Setting Up Test Environment for A4 B6 Concert II Diagnostics with VCDS via L-Line

I have trouble diagnosing Concert II on A4 B6. It was retrofitted to a car with aftermarket radio preparation only.
I am unable to communicate with VCDS through the L line to radio address 56.
The radio is the only controller on the L line in this car. The L line is connected from the OBD connector pin 15 to radio T8 pin 3 as specified.
The L line voltage level is approximately 9V when connected to the radio and 0V without.

How do I set up a test environment on a table? Are there any line terminations/pull-downs on the K/L lines? Or is it just power, ground, and the L line?

Setting up a test environment on a table can be a practical approach for diagnosing communication issues with the Concert II on an Audi A4 B6. Here's a step-by-step guide:

1. Test Bench Setup:
Set up a clean workspace on a table where you can comfortably work with the components. Ensure you have access to power sockets and a stable surface.

2. Power Supply:
Use a stable power supply to provide power to the Concert II unit. The power supply should match the required voltage and current specifications of the unit. Typically, automotive systems operate on 12V DC, but double-check the specifications for your specific unit.

3. Ground Connection:
Ensure proper grounding for the Concert II unit. Connect the ground wire of the unit to a suitable ground point on your test bench setup. This helps stabilize the electrical connections and prevent potential issues.

4. Communication Lines:
Connect the communication lines, including the L-line, between the Concert II unit and your diagnostic equipment. Use appropriate cables or adapters to make the connections securely.

5. Diagnostic Equipment:
Use a diagnostic tool like VCDS (VAG-COM Diagnostic System) to communicate with the Concert II unit. Connect the diagnostic tool to your computer and ensure it's configured correctly for Audi A4 B6 models.

6. Line Terminations and Pull-downs:
Check for any line terminations or pull-down resistors on the K/L lines. These components are often used in automotive communication networks to improve signal integrity and prevent issues like signal reflections. Refer to the vehicle's wiring diagrams or technical documentation to identify any such components and ensure they're properly connected in your test setup.

7. Voltage Measurement:
Measure the voltage levels on the L-line using a multimeter. Compare the measured voltages with the expected values to ensure proper communication signal levels.

8. Testing and Troubleshooting:
Once the setup is complete, attempt to communicate with the Concert II unit using the diagnostic tool. Monitor the communication interface for any errors or issues. If communication fails, troubleshoot systematically by checking connections, voltage levels, and potential signal integrity problems.

By following these steps and maintaining a systematic approach to testing and troubleshooting, you can effectively diagnose communication issues with the Concert II unit on your Audi A4 B6.

Show the cubes on the radio.

Copilot gave answer bit more to details. 10Kohm between L-line and GND
Have to try that.

As side note, Audi A3 Concert unit would not work on A4. Now I have right unit and everything works. Have to test A3 unit on bench though.

Copilot
"Setting up a test environment for diagnosing the Audi A4 B6 Concert II radio using VCDS via the L-Line requires careful attention to wiring and voltage levels. Let’s break it down:

Test Environment Setup:
Concert II Radio: If you’re diagnosing the Concert II radio, ensure it’s properly connected to your test setup.
VCDS (VAG-COM): Connect your VCDS diagnostic tool to the car’s OBD connector.
L-Line Connection: The L-Line (diagnostic communication line) connects from the OBD connector pin 15 to the radio’s T8 pin 3.
Power and Ground: Make sure the radio has power and ground connections.
L-Line Voltage Levels:
When connected to the radio, the L-Line voltage level should be approximately 9V.
Without connection to the radio, the L-Line voltage should be 0V.
Pull-Down Resistor:
The L-Line typically doesn’t require a pull-down resistor.
However, if you encounter communication issues, consider adding a 10kΩ pull-down resistor between the L-Line and ground. This can stabilize the signal.
Remember, this is a test environment, so ensure proper connections, check voltage levels, and troubleshoot any issues. Good luck with your diagnostics!