AHU immobiliser stuck on. Help please

[hotpod]

1997 VW Golf AHU tdi in a T3 van. Installed in 2014


I went to leave work the other evening and the van started for a second and then died. I am used to this happening occasionally when I forget to insert the golf key into the golf barrel and transponder ring in its hidden position. I checked the key and it was where it should be.
No amount of in, out, wire wiggling, battery disconnecting etc has helped.
I put VCDS on it and it clearly told me that the immobiliser is preventing engine from starting. I can clear the code and without touching anything, it returns after a few seconds.
I have scoured forums for the last two days, tidied up the engine bay, replaced all the vacuum pipes (not related but made me feel proactive), opened up the ecu (and replaced the pipe with a hole in it leaking oil in there ??? which may help with my boost issue  ) but things are exactly the same.
Luckily it is in a private car park  and not by the side of the road/at tescos/etc, so I'm not worried about it but it is currently just an expensive useless lump. All because of some electrical issue that I cant solve.
I have been reading about deleting the immobiliser from the ECU myself but I quite like having it plus it is all a little to nerdy for me.
I have ordered a £14 immo delete circuit board from ebay which I will try but I dont think I like it as a permanent solution. If it works then I will likely get myself a second hand ECU and get the immobiliser deleted professionally and keep it as a backup unit.

QUESTIONS:
The most likely culprits currently appear to be the key chip (do they fail?) or the transponder ring.
I presume the transponder ring doesn't need to be matched to the ecu (which I also cant work out how to do anyway) so a second hand or even new one if available might be worth a try? is there a way to test the one i have?

The key...there is unfortunately only the one (my bad, it was on my to do list to get it cloned  ) is there any way to test it? I tried with an rfid app on my phone and there was nothing but I have checked our caddy keys and there's nothing there either (but it does read credit cards etc)

I am making the assumption that if I can communicate with the ECU via vcds the the k line is not at fault
also that if the immobiliser is creating a fault then it still functions?
and therefore that either the key or transponder ring are at fault???

Thoughts and knowledge from the hive mind please



 

[cobblers]

You're right - if you have communication via K line then that's OK.
Measure the voltage on the K line as you move the key closer to the reader - you should see a distinct change as you get the key in range and it starts transmitting the code to the ECU.

The chips in the key don't fail unless physically damaged. The key reader coils also rarely fail on their own. Make sure there is power and ground to the key reader.

There's no way to test the chip without some equipment. It's ten minutes work for an automotive locksmith to test your chip and clone you a spare key, that would probably be where I'd start

The immo off boards do work fine.

[hotpod]

Awesome, thank you. I'll try not to forget my multimeter tomorrow and report back.

[hotpod]

Thank you very much for your very useful response cobblers.
I had assumed (like with the K line) that if I could talk to the ecu via the obd port which is tied into the immobiliser unit, then it must all be getting power. It takes power from the ignition live feed for the stereo. Which also happens to be on the same fuse as the lighter socket which I had removed and refitted a couple of times on Wednesday
I remember having to bend the internal tabs to remove it and getting a spark when the screwdriver I was using slipped. I though nothing of it until today when I traced the power to the immobiliser. Then it all came flooding back.
So it was just a blown fuse after all that. Possibly made worse by the fact that there was a 10a fuse where it should have been a 15.
Assumptions are the enemy of sound diagnosis   .


I fitted the engine in 2014 and many things were done on a temporary basis to get it running with the idea that I would need to go through everything and tidy it all up. It never happened, has run beautifully since and so remembering what was done and how 11 years later is a time consuming process.

I'm enjoying putting things right though and its making me realise the need for some backup plans ready for next time.
For example, I am getting a spare key programmed this week and I will be installing the immo delete in an old immobiliser housing so it can be plug and play. but I do like having the immobiliser in place and working as peace of mind for my increasingly valuable truck.

Thanks again.
I do love this forum.

[silverbullet]

FWIW I seem to recall that the ignition lock transponder coils are known to give trouble with age, they were known for being a known weak point with AGG swaps? Might be worth keeping a VAG spare on board.

[hotpod]

I think I will buy a new one and keep the current one as a spare.
In my panic last week I bought a brand new immobiliser unit which I shall be fitting and keeping my currently still working but 28 year old one as a spare.

I have a whole new threads worth of questions now having had my head in tdi mode for a week.

[hotpod]

OK...new question 
When i fit my brand new immobiliser box with the original key and coil thingy, I cant start the van so I am guessing that it will need to be programmed to the ECU.
Does anyone here have a clue as to how to do that? I have Vcds lite, registered version and it happily talks to the ecu and immobiliser but beyond that, I have not got a clue.
The van is running so its not a massive problem but my brain would like to know ready for next time