DIY OBD Adapter: Build Your Own Diagnostic Tool (Articles)

Translated on 28-07-2026, 20:13.

KKL Diagnose Adapter - OBD DIY

Bei allen Fahrzeugen des Volkswagen Konzerns (Audi, Bentley, Seat, Skoda und VW) ab etwa Baujahr 1993 lassen sich über die Diagnoseschnittstelle mehr oder weniger wichtige Informationen der Motormanagements abfragen. Bei aktuellen Fahrzeugen ist eine Diagnosesoftware zur Servicerückstellung sogar zwingend notwendig. Dies kann mit einem Adapter (Diagnosesystem) für die USB/serielle Schnittstelle des PC bewerkstelligt werden, dann braucht man nur noch einen preisgünstigen Laptop/Notebook und die entsprechende Software dazu. Dies ist vor allem für kleinere Werkstätten und Bastler interessant, ohne die Möglichkeit des Zugangs zur Onboard Diagnose artet die Fehlersuche an der heutigen komplexen Systemen der Motorelektronik leicht zum Ratespiel aus, obwohl das Auslesen des Fehlerspeichers schnell Klarheit verschaffen könnte. Für ältere Fahrzeuge reicht ein Diagnoseadapter mit KKL (K-Line), hier ist die Bauanleitung dazu. DiThe attached list, under the section "HEX-USB Diagnostics," mentions specific vehicle models for which professionally manufactured diagnostic systems are available.

There are several other programs that allow individuals to gain insight into modern engine electronics at affordable prices. One of them is VCDS from Ross-Tech, for OBD2 via CAN-Bus (manufacturer-independent vehicles from 2008 onwards), and another is the dieselschrauber KOBD2Check.

The last VCDS software version that works with self-built interfaces (e.g., our B1 or U1 kits) is the manually unlockable VCDS-Lite from Ross-Tech, which is still being maintained (label files, Autoscan, plugins where possible).

In order for VCDS-Lite to work with a self-built adapter, the adapter must meet certain criteria in its transmission properties. Starting with version 504.1, VAG-COM/VCDS always queries the license data within the diagnostic system; manual activation is no longer required (dongle function).

The same applies to the diesel repair OBD2 software KOBD2Check (dongle function): https://shop.dieselschrauber.org/en/obd2-diagnose-kit-p-321.php
You can purchase repair guides on how to use the diagnostic tools, for example, directly from VW (

Erwin). The most common procedures can be found on the VCDS, information about the procedure, repair manuals overview.

Now, regarding the adapter problem - what should be considered when building it yourself?
Since the vehicle's diagnostic signals are not compatible with those on the computer's COM port/USB port, both VCDS-Lite and an adapter are required for communication.

For DIY soldering projects,
schematics designed with a minimalist approach are often suggested. These work flawlessly in many cases, but sometimes they don't. In some instances, increasing the value of resistor R7 to approximately 10 kOhm proves to be necessary.
effective remedy. Other details of the standard circuit can also lead to problems:

for each bit.
When the 0.6 volts are just reached, individual characteristics of optocouplers 3 and T2, operating temperatures, the current onboard voltage, impedance ratios of the COM port, and the limited slew rate on the K-line can lead to timing shifts in the high-low transitions detected by the PC, or bits may be completely lost, which manifests as synchronization problems, various error messages, etc.

Instead of adapters with optocouplers, it is also fundamentally possible to use types without galvanic isolation between the PC and car circuits. These are generally superior in terms of their transmission characteristics compared to adapters with optocouplers, provided that a sensible circuit design is used.
Only in cases of problematic combinations of ST generators and diagnostic computers can adapters without optocouplers sometimes enable more stable communication.
However, such adapters can cause problems if they are

not the only connection between the car and the PC, for example, if a diagnostic laptop is powered from the vehicle's electrical system. This can happen when errors or electrical incompatibilities occur.
In the worst-case scenario, the laptop and/or the vehicle's electronics could be damaged.

Those who want to have the greatest possible security in such cases should still use an adapter with galvanic isolation between the circuits of the computer and the car.

To reduce or minimize the problems associated with optocouplers, the...

Standard circuit redesigned and expanded. The new design includes:

. Furthermore, the following were provided:

. Note:

At high voltages and low impedance of the RXD pin on the COM port, LED 3 may constantly glow faintly, even if the OBD connector is not plugged in or no data is being received from the vehicle.This is not a defect in the adapter and does not affect its function; it is simply caused by the relatively simple circuit of LED 3 and can be ignored. Nevertheless, data reception from the vehicle can still be recognized by observing a brighter flashing pattern of LED 3.

If the diagnosis is only to be performed on vehicles without an active L-line, then components R 1 through 4, VR 1, OK 1, LED 1, and T 1 can be omitted during the adapter assembly.

Adjustment of the trimmers

(Note: If the diagnostic PC is replaced with a different model, readjustment may be necessary for reliable communication.)
Set all trimmers to their maximum values. In the VCDS-Lite LED test, LEDs 1 to 3 must blink in sequence - sometimes individually.
partially together. Using VR 1 and 2, the brightness of LED 1 and 2 should be adjustable.

green range. If necessary, adjust VR1 optically again like VR2.

If the adapter behaves differently, the functions on the test bench can be checked as follows.
A voltage source of approximately 12 volts is required, along with a voltmeter and a set of measurement cables with alligator clips – or alternatively, some pieces of wire and a soldering iron to create the connections. All measured voltages are referenced to...
Vehicle-side mass.

Preparation:

Tests (measure voltages against ground; the voltages to be supplied are also to be understood as being relative to ground):

logical lighting behavior of the adapter will lead to malfunctions.

Christian designed a

PCB layout for the above circuit diagram.
"Thank you very much!"

Adapter test software from Andreas (AST) and a user manual from Ulf, available for download

.

For vehicles with control units divided into two K-lines, the following

designapplies.

Johannes has created files for Eagle

: schematic, board.

Based on previous experience, a properly configured 2.4 adapter is usually sufficient for a stable connection in most cases.
Due to its relatively simple basic concept, it has some weaknesses that can make the communication unstable under extreme conditions (e.g., bit errors caused by significant fluctuations in the on-board voltage when starting the engine, extremely long cable lengths with high capacitance between the PC and the adapter, very unfavorable data from the COM port). Furthermore, it can only communicate with a K-line, and the data lines on the vehicle side are not protected against voltage spikes from the on-board network.

The

"Worst-Case Design" 4.0 offers advantages compared to...
The following improvements are available from version 2.4:

The principle of the protection diodes D11 to D16 can also be applied to other adapters.

If dual-K diagnostics are not required, then D4, 12 and 16, LED 4 and R10, as well as the switch can be omitted; cable 15 is connected directly to the L-line.
For vehicles without an L-line, D1, 13 and 16, R1 - 4, VR1, OK1 and T1 can be omitted.

If a simple plug-and-play adapter is needed to facilitate communication, for example, in critical combinations of vehicle, diagnostic PC, and software, by minimizing signal delays and distortions, then this one can be used.

Circuit for a diagnostic adapter without galvanic isolation and without MAX232.
They are used and operate stably at up to approximately 500 KB, depending on the cable capacities and characteristics of the COM port. However, as with all adapters without galvanically isolated circuits for the PC and vehicle sides, there is an increased risk of damage to the PC and vehicle electronics under unfavorable conditions (e.g., faulty adapter construction, powering the PC from the vehicle's electrical system using unsuitable power supply circuits or transition resistors, loose connections, etc.).

LED1 displays the data sent to the vehicle, LED2 displays the received data, LED3 indicates the control of the L-line, LED4 shows whether the operating voltage is sufficient (it goes out below 8 volts), and LEDs 5 and 6 indicate interference voltages between the vehicle and the PC, which can affect communication.

For vehicles without the L-Line, parts D 8, LED 3, T 4, and R 10 to 12 can be omitted.
will be.

For owners of older optocoupler adapter versions from Ulf...

Common problems
(Authors: Ulf, Rainer)

Unfortunately, the data exchange between the PC and the vehicle is so complex due to the involved hardware and software that the initial attempts using VCDS-Lite are often described as "plug and pray" (i.e., simply plug it in and hope for the best).

"I can't connect to any STG at all!"

The most common causes are:

1. Adapter incorrectly designed/constructed/assembled or defective.

2. The electrical specifications of the adapter and the PC interface are not compatible. This can happen relatively easily due to the wide variation in characteristics of COM ports, especially when using adapters with optocouplers, which are often not very tolerant regarding the data transmitted through the COM port.
Adapters with corresponding adjustment options can provide a solution, such as the versions 1.1, 2.4, and 4.0 shown here.

3. On-board errors (ignition not switched on, cable breaks in the area of the diagnostic connector, etc.).

4. störende Software-Konfiguration des PC (vgl. No.

5. Computer malfunction / VCDS-Lite installation error.

"I cannot access all control units (STG), only some functions work, and VCDS-Lite constantly displays error messages."

This can mainly be caused by the following reasons:

6. Significant bit-length errors in the adapter impede communication, particularly with STGen devices operating at higher baud rates.
Design or adjustment errors in the adapter are most likely to occur in circuits using optocouplers.

7. The adapter's onboard logic-low detection threshold is lower than the logic-low output level of the "silent" STGs, which often (and still compliant with ISO standards!) provide 1 to 1.5 volts instead of the ideal value of 0 volts.
Design or calibration errors in the adapter; ideally, a logical "0" should be recognized as soon as and while the K-line voltage is less than half of the vehicle's supply voltage.
Attempt to fix the issue (if the actual adapter circuit cannot be corrected): Connecting 1 to approximately 4 diodes, such as 1N4002 or similar, in series and in forward bias within the ground wire leading to the OBD connector (pin 4) might resolve the problem with some luck.

8. The overall software configuration of the PC is interfering with the VCDS-Lite dialog.
Solution: Close or delete as many background programs as possible, or verschandeln up the startup files.
If necessary, try a PC with a different software configuration; using identical hardware allows you to more reliably pinpoint the cause of the problem within the software.
More information can be found on Uwe Ross's website.

9. The data format of older STGe devices (especially those from 1Z engines) is not fully compatible with the version of VCDS-Lite being used.
Joergs has come up with the following solution:
It can be helpful to gradually decrease the value for 'Blk Int' in the program options.

Regarding the general settings for 'Char Int' and 'KW2 Delay': decreasing these two values can improve performance on slower computers, while increasing them may help on faster machines.

Thank you, Jörg!

"I'm getting inconsistent text when I read the error memory of the STGe device!"

Take some time to reflect and make sure you have the proper authorization for your VAG-COM/VCDS-Lite version!

When purchasing adapter kits without an included wiring diagram, you are essentially buying a "black box" regarding potential (but not necessarily guaranteed!) problems related to items 1, 2, 6, and 7.
This also applies to (www-)auctions, flea markets, etc., where design flaws from private/unknown manufacturers are apparently sold even more openly than in regular retail

.

If an adapter is recognized by VCDS-Lite or if a check using the provided adapter test software yields a positive result, then only adapter problems listed under numbers 1 (excluding the cable and connector on the vehicle side!), 2, and 6 can be largely ruled out.
Error number 7 can still occur, especially when using the widely used optocoupler minimal circuit and related designs, which may also be included in various commercially available adapters.

In addition to everything else, items 6 and 7 in particular can also vary depending on the onboard voltage, ambient temperatures, etc. (which makes them quite unpredictable).

For these reasons, the purchase of pre-made adapters from unknown manufacturers or suppliers, or with unclear designs, should be carefully considered.
If high reliability is desired from the outset, circuits like those described or linked here should be preferred (with the exception of the minimal designs). Furthermore, support is often available in this forum, and various problems have already been addressed previously, which can be found using the search function.

But:
Manufacturing defects and flaws can occur in any individual component, therefore creating an almost overwhelming variety of error patterns. If measuring instruments are also missing, and/or the person seeking help lacks a basic understanding of electronics, then providing meaningful support via remote diagnosis is practically impossible!

For training purposes, teaching, or professional work on the powertrain, the OBD2 diagnostic software KOBD2Check is a good option and is available in the shop: https://shop.dieselschrauber.org/en/rks-can-can-bus-interface-c-24.php

Here is an example of boost pressure, measured with VAG-COM (Excel file).