The background

Many 1997 and newer VW/Audi/Seat/Skoda (VAG) radios are 'diagnostic capable' - meaning that the dealerships can communicate with the radio to, for example, set whether a CD changer is present, or to test the speaker outputs.

This connection is provided by a wire between the radio and the diagnostic connector on the car (known as the K-line).

The problem

When fitting an aftermarket radio to a VAG car, many owners use an off-the-shelf wiring harness to connect between the car and radio wiring.

Certain wiring harnesses do not deal with the K-Line wire correctly, and short it to the 12V supply from the battery. Whilst this doesn't affect the car at all, it can badly damage diagnostic equipment when it attempts to connect to the car.

Diagnostic tools ground the K-Line to initialise communications, but if a hard 12V is present on the K-Line, this can't be grounded, and will damage the K-Line driver on the diagnostic tool.

The solution

Many modern diagnostic interfaces (e.g. Ross Tech's HEX or KEY interfaces for VAG-COM) contain protection against this problem, and will simply not connect to such a car.

Care should however be taken when connecting any tool to a VAG car with an aftermarket radio present. Many garages still refuse to connect to such a car if an aftermarket radio is present - replacing a garage-based diagnostic tool is very expensive.

Testing for the problem

It is possible to test for the problem.

• If the car is a pre-1996, then it is most likely to be fine. The K-Line did not go to the radio before then.
• On a post-1996 car:
  • measure the voltage between pins 4 and 7 on the diagnostic connector (layout shown here)
  • if the voltage is below 9V, then it is fine
  • if the voltage is over 9V, then more testing is required. A 1k Ohm resistor should be temporarily connected between pins 4 and 7, and the voltage should again be measured (in parallel to the resistor). If the voltage is now down to about 1V, the car is fine. If it is still at around 12V, then the Aftermarket Radio Problem is present (the resistor will also get hot if this is the case).