Immobiliser transponders/RFID frequencies/making your own
Sci
motorbike/scooter. Specifically a 1999 Vespa ET4 125cc.
It worked when it went into storage and doesn't now. After a day of
checking the mechanics it's just not getting a spark. And after finding
the LED diagnostic port, it seems to be to do with the immobiliser.
The battery ran flat while in storage so I think that's reset the
immobiliser's "handshake" if that's the right term. I have the all
important Red Key (thinking Doom here), but it doesn't seem to be
reading from it now.
It might be (read; hoping) it's just the antenna coil. Found a document
saying it's supposed to be a resistance between 7 and 9 ohms. Best I
managed to read on it was 10 ohms though, so if that's right then it's a
little out of spec.
Is that likely to be far enough out of spec to be the cause of the
issue? It is a very small transponder chip.
I also can't find anything about the frequencies used in these
transponders (nor their data format). I haven't done very much at all
with RFID, but I think the frequency would determine the number of loops
I'd need to make in a temporary test-coil?
Ultimately I'm still guessing here. It might be the antenna, the key
transponder, the immobiliser itself, or a combination.
All the replacement parts would cost more than was paid for the bike
(except the antenna, which is about half what the bike cost).
Basically I want to avoid having to pay for a new immobiliser, keys and
barrels.
The immobiliser connects to the ignition control system (CDI) by a
single serial wire (chassis as ground), and none of the guides I've
checked say you have to change that as well, so I presume when the key
is recognised the immobiliser sends a series-standard set of data down
the serial line to say it's ok to turn on the ignition.
Of course that means finding someone with the same model CDI to sniff
what that data is when switched on, and create a circuit to send it. Or
possibly build my own keypad-bassed immobiliser instead.
Beyond that the options are hunting down a bike with the same engine but
no immobiliser option to get the CDI off that or building my own CDI
from scratch (for which there are a couple of guides, but for very
different bikes).
Anyone got any advice on the subject of repairing/analysing or bypassing
an immobiliser that's acting up?
~ Sci
Nigel Worsley
> saying it's supposed to be a resistance between 7 and 9 ohms. Best I
> managed to read on it was 10 ohms though, so if that's right then it's a
> little out of spec.
I wouldn't be worried about it being that little out. Far more important is the resonant frequency, there will be a
capacitor in parallel with the coil to set this.
> I also can't find anything about the frequencies used in these transponders
It will probably be 125kHz, if not then 134.5kHz or something close.
> (nor their data format)
For these low frequency RFID devices there aren't really any standards for data format. Every chip manufacturer
is different, and they often have several slightly different formats for their ranges of chips.
Somewhere at home I have a board that has 125kHz reader electronics on it, it can easily be retuned a bit. This
should be enough to verify that the transponder is functioning, not sure what your next move should be if it is.
Nigle
Sci
Most follow-up moves require finding someone else with a similar model
bike so I can either half-dismantle it to sniff the serial line or see
if my bike responds to their key with the expected "wrong keycode" LED
sequence. :/
Sci