Shown below is a 2003 VW data bus network diagram for the DLC connector. To test NO COMMUNICATION fault message from your scanner, you can use this diagram as your guide. Using a digital voltmeter, you can check problems related to NO COMMUNICATION by measuring the voltages shown in the DLC pin terminals.
This 2003 VW data bus wiring above has CAN high and Lo (hi speed) bus wires at pin 6 and pin 14 respectively. There is also a low speed bus wire (K-Line) at pin 7. These pins at DLC can be easily diagnosed using a voltmeter in minutes! For more details, you can get this exact wiring diagram with correct wire colors and pin connections from your local parts stores or get it online from ATS with one on one assistance .
2003 Data Bus Network Description:
Looking at the 16 pin terminals of the DLC, there are basically 3 pins to worry about:
PIN 6 which is the CAN Hi speed bus communication signal. When you measure it it with a voltmeter using PIN 5 as your ground, you should get 2.5 volts at key on engine off (KOEO). Take note that this voltage reading actually fluctuates between 2.5 volts to 3.5 volts at KOEO.
PIN 7 which is the K Line bus communication signal. This is a low speed bus signal and usually is a lone wire going to the accessory modules like Climatronic Control Module and the Radio as shown on top left. When testing for voltage, probe PIN 7 against PIN 5 and you should get about 5 volts at KOEO. If out of range, disconnect the 2 accessory modules one at a time along with TCM, Memory Seat module and Comfort System module.
PIN 14 is the High Lo bus communication signal. This bus is paired together with the CAN Hi bus wires which are twisted together. To measure the voltage of this bus at PIN 14 , probe this pin against PIN 5 and you should get 2.5 volts average reading at KOEO. This voltage is also fluctuating between 1.5 volts to 2.5 volts.
Troubleshooting 2003 VW data bus network:
Looking at the diagram, you can see the CAN bus wires (both CAN Hi and CAN Lo) entering the Instrument cluster. This module acts as a gateway module which feed to all the modules except the 2 accessory modules. So if we are to check if the Air Bag module is defective, we can measure the resistance between PIN 6 and PIN 14 with battery disconnected.
Isolate the Air Bag module by disconnecting it at the green and red terminals of the Air Bag module. Do the same when testing ABS, TCM (transmission control module) and steering angle sensor. If you find these modules to be OK, by process of elimination, what is left is the ECM or Diesel Direct Injection (DFI) module. Anytime the resistance reading is out of the 60 ohms range, this is the best strategy to use.
I usually verify the ECM or DFI by going straight to the sensor like TPS or crank sensor to check for presence of 5 volt VREF. Make sure to verify all terminal connection fits for loose connection which can cause lost of communication.
1.) The actual bus network diagram will show you the exact colors of the bus
wires and locations of the modules in the circuit.
2.) This DLC diagram can be used for 2001-2006 VW models. The only difference is the number of modules being used on later models. Also, the Instrument cluster is used all the time as a gateway module.
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