Immobiliser Hell

Delbourt wrote:

As I stumble through an uphill learning experience with a TF this topic is of interest. Has it been possible to find a means to renew the battery in this horn please? Also where do I find this horn and what is involved in removing it.
I ask because we think that the car alarm that woke us up at 4 AM two nights ago might have been our locked up TF in the locked garage. By the time I had got outside to check which car it was the horn had stopped. So all we have to go on was a slightly distant horn sound that was neither strident or with a deep note, but it had some similarities with the sound one hears when initiating the horn test prior to coding in key fobs using pscan.

OK. I'm just trying to be helpful here but, be warned, I have little confidence in what I am saying, so make your own checks!

The battery-backed sounder referred to above (YWK000041) - I think it is probably only fitted to the late SAIC cars. I'm pretty sure my car ('04 TF) doesn't have it as I have had the windscreen washer bottle out and didn't notice it (It is allegedly there somewhere).

minimax replied by saying that the battery-backed alarm is present in all TFs and it is shown in the wiring diagram. There is definitely a 'Sounder-Alarm-Battery backed up (D154)' shown at the top of page 4, ANTI-THEFT ALARM. I may be wrong, but I think this may be the beeper under the dash which sounds when the immobiliser is activated. There is no way that you would confuse this sounder with the car's alarm or horn as you would not be able to hear it from anywhere other than in the car (with my hearing!). It certainly wouldn't wake you up, unless you sleep in the garage!

Get the TF wiring diagram from the Community/Downloads section of this site. It is extremely useful but there will be differences in some cars as things will have changed a bit over a nearly 10-year period.

Well, what I can see fron electric diagrams mk1 MG TFs which have Lucas alarm system DO NOT have BBUS unit, but use horns.
Mk2 MG TFs which have Pektron alarm system DO have BBUS unit and use it instead of horns.

Roverlike wrote:

Well, what I can see fron electric diagrams mk1 MG TFs which have Lucas alarm system DO NOT have BBUS unit, but use horns.
Mk2 MG TFs which have Pektron alarm system DO have BBUS unit and use it instead of horns.

I'm no expert but I can say with confidence that my 2004 MG TF with a Pektron system uses the vehicle horn as its alarm sounder. I've set it off enough times whilst trying to make the wretched EKA work. It is entirely possible that this could be a change from standard, carried out by a previous owner, I suppose.

The only wiring diagram I have is for MGTF RHD dated 01/08/2003

As good as the wiring diagrams are the complexity does not appear to be entirely revealed as the logic in the various ECU “black boxes” is not shown.
As far as we can believe the car horn went off in the early hours whilst static in a locked garage. It has been suggested that voltage and presumably battery voltage variation with temperature might trigger the alarm.
For practical reasons it is not possible to check the under bonnet switch that is also thought to be a possible issue.

However, can anyone tell me if pscan can interrogate the Lucas 5AS system for “history” with the intention of establishing which sensor triggered the alarm?

You really need to post in the pscan forum section for an answer to that.

However a workaround for the alarm going off would be to lock the car with the key in the lock rather than using the remote. This isolates the internal movement sensor which is responsible for the majority of false alarms. The doors etc remain alarmed.