Diagnostic Cable Issues: 20m Cable Not Working.

Translated on 09-09-2026, 5:40.

Hello,
"Today, I built the diagnostic interface version 2.4 using optocouplers. VCDS recognizes the adapter, and the LEDs and other functions work as expected, just like the baud rate test with the adapter test software. However, the control unit of my Golf 4 TDI, manufactured in 1998, is not responding. Since I don't have a laptop, I connected everything via a 20-meter network cable. Could this be the reason why it's not working? If so, what can I do to make it work?"

-regards-

Translated on 09-09-2026, 5:42.

Sven@G4H wrote:

Hello,
"Today, I built the diagnostic interface, version 2.4, using optocouplers! VCDS recognizes the adapter, and the LEDs and other functions work as expected, just like the baud rate test with the adapter test software! However, the control unit of my Golf 4 TDI, manufactured in 1998, is not responding. Since I don't have a laptop, I connected everything using a 20-meter network cable. Could this be the reason why it's not working? If so, what can I do to make it work?"

-mfg-

Hi,

It's better to move the PC to the garage. Is the OBD2 connector properly connected? My customers sometimes get the pins on the diagnostic connector mixed up (reversed).

Best regards, Rainer.

Translated on 09-09-2026, 5:43.

Hi,
The pins are definitely connected correctly! Why does it stop working when using a longer cable?

-regards-

Translated on 09-09-2026, 5:43.

Sven@G4H wrote:

The pins are 100% correctly connected! Why does it stop working with a longer cable?

Perhaps the trimmers on the adapter are adjusted in such a way that the communication with a shorter cable just barely works, but the capacitance of longer cables leads to unacceptable delays in the signal edges.

Translated on 09-09-2026, 5:44.

Hi,
Okay, it's working now! I experimented with the trimmer setting again, and now I can connect either a 1.5-meter cable or a 20-meter cable, and it works with both!

-regards-