To use a cheap POS ebay USB-KKL interface with the older Motronic ECUs they need a slight modification to work reliably.
The KKL interface uses two lines, a 'K' half duplex line, and a 'L' low speed init line.
It's very similar to an ALDL one wire interface, the K line is used for both tx and rx. The L line is simplex and only used to "wake up" the ECU by sending a 5 baud ID byte (yes 5 baud, i.e. 2 seconds per byte!) then it's not used any further.
The cheapest KKL cables use a WinChipHead CH340T USB-serial bridge, which is quite decent - but for some reason the interfaces are designed with the K and L lines effectively tied together which makes the connection very unstable.
The fix is to separate the L line, and connect it to the CH340T chip's RTS pin so it can be driven independently of the K line:
And here's a schematic I traced out for the un-modified interface, you can see that the K and L lines are tied together:
Fixing cheap ebay USB KKL cables
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Re: Fixing cheap ebay USB KKL cables
Will this modification work on this cheap usb kkl cable?
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Re: Fixing cheap ebay USB KKL cables
It looks like the same design down to the same component numbering in the same place. I think it would work.
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Re: Fixing cheap ebay USB KKL cables
besiseitaj's cable is missing C1 in comparison to festy's original post. What is the significance of this capacitor?
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Re: Fixing cheap ebay USB KKL cables
Its to reduce electrical noise. The board with it will likely be slightly better in marginal conditions, and also introduces less of its own noise in to the circuit.
Have you read the FAQ? For lots of information and links to significant threads see here: http://pcmhacking.net/forums/viewtopic.php?f=7&t=1396