DIY Car Diagnostic Tool: Build Your Own!

Hey everyone,

I'm considering building my own diagnostic connector, but I'm having trouble accurately translating the English instructions and I don't know what components I need. Also, I have no idea where to find a cheap adapter plug and the software.

Perhaps someone has a tip for me on where I can get it very cheaply, maybe used, because I can't and don't want to spend much money on it.


Thank you in advance.

Hi,
The link to the software is located on the left side, under 'Diagnosis Adapter...'. You can find the parts, for example, at Reichelt (or possibly at your local electronics store).
Den Original Optokoppler, der in der Schaltung angegeben ist, gibt es nicht (mehr). Es sind aber einige Ersatztypen angegeben. Ich habe das Teil mit einem ILQ615-4 (mindestens 100% Übertagung) auf Lochraster aufgebaut. Works.
The diagnostic connector is not available (at least not at a reasonable price). You could try to improvise something using a connector housing similar to the one used in cars, or you could tap into the three to four relevant wires of the diagnostic connector and connect them to a D-sub 9 connector that matches the pinout specified in the wiring diagram.

Hi there,

I patched the adapter onto the grid of holes using a piece of wire threaded through them.

Optocouplers: ILQ5 (quad). Alternatively, the CNY17-3 would also work.

transistors: BC-546

works perfectly...

Unfortunately, I don't have the correct connector for the diagnostic interface.
So, I guess I'll just stick the bare wires in. :/

If anyone has any sources for that?

CU Gremlin.

Hi,
I am not aware of any sources for inexpensive diagnostic connectors.
Expensive sources would be DRV in Berlin (German distributor of VAG-COM), but they sell the complete adapter with connector (and software?), or directly from VAG (connecting cable for models 1551/1552).
Here's an idea that I haven't tried myself yet: You could take a connector housing from the vehicle (for about 2-3 EUR) and glue in matching pin contacts (or, if easier, use pre-made cables with connectors and 10cm of wire from VW). A housing for the connector could be made by folding thin plastic (like Makrolon) and securing it to the main housing with silicone adhesive.

Für 50 € könnte ich einige komplette OBD II Stecker mit Innenleben besorgen. Mit Invoice and related documents.

Yes, it's very expensive. But I can't do anything about it.

I'm currently building a diagnostic adapter and I read in a forum that you have experience with it...

I built the adapter almost exactly according to Jeff's circuit diagram, but it doesn't work.
'As optocouplers, I had to use three CNY17s because I couldn't find any others. I suspect the problem lies with the optocouplers themselves, as I'm not sure how to connect them correctly. The CNY17 has three connections on the transistor side (base, emitter, collector), but Jeff's schematic only shows two connections.'

It would be nice if someone could help me.

Sure, here is the translation of the text from German to English:

'Hi,'
It's possible that the CNY17 still won't work. However, I won't have access to the datasheets until Monday.

I skipped the base part. (I thought that right away.) I just checked the optocouplers and the circuit. It works electrically; i.e., the transistors and optocouplers are switching. And I'm also getting signals on the 'L' line when I test with VAG-COM, but the K-line isn't changing...

Furthermore, I've noticed that my Golf doesn't have an L-line at all, or rather, it seems like the pin for the L-line isn't connected on the connector.

Hi,
The L-line was only required in older models (development stage prior to the Golf 3; e.g., Audi 80 TDI).
The K-line should also be moving (assuming the 12V is connected). If you have a continuity tester, please check all the connections again (and make sure they are only connected to where they are supposed to be).

At the time, I had to increase the pull-up resistor of the K-line from 1k to approximately 10k in my interface.

I've double-checked everything and replaced a faulty transistor. All the connections are fine and in the correct places...

The K-line is currently grounded in its idle state. However, when I test it with VAG-COM, the K-line voltage occasionally fluctuates to 12 volts.

The interface in the car still doesn't work.

I'm slowly going crazy. If it weren't so (unnecessarily) expensive, I would buy a ready-made adapter from Auto-Intern. But it can't be that difficult to build this interface myself...

I've put together more complex circuits than that before.

My problem is that I don't really know much about it. I can assemble the parts and understand how each component works, but that's pretty much all I know.

Hello.
What laptop do you have and what software do you use?

I have an old Toshiba laptop with a Pentium 133 processor and I'm using it to test with VAG-Com.

I've since received the tip that the optocouplers I'm using may not be the right ones, as their gain seems to be too low.

Hello.
I have the same laptop.
If you don't have the latest version of VAG software, you won't be able to connect!
VAG actually had a bug in Toshiba laptops.
I was also frustrated with it for several days, and then I just used a different laptop, and it worked.
Try version 0.81; if you haven't encountered any other errors, it should work.
See you, Thomas. / Bye, Thomas.

I have version 0.81.

'I'll double-check everything again this afternoon, and just to be safe, I've ordered some more optical isolators. I'm going to test them with the new components this afternoon.'

And if nothing else works, I'll throw a cable out the window and connect it to my regular PC.

VAG actually had a bug in Toshiba laptops.
I was also frustrated with it for several days, and then I just used a different laptop, and it worked.
Try version 0.81; if you haven't encountered any other errors, it should work.
See you, Thomas.


Okay... I was running version 0.78 on a Tecra 540CDT under Windows 98.

Absolutely without any problems...

CU Gremlin.

Okay... I gave up and ordered the damn thing from the store.

I've rebuilt and retested the circuit again, and it still doesn't work. I even used new components and sockets to make sure nothing would get damaged during soldering...

Even so, it's not working. I'm really going crazy!!!!