DIY OBD to PC Cable: Build Your Own for Under $20

Are you looking to diagnose your older vehicle using your PC? An Obd To Pc Cable is essential for connecting your computer to your car’s On-Board Diagnostics (OBD) system. While you can purchase a pre-made OBD1 cable for $50-$70, you can save a significant amount of money and gain a satisfying DIY project by building your own for less than $20. If you have basic soldering skills and a few tools, this guide will walk you through creating your own OBD to PC cable.

Before you begin, consider this potentially easier and cheaper alternative: http://www.firebirdnation.com/forums/topic/457761-make-an-obd1-cable-for-under-20/page__view__findpost__p__888714 for a potentially simpler solution. However, if you’re ready to build, here’s what you’ll need:

Parts and Tools You Will Need

To create your OBD to PC cable, gather these components:

1. USB-to-Serial Port Adapter (FTDI-based): This is the core of your cable, allowing your PC to communicate with your car’s OBD-I system. It’s crucial to choose an adapter based on an FTDI chip, as Prolific-based adapters are not compatible with this application. The Sabrent CB-FT1K is a recommended option, but any equivalent FTDI-based adapter will work. These typically cost around $20, but you can often find them for about $10 if you shop around. You can find options at online retailers like Newegg. Ensure you install the driver for the adapter, usually provided on a CD or downloadable online. It’s generally recommended to install the driver before plugging in the adapter.

2. Electronic Components: Obtain these parts from electronics supply stores like Radio Shack or online retailers:

   • 1K ohm Resistor: This resistor is a standard 1/4 watt resistor and is crucial for signal conditioning.
   • Diode: A silicon or switching diode is suitable (e.g., 1N4148 or 1N4001). Avoid Zener diodes as they are not appropriate for this application. The diode helps protect the circuit and ensures proper signal flow.
   • DB9 Connector (9-pin Female/Socket Type): This connector will interface with your USB-to-Serial adapter. Choose a DB9 connector with solder terminals for easy wiring.
   • Wires: You’ll need at least two insulated wires, each about 6 inches long. Stranded AWG 20 or 22 wire is recommended for flexibility and ease of soldering. Different colored wires are helpful for easy identification, or you can mark one wire for polarity.
   • Optional OBD-1 12-pin Plug: While not strictly necessary, an OBD-1 12-pin plug provides a professional and secure connection to your vehicle’s diagnostic port. However, these can be harder to find affordably. You can initially use individual pin connectors or carefully insert the wire ends into the OBD-I port if you choose to skip this part initially.

3. USB Extension Cable (if needed): If your USB adapter lacks an integral cable, a USB extension cable will be necessary to connect it to your PC. These are inexpensive and widely available online or locally.

4. Soldering Tools:

   • Soldering Iron and Solder: Essential for creating reliable electrical connections.
   • Wire Stripper: For removing insulation from wires without damaging the conductors.

Step-by-Step Guide to Building Your OBD to PC Cable

Follow these steps to assemble your DIY OBD to PC cable:

Step 1: Cable Fabrication

Carefully assemble the components as illustrated in the diagram below. Solder the components to the DB9 connector as follows:

• Connect one end of the 1K ohm resistor to pin 7 of the DB9 connector (RTS).
• Connect the other end of the resistor to the anode (non-banded end) of the diode.
• Connect the cathode (banded end) of the diode to pin 2 of the DB9 connector (TXD).
• Connect one wire (e.g., black wire) to pin 5 of the DB9 connector (Ground).
• Connect the second wire (e.g., white wire) to pin 3 of the DB9 connector (RXD).

Ensure your solder joints are clean and solid. It’s crucial to properly insulate the components and wires at the DB9 connector to prevent short circuits. Heat shrink tubing or electrical tape works well for insulation. “Tin” the ends of the wires that will connect to the OBD-1 connector with solder. This makes them sturdier and prevents fraying when inserting them into the OBD-I connector. Pay close attention to the diode’s polarity; the cathode (marked with a black band) must face towards the DB9 connector as shown in the diagram. The resistor is non-polarized and can be connected in either direction.

Step 2: Reprogramming the FTDI Chip

To ensure proper communication, you need to reprogram the FTDI chip within your USB-to-Serial adapter. This involves inverting the TXD and RXD signals.

1. Download FT_Prog: Obtain the FTDI utility “FT_Prog” from the official FTDI website: http://www.ftdichip.com/Support/Utilities.htm#FT_Prog.

2. Install and Run FT_Prog: Unzip the downloaded file, install the FT_Prog utility, and run the application.

3. Connect USB Adapter: Connect your USB adapter to a USB port on your PC.

4. Detect Device: Press the F5 key in FT_Prog to scan for connected devices. Your adapter should appear in the left-side box. If multiple devices are listed, disconnect any other USB-to-Serial adapters. If no devices are detected, double-check your driver installation for the adapter.

5. Invert Signals: Click on “Hardware Specific” and then “Invert RS232 Signals”. Initially, no checkboxes should be selected. Check the boxes for “Invert TXD” and “Invert RXD” as shown below.

6. Program Device: Press Ctrl + P keys together (or navigate to “Devices” > “Program”). In the “Program Devices” window, click the “Program” button. Wait for the “Finished Programming” message to appear.

7. Close and Exit: Close all FT_Prog windows and exit the program.

To revert the adapter to its original settings, repeat these steps, but uncheck the “Invert TXD” and “Invert RXD” boxes during reprogramming.

Connecting to Your Vehicle and Running Diagnostics

Your DIY OBD to PC cable is now complete. To use it:

1. Connect to OBD-I Port: Locate the OBD-I diagnostic port in your vehicle. The black wire from your cable connects to pin A (Ground) in the OBD-I connector, and the white wire connects to pin M (ALDL Data). Refer to your vehicle’s service manual for the exact location and pinout of the OBD-I connector.
2. Install Diagnostic Software: Install compatible OBD-I diagnostic software on your PC. Popular options include ScanTool.net, TunerPro RT, and others depending on your vehicle make and model.
3. Run Diagnostics: Launch your chosen diagnostic software, configure it to use the correct COM port for your USB-to-Serial adapter, and initiate communication with your vehicle’s computer.

Important Notes:

• Accidentally shorting the black and white wires will not damage anything but will temporarily interrupt communication with the vehicle’s Powertrain Control Module (PCM).
• This cable configuration is primarily designed for 4th Gen GM vehicles. For earlier vehicles or other makes, modifications, such as adding a resistor and connecting the white wire to a different pin, might be necessary. Consult your vehicle’s specific OBD-I wiring diagrams and diagnostic software documentation for compatibility and specific connection instructions.

By following these steps, you can create a functional OBD to PC cable, enabling you to perform diagnostics on your OBD-I equipped vehicle using your computer, all for a fraction of the cost of a pre-made cable.