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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 KII
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).
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