Diagnosis Adapter - DIY - On Board Diagnosis (Fachartikel)

Zuletzt bearbeitet am 24-10-2021, 17:56, insgesamt 40-mal bearbeitet.

KKL Diagnostic Adapter - OBD DIY

This adapter allows you to access more or less important information from the engine management system of all vehicles in the Volkswagen Group (Audi, Bentley, Seat, Skoda, and VW) from around 1993. For current vehicles, diagnostic software for service reset is also necessary. This can be achieved with an adapter (diagnostic system) for the PC's USB/serial interface, then you only need an inexpensive laptop/notebook and the corresponding software. This is especially interesting for smaller workshops and hobbyists, as without the ability to access the on-board diagnostics, troubleshooting complex modern engine electronics quickly becomes a guessing game, although reading the error memory could quickly provide clarity. For older vehicles, a diagnostic adapter with KKL (K-Line) is sufficient, and the instructions for this are included. The list attached under the "HEX-USB Diagnosis" section lists such vehicles, including professional-grade diagnostic systems.

There are also a number of other programs that allow private individuals to access modern engine electronics at affordable prices. One of these is VCDS from Ross-Tech, for OBD2 via CAN-Bus (manufacturer-independent vehicles from 2008), the dieselschrauber KOBD2Check.

The latest VCDS software version that works with self-built interfaces (e.g., our B1 or U1 kit) is the VCDS-Lite, which can be manually unlocked by Ross-Tech, and which is also further developed (label files, Autoscan, plugins where possible).

For VCDS-Lite to work with a self-built adapter, this must meet certain criteria in its transmission properties. From version 504.1, VAG-COM/VCDS always queries the license data in the diagnostic system; manual unlocking is no longer necessary (dongle function).

The same applies to the dieselschrauber OBD2 software KOBD2Check (dongle function): https://shop.dieselschrauber.org/obd2-diagnose-kit-p-321.php

Repair manuals for using the diagnostic tools can be purchased, among other places, directly from VW (Erwin). You can find the most common procedures on the VCDS, information on the procedure, repair manuals in an overview.

Now, let's talk about the adapter problem – what should you keep in mind when building one yourself?

Since the vehicle-side diagnostic signals are not compatible with those on the COM-port/USB-port of the computer, not only VCDS-Lite, but also an adapter is required for communication.
For DIY solderers, minimalistically designed circuits are often proposed. These often work without problems, but sometimes they don't. In some cases, increasing R7 to approximately 10 kOhm proves to be an effective solution.

Other details of the standard circuit can also cause problems:

The high-low switching threshold of T 2 is in the range of 0.6 volts, which is only about 5% of the nominal 12-volt total voltage. This is particularly problematic when combined with the relatively small original value for the pull-up resistor R 7, which can cause problems for the on-board electronics, preventing the K-line from being pulled low enough for each bit.When the 0.6 volts are just reached, the individual properties of optocoupler 3 and T 2, operating temperatures, the current on-board voltage, the impedance ratios of the COM-port, and the limited slew rate on the K-line can lead to time delays in the high-low transitions detected by the PC, or bits can be completely lost, which can be noticeable in synchronization problems, various error messages, etc.
Especially laptops with weak outputs on the COM-port can experience problems with the adapter-side wiring of the RTS and TXD wires: due to the antiparallel diodes at the optocoupler inputs, high currents flow at the low level, just as during the control of the optocouplers. Since the RTS output, which is switched to low when receiving data, is further loaded through R 3 and the output of optocoupler 3, the so-called "excessively high" current on the K- and L-lines can overwhelm the COM-port and cause communication errors.
Depending on the optocoupler and input current, saturation-related delays can occur in the signal flanks, where the rising and falling flanks are delayed differently: this is a typical characteristic of optocouplers that can lead to communication problems due to bit length errors.

Furthermore, there is no optical functional check during operation, and ultimately, the selection of optocouplers can cause problems: sometimes, difficult-to-obtain types are specified.

In principle, adapters without galvanic isolation of the circuits from the PC and car can also be used instead of adapters with optocouplers. These are generally superior to adapters with optocouplers in terms of their transmission properties, provided that the circuit design is appropriate.
However, in problematic combinations of STGen and diagnostic PCs, adapters without optocouplers can 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 on-board network. In the event of errors or electrical incompatibilities, the laptop and/or the vehicle electronics could be damaged.

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

To minimize problems associated with optocouplers, the
standard circuit has been redesigned and expanded.

The new design includes:

a fixed voltage divider that sets the high-low detection threshold for the K-line to approximately 6 volts.
D 1 and 2, which reduce the load on the COM port when the K and L lines are not controlled.
VR 1 to VR 3, to adjust the control currents for the optocouplers so that the shortest signal propagation times are achieved with the same rising and falling signal edge delays. The optimum depends individually on the optocouplers used and the impedances of the PC interface. For VR 1 to VR 3, it is recommended to use dust-resistant, encapsulated versions; trimmer potentiometers are not required.
LEDs 1 - 3 as optical controls for the signals on the L-line (LED 1) and K-line (LED 2 lights up when data flows towards the car, D3 when data flows towards the PC)
a transistor output stage also for the L-line, to make the selection of optocouplers (coupling factor!) less critical.

Furthermore, the following have been provided:

R 13, C 1 and C 2, to better suppress interference from the power supply.
R 16 and R 17, which divert any static charges through the PC chassis instead of the signal pins (this requires first connecting the diagnostic plug to the vehicle).
a transistor output stage also for the L-line, to make the selection of optocouplers (coupling factor!) less critical and to align the propagation times with the K-line.
an optional voltage control with R 14, R 15 and LED 4 (turns off at 8 volts).

Note:
At high voltages and low impedance of the RXD pin on the COM port, LED 3 may constantly light up, even when the OBD plug is not inserted or when no data is received from the vehicle. This is not a fault in the adapter and does not affect the function; it is due to the relatively simple circuit of LED 3 and can be ignored. Data reception from the vehicle can still be detected by the brighter blinking of LED 3.

If the diagnosis is only to be carried out on vehicles without a confirmed L-Line, the components R 1 to 4, VR 1, OK 1, LED 1 and T 1 can be omitted during the assembly of the adapter.

Trimmer Adjustment

Connect the adapter to the diagnostic PC and the OBD port.
(Note: If the diagnostic PC is replaced with a different model, a re-calibration may be required for safe communication.)
Set all trimmers to maximum. In the VCDS-Lite LED test, LEDs 1 to 3 should blink in sequence - sometimes individually, sometimes together. With VR 1 and 2, the brightness of LEDs 1 and 2 should be adjustable.
In the repeated VAG-COM adapter test, find the position of VR 2 where the adapter is no longer recognized. Then, adjust VR 2 halfway towards the "recognized" terminal. Repeat with VR 3. Adjust VR 1 visually to the same position as VR 2.
In the diagnostic mode, determine the ECU with the highest measurement value sampling rate. Determine the adjustment ranges of VR 2 and 3, where communication is currently stable. Adjust both trimmers to the center of the respective green range, and optionally adjust VR 1 visually like VR 2.

If the adapter behaves differently, the following functions can be checked at the workbench. A power source of approximately 12 volts, a voltmeter, and a set of measuring cables with crocodile clips - or alternatively, a few pieces of wire and a soldering iron to create the connections are required. All measured voltages refer to the vehicle's ground.

Preparation:

Connect pin 5 of the PC connector (GND) with the vehicle-side ground wire.
Connect pin 4 of the PC connector (DTR) with the vehicle-side 12V wire.
Set all trim pots to maximum.

Tests (Measure voltages against ground, the voltages to be supplied must also be measured against ground!):

- Connect pins 3 and 7 of the PC connector (TXD and RTS) to the vehicle-side ground wire and supply 12V from the vehicle side. The K- and L-lines must each carry 12V. The collector of T 3 must not carry any voltage. LEDs 1 to 3 must not light up.
- Instead of ground, supply 12V to pin 7 of the PC connector. LEDs 1 and 3 must light up, this corresponds to a switch state of the LED test in the VAG-COM menu. Subsequently, reconnect pin 7 with ground.
- Supply 12V to pin 3 of the PC connector (TXD). LED 2 must light up. The collector of T 3 must carry 12V. This corresponds to the data flow from the PC to the vehicle.
- Connect pin 3 of the PC connector (TXD) and the K-line to ground. LED 3 must light up as long as the K-line is connected to ground. This corresponds to the data flow from the vehicle to the PC.

Any other logical lighting behavior of the adapter will cause malfunctions.

Christian has designed the PCB layout for the above schematic, thank you!

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

For vehicles with control units split into 2 K-lines, the following design is available.

Johannes has created files for Eagle: schematic, PCB.

A properly configured 2.4 adapter is usually sufficient for a stable connection based on previous experiences.
However, due to its relatively simple basic concept, it has some weaknesses that can make the connection unstable under extreme conditions (e.g., bit length errors due to fluctuating power supply during startup, very long cable lengths with high capacitance between PC and adapter, and very unfavorable COM port data).
Furthermore, it can only communicate via K-Line, and the autside data lines are not protected against voltage spikes from the power supply.

The "Worst-Case-Design" 4.0 offers the following improvements compared to version 2.4:

- High slew rate and low output impedance on the RXD pin for problematic conditions towards the COM port.
- Constant bit lengths from 9V operating voltage.
- Switchable to 2 K-lines with separate receiving LEDs that indicate which K-line the STG is connected to (not practically tested so far).
- Protection against voltage spikes on all diagnostic lines.
- More evenly distributed current load on the COM port.

The principle of protection diodes D11 to D16 can also be applied to other adapters.
If a double K-line diagnosis is not required, D4, 12 and 16, LED 4 and R10, as well as the switch, can be omitted. Cable 15 is directly connected 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 required, for example, to facilitate communication in critical combinations of vehicle, diagnostic PC, and software through minimal signal delays and distortions, a circuit like this can be used for a diagnostic adapter without galvanic isolation and without MAX232. It can operate stably up to approximately 500 KB, depending on the connected cable capacities and the properties 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 on-board electronics under unfavorable conditions (incorrect adapter construction, PC power supply from the vehicle's power network with unsuitable power supply circuits or transition resistors, loose connections, etc.).

LED 1 displays the data sent to the vehicle, LED 2 displays the received data, LED 3 controls the L-Line, LED 4 indicates whether the operating voltage is sufficient (turns off below 8 volts), and LED 5 and 6 display interference voltages between the vehicle and the PC, which can affect the communication.

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

For owners of older Optocoupler Adapter versions from Ulf...

[color=red][size=16][u]Common Problems
(Authors: Ulf, Rainer)

Unfortunately, the data exchange between PC and vehicle is so complex due to the involved hardware and software that the first attempts with VCDS-Lite are often described as "plug and pray" (= plug in and hope).

:-(( [b]"I'm not getting any connection at all!"

(

The most common causes are:

1. Adapter, including the connecting cable, are incorrectly designed / assembled / defective

2. Electrical data of the adapter and PC interface do not match. This can happen relatively easily due to the wide range of properties of COM ports, especially when adapters with optocouplers are used, which are usually not overly tolerant of COM port data.

Solutions include adapters with appropriate adjustment options, such as the versions 1.1, 2.4, and 4.0 shown here.

3. Errors on the side panel (ignition not switched on, cable breaks in the area of the diagnostic connector, etc.)

4. Disturbed PC software configuration (see No. 8)

5. PC defective / VCDS-Lite installation faulty.

:cry: "I can't address all STGe / only some functions work for me, VCDS-Lite constantly displays various error messages." :cry:

This can mainly be caused by the following:

6. Significant bit length errors in the adapter hinder communication, primarily with STGen at higher baud rates:
Construction or adjustment errors in the adapter, most likely to occur in circuits with optocouplers.

7. The logic-0 detection threshold on the adapter side is below the logic-0 output level of the "silent" STGe, which often (and still ISO-compliant!) provide 1 to 1.5 volts instead of the ideal value of 0 volts:
Construction or adjustment errors of the adapter; ideally, logic 0 should be detected as long as the K-Line voltage is less than half the main power supply voltage.
Attempt to rectify the problem (if the actual adapter circuit cannot be corrected): 1 to approximately 4 1N4002 or similar diodes in series in the direction of current flow in the ground line to the OBD connector (pin 4) can solve the problem, with some luck.

8. The overall PC software configuration interferes with the VCDS-Lite dialog.
Solution: close/delete as many running programs as possible / verschandeln up autorun files.
If necessary, try a PC with a different software configuration; with similar hardware, it is possible to pinpoint the causes more reliably in the software area.
Further information can be found on Uwe Ross's homepage.

9. The data format of older STGe (especially from 1Z engines) does not match the version of VCDS-Lite being used.
Jörg has found a solution to this:
It can help to gradually reduce the value for 'Blk Int' in the program options.
For the general settings of 'Char Int' and 'KW2 Delay': reducing these two values can help on slower computers, while increasing them can be helpful on faster ones.
Thanks Jörg!:D

:cry: "I'm getting inconsistent texts when I read STG error codes!"

[b]Think about it and make sure you have a proper unlock for your VAG-COM/VCDS-Lite version!

When buying ready-made adapters without a corresponding diagram, you're essentially gambling with numbers 1, 2, 6 and 7.
This also applies to (www-)auctions, flea markets, etc., where poorly constructed adapters from private/unknown manufacturers are apparently sold even more carelessly than in regular trade :evil:

If an adapter is recognized by VCDS-Lite, or if a check using the adapter test software provided here yields a positive result, then only adapter problems numbered 1 (excluding the cable and connectors supplied with the vehicle!), 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 found in various aftermarket adapters.

Furthermore, items 6 and 7 can also vary depending on the voltage, ambient temperatures, etc. (i.e., they are quite unpredictable).

Angesichts dieser Gründe sollte der Kauf von vorgefertigten Adaptern unbekannter Hersteller / Anbieter / Konstruktion besonders sorgfältig überlegt werden.

Wenn von Anfang an eine hohe Zuverlässigkeit gewünscht ist, sollten z.B. die hier beschriebenen bzw. verlinkten Schaltungen bevorzugt werden (mit Ausnahme der Minimal-Designs), da auch im hierigen Forum oft Unterstützung möglich ist bzw. diverse Probleme bereits behandelt wurden und über die Suchfunktion recherchiert werden können.

However:
Imitations errors and defects can occur in any individual component, and therefore create a nearly insurmountable variety of error patterns. If, in addition, there are no measuring instruments and/or the consultant lacks basic electronic understanding, then meaningful remote diagnosis is practically impossible!

[b]For training purposes, lessons, or professional work on the powertrain, the OBD2 diagnostic software KOBD2Check is ideal, available in the shop: https://shop.dieselschrauber.org/rks-can-can-bus-interface-c-24.php

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