Technical Immobiliser Fix

[rceddie123]

Hope this helps someone! I have a 2.8jtd 244 based motorhome. Began getting intermittent issue with immobiliser light not going out and engine management giving u1600 (coded key not present) fault. After A lot of money and frustration I eventually Identified that if I move the key coil/ Ariel so it sat around the key it seemed to work ok. Bought a new Ariel/coil thinking it must be faulty (40 ohms resistance ) new coil (38 ohms ) fitted made no difference. 12v at immobiliser box seemed ok , wires not trapped etc. But output to key coil only 5v (is this correct?? Plugged in another immobiliser box and it also gave 5v). By moving key coil 5mm away from the key barrel it seems to now give enough induced current to make transponder work ok. This problem occurs with both keys..For now it is fixed/bodged but I would love to Know why / what has failed/ changed to make this necessary.

 

[mj2k]

No idea what would cause that issue, I had the same on an Alfa 916 GTV and never got to the bottom of it, though ultimately the car barbecued it's ecu, so it could have been the first signs of ecu failure for mine at least.

A permanent 'bodge' would be to remove the transponder chip from one of the keys and glue it inside the coil (once you have tested it works of course). Then you can move the coil to wherever you want safely out of the way.

You'd need to make sure you had a Thatcham approved 3rd party alarm / immo if you did that though, since you'd effectively be defeating the built-in immobilizer.

 

[Communicator]

My vehicle is a 2006 x244 2.8 JTD PVC, which I have owned from new.

No problems until recently, when after having had the battery disconnected while working on the vehicle, I got the padlock symbol when switching the ignition on.
The fault did not persist
No problems for several hundreds of miles over several weeks, until a week ago when I had a refusal to start after booking into a campsite. Again the problem did not persist, but I have only started the, or turned the ignition on, about ten times since the last failure.

If I remember correctly, Anthony489, a regular contributor to this forum, recently suffered a complete failure on his (I think) 2007 x250.

I have also read a post where the problem was temporarily cured by moving or reconnecting the code receiver connector. I was thinking of trying that on my vehicle.


I have been wondering about the possibility of obtaining the emergency code details from Fiat. It is my understanding that this information is witheld from owners of vehicles built for the UK market, at the request of the Thatcham insurance centre.

As regards the quoted output of 5V from the code receiver to the aeriel coil, please note that this is an RF signal to supply power to the chip, interrogate it, and receive the coded reply. I doubt that the a simple multimeter could measure this, even on an AC range.

 

[rceddie123]

Thanks for the info on the 5v that makes sense. My problems started with intermittent failure and got worse. Disconnection of the battery would seem to fix it initially but then that got worse and needed multiple reconstructions to overcome for just one or two starts. I hope this fix lasts!

 

[Anthony489]

Hi rceddie

This (to the best of my knowledge) is how an immobiliser of this era is supposed to work. The immobiliser section in the vehicle sends an AC signal at about 125 kHz (a low radio frequency) to the "reader" antenna coil mounted around the key opening in the lock barrel. This generates a magnetic field near the coil, the strength of which diminishes quite rapidly with distance.


This magnetic field is picked up by a tiny ferrite cored "transponder" coil within the key, which turns it back into an electrical signal. It induces just enough power that it can be rectified to DC and used to temporarily power up the associated transponder chip. The chip then modulates the transponder coil by rapidly changing its tuning in binary fashion in accordance with its unique pre-programmed code.

Back at the vehicle, the modulation can be sensed as a slight variation in the AC voltage on the reader coil. This can then be processed into a data stream and decoded, i.e. compared with the stored good code(s). If they match the immobilisation is released. The key interrogation stops at this point.


The whole setup is reliant on the reader coil and the transponder coil being close enough together to get sufficient coupling in both directions. Both coils also need to be electrically tuned to resonate fairly close 125 kHz. The physical distance is normally fixed once the key is home in the lock barrel. However, the tuning is reliant on electrical components which will have an initial factory tolerance, plus any drift in value with age.

As you have the same problem with two different keys, and have changed the reader coil, the finger of suspicion points to the immobiliser drive circuit. I suspect that the tuning is (and maybe always was) a little bit out, and has grown worse with component drift over the years. This will result in not enough signal getting to the transponder chip to power it up, and/or not enough signal getting back to the reader to be decoded. For the time being, you have been able to compensate for this by moving the coils closer together than standard, to increase the coupling between them.