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Tim West
Tim West, ASE Certified Technician
Category: Car
Satisfied Customers: 4638
Experience:  ASE Certified Service Manager for Bridgstone/ Firestone
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I have a 2001 vw Jetta 1.8T. My check engine light is on and

Resolved Question:

I have a 2001 vw Jetta 1.8T. My check engine light is on and I had to take it in to have the emission inspection. The mechanic said that he could not get the OBD to communicate. He also tried to do an overall diagnostics to see why the check engine light came on, but still no luck. It would not communicate with the diagnostic mechine. He said all the fuses were fine. The only thing different on the car is that an aftermarket radio has been installed. Can you help me out here?
Submitted: 5 years ago.
Category: Car
Expert:  Tim West replied 5 years ago.
Thanks for choosing JA,
It's possible his scan tool is out dated. Where was this mechanic? Was it a "big" name place, or a local "ma and pa" shop?
Customer: replied 5 years ago.
I took it to 2 places. the first was a loacal mechanic and the second was an Exxon station mechanic. He hooked his diagnostic machine to another vechicle to show me how it worked. and then he hooked it to my vw to show me that it wouldn't communicate.
Expert:  Tim West replied 5 years ago.
If that's the case then the new stereo has been installed incorrectly.
Here is

Documentation from Volswagen

provided in PDF

Take it back to whoever installed the stereo and give them this information:

1. Use a Digital Volt Ohm Meter (DVOM) to measure voltage at pin 7 of the Diagnostic Link Connector (DLC).

2. If full battery voltage is present, a problem in the K data communication line is indicated (normal voltage will run from about 2.0 volts to a maximum of about 10.5 volts).

3. Check for installation of an aftermarket radio.

4. If an aftermarket radio has been installed, the radio wiring harness must be modified to prevent the aftermarket radio from applying battery voltage to the K data line.

5. Remove the radio and look at the Black 8-pin connector. Pin 3 of the Black 8-pin connector is the K data line.

6. Remove pin 3 from the connector and tape it back on the harness in such a manner as to preclude shorting.

7. Replace the radio and check for correct voltage at pin 7 of the DLC.

8. If 0 Volts is present on Pin 7 of the DLC, a potential short to ground in the wiring harness, or a shorted controller on the K data line is indicated.

9. Disconnect the controllers on the K data line one at a time, while monitoring the DLC pin 7 voltage.

10. If the DLC Pin 7 voltage returns to normal after a controller is disconnected, an internal short in the disconnected controller is indicated.
Tim West, ASE Certified Technician
Category: Car
Satisfied Customers: 4638
Experience: ASE Certified Service Manager for Bridgstone/ Firestone
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