode is a feature built by GM for troubleshooting purposes, effectively bypassing the Passlock system to allow for easier vehicle diagnostics.
The issue arises because this same data wire, known as the J1850 bus, is also used by diagnostic scanners and programmers to monitor vehicle data. Unfortunately, this data wire doesn’t always handle dual functions seamlessly. Attempting to use it for both remote start (in diagnostic mode) and data communication simultaneously can lead to conflicts and unreliable operation.
The Relay Solution: Simulating Key Start for Reliable Remote Start
To overcome this data wire conflict, a more robust and reliable method for remote starting these older GM trucks involves using relays to simulate the standard key-initiated start process. Instead of relying on the OBD2 data wire to put the truck into diagnostic mode, this relay-based system mimics the electrical signals that occur when you physically turn the key in the ignition.
From the truck’s perspective, it’s as if you are sitting in the driver’s seat and starting the engine with the key. This approach completely bypasses the diagnostic start mode, eliminating any potential interference with OBD2 data communication and ensuring a clean and consistent remote start operation.
Addressing the Immobilizer System: The R-Code
Trucks manufactured before mid-2007 utilize a resistive immobilizer system, often referred to as the “R-Code” system. Unlike newer systems with chip-embedded keys, these older trucks employ a resistor within the ignition cylinder. This resistor has a specific, known value (the R-Code) which the truck’s Passlock system recognizes to authorize starting.
To integrate a remote start system with these R-Code immobilizers, a clever workaround is needed. The solution involves reading the R-Code value of your truck using the method described in the image above and then setting a trim pot (potentiometer) to match this resistance value.
When the remote start is activated, the first signal sent is a status (-) wire. This signal powers up relays and, crucially, activates the truck’s ignition wire. At this point, the system interrupts the factory R-Code wire and injects the R-Code value from your pre-set trim pot into the Passlock module. This effectively fools the system into thinking a valid key is being used, allowing for a normal engine start, just as if you were using the key.
Door Lock and Door Pin Considerations
A potential side effect of disconnecting the factory R-Code wire is that it can impact the functionality of your door locks and possibly door pin monitoring. These systems are sometimes routed through the same wiring, and disconnecting it for the R-Code bypass can render them inoperable.
To restore door lock functionality, installers need to tap into the analog door pin wires, similar to the wiring approach used on older 2001 trucks. This typically involves using diodes to isolate the signals. Furthermore, the door lock motors will require a “5-wire” configuration, often achieved using two relays or a single 451M module. This ensures proper control and operation of the door locks independently of the remote start system.
Guidance for Your Installer
If you are planning to have a professional install your OBD2 remote start system on an older GM truck, sharing this information with your installer can be highly beneficial. The key components required for a successful installation using this method are:
- Relays: A total of four relays (or two relays and a 451M module) are needed to manage the key signal simulation and door lock integration.
- Diodes: Four diodes (or tapping into the dome light wire as an alternative) are necessary for isolating door pin signals.
- Trim Pot: A trim pot is essential for replicating the truck’s R-Code resistance value to bypass the immobilizer.
By implementing this relay-based solution, you can ensure that your remote start system operates reliably and without any conflicts with your truck’s data communication systems. Furthermore, with the proper wiring for door locks and door pins, you can maintain full functionality of all your vehicle’s systems. If any issues persist after this type of installation, you can confidently rule out the remote start system as the source, as it will be completely isolated from the truck’s data wires.
