[Roddrz]

CAN Bus Security Dongle Design Guide

Security Dongle Design Summary

Overview

A cost-effective DIY security solution ($35 in components) contained in a single OBD2 dongle that prevents vehicle theft by disabling CAN bus access to the OBD2 service port.

Design Philosophy

• Keep your doors unlocked approach: Focuses on preventing key programming rather than preventing window break-ins
• CAN Bus vulnerability mitigation: Disables source of vulnerability through service port
• No stock wires or connectors are cut

How the Security Dongle Functions
Core Mechanism

The dongle operates through a "miniature DPDT wireless relay" that controls CAN bus access to the service port:

1. Default State: When unplugged or disarmed, the relay disconnects the CAN bus signal to the service OBD2 port
2. Armed State: User presses the FOB button to activate the relay, connecting the CAN bus to the service port
3. Set and Forget: Once armed, the relay remains latched until power is removed from the dongle or the dongle is unplugged

Installation Process

1. Connect the relay control wire to the CAN bus twisted pair that controls access to the service OBD2 port
2. Relay gets it's power by 12V and ground pins from dummy OBD2 port
3. Add 120 Ohm terminating resistors across GMLAN pairs to trick FOB programmer into detecting a data link
4. Solidly tape the Data Link Connector to BCM and tye wrap the connector under the dash
5. Install security plate using security screws over BCM wire connectors

Build & Installation Time

• Harness and Relay Build: 8 hours
• Security Plate Build: 5 hours
• Installation: Wire and security plate installation takes approximately 1 hour
• Total: Approximately 14 hours

Parts List ($35 total)

• 12V Wireless Remote Control Switch: $13.00
• Harness: $12
• Small OBD2 connector dongle case: $8.00

Benefits & Safety Advantages
Security Design Philosophy

• CAN Bus Protection: Blocks unauthorized key programming through OBD2 service port
• Dummy Port Concept: Uses a dummy OBD2 port to obscure the actual service port location
• Physical Barriers: Solid aluminum cover (1/8" thick) over BCM wire connectors with security screws prevents vampire clamp access

Operational Advantages

• No User Friction: Set and forget operation—arm once, no re-arming required until power interruption
• Integrated Concepts: Combines two proven security methods:
• "Keep Hope Alive's" Dummy Plug concept
• Ravelco's anti-theft device principles
• Compact Design: Smaller form factor dongle case (half previous size, not shown) that fits the wireless relay perfectly, enabling discreet installation

Theft Prevention Scenario
Even if a thief:

1. Pulls the dongle from the dummy port
2. Finds and accesses the live OBD2 service port
3. Cannot program a new key because CAN bus access is disabled until dongle is reinserted and armed with the FOB
4. However, if they manage to find the OEM OBDII port or vampire clamp onto the BCM wires then they are away to the races.

Test Results (1 Year)
Performance Validation

• One Year of Flawless Testing: The concept has been tested and working without failure for a full year
• Component Reliability: Test parts fit perfectly with a very small case for the wireless relay

Relay Durability Analysis

• Rated Operations: 100,000 minimum operations at heavy load (30VDC, 1 Amp)
• Actual Usage Estimate: Only 20-40 relay toggles over vehicle service life
• Margin of Safety: 2,500x safety margin (100,000 rated vs. 40 estimated operations)
• Holding Current: Only 8 mA when latched (12V / 1440Ω coil resistance)

Power Consumption

• Parasitic drain comparable to vehicle baseline (20-50 mA range)
• Minimal impact on battery when vehicle is off

Data Flow & Signal Control

Signal Path
1. CAN Bus twisted pair controls access to service OBD2 port
2. Dongle Relay Interruption: Controls whether CAN bus signal reaches the service port
3. OBD2 Port Communication:

• Terminating resistors simulate data link presence
• Prevents scanner/programmer from detecting open circuit
• Dummy port masks actual service port location

Backup System

• Spare dongle kept in vehicle as backup
• Manufacturer supports up to 3 FOBs
• Spare dongle can operate with simple wire connection as failsafe. No relay.

Enhanced Security Features
Additional Layers

• Hidden hood release: Prevents easy engine bay access
• Smart motion sensor alarm: Integrated into 12V charger socket with wireless siren under hood
• Soler Performance Accelerator Disable
• Security screws: Prevent dongle disassembly and BCM Security Cover Plate Removal

NOTE: The pair of wires (2 feet length) hanging on the completed harness will not be on your harness. Those wires were for Capricio's Design and connected to the wire leading to pin #15 on the BCM. See picture below. Ignore references to Pin #15 to Starter Relay Coil because that is wrong and Pin #15 DOES NOT control the starter relay, period.

I reused the harness as shown in the final schematic. Remember when I connected to the wire for Pin #15 and removed the dongle, my car shut off entirely, dead. Pin #15 is NOT the Starter control.

IMHO.....GM Chev have a very serious design flaw in the BCM that shuts down the car when Pin #15 is disconnected, open circuit, loose connection, etc.

[radz28]

This is frickin' sweet!!!

So - it would seem the "weakest" part of this layout would be access to enough of the BCM wire harness to connect vampire wires to so they can complete communication to the BCM to duplicate the fob. If I have that correctly, if we add enough layers of physical security (in addition to the block-off plate you describe) for the harness, it would take MORE effort for the POS and might discourage them from continuing to steal this car and move on to others? No?

Thanks for posting this!

[Roddrz]

The weak point is the exposed BCM connectors and wiring.

The photos illustrate how to create a protective block-off plate for these wires.

You nailed it radz28, the BCM connector is exactly what those POS' target to steal GM and Chevrolet vehicles.

There are lots of security solutions out there, but I went with a security DIY harness (CAN Bus Security Dongle Design) so I can still use the well hidden OBD port for my Range Technology AFM and connect a scanner for diagnostics when needed.

BTW. The pad lock on the hood latch in the first picture is a deterrent for the Service Technician if I need it. That latch actually doesn't work anymore because I installed a second hood latch somewhere else.

[Jetrep]

excellent work RODDRZ !!!

[2023-SS]

are you going into production to sell these pieces as a kit?

I found a guy who will cut the aluminum plates for us