Alarm going off in the middle of the night - now solved!

[DWPilbeam]

At last....I've now solved the riddle of the horn sounding at random intervals ...mostly in the middle of the night...

I have a 1998 300tdi Defender 90...which I bought last year. It has the standard Lucas 10as alarm ecu and spider.

I wasn't aware that I had a BBUS - Alarm Sounder with battery backup as well as the normal horn. I worked on my Def fitting tow hitch electrics etc and such like..had to disconnect the battery and it behaved perfectly. I wasn't aware the BBUS should have gone off...but it didn't anyway so I was blissfully unaware of it's existence until recently.

A few months ago the horn (not the BBUS - just the normal horn) started sounding in the middle of the night..a long very loud continuous horn sound that didn't endear me to the neighbours in any way at all. I hadn't set the alarm. It happened twice. Each time I disconnected the main battery and when I reconnected it everything was fine...After the last time I removed the alarm sounder fuse which was the only way to stop the noise permanently apart from disconnecting the battery. I started looking more seriously at the problem when it was impossible to reconnect the battery without the horn sounding continuously.

Long story short...The horn is controlled by the horn switch BUT can also be turned on by the alarm relay. The alarm relay is connected to the 10as alarm ecu by an Orange / White wire. Some way along that wire there is a black wire spliced in which goes to the BBUS. This wire is supposed to be continuously held at +12v. If the alarm detects an alarm event it switches this connection to low, which energises the horn relay AND the BBUS. The problem is that the alarm was never turned on yet this wire was always now -ive. As the splicing of the black wire into the relay wire looked a tad amateurish (crimped wires covered with bright blue insulating tape - I've since been told that this is standard LR practice for splicing) I cut it. Low and behold the OW wire became +12v and the horn didn't go off. everything else worked fine.

If I reconnected the black wire (which goes to No. 3 terminal on the BBUS) to the O/W wire then the horn sounded continuously.

I now suspect that the BBUS as well as being non-functioning over time started to further degrade and intermittently initially at least started to short the black wire to earth thereby causing the alarm relay to energise and the horn to sound. When it finally died or corroded the earthing was permanent.

I've removed the BBUS and everything is fine now. As the BBUS will never be refitted I haven't re-spliced the black wire to the O/W..

So..if you have a problem with the horn going off in the middle of the night for no reason at all..pull the connectors off the BBUS. It might just stop the horn going off.

 

[thebear843]

Wow, to say its only your fourth post on this forum, I dont think anyone else here has posted anything as informative so quickly .

I have the same problem, the alarm sounding at night, so I stopped arming the alarm. But even then, it sometimes goes off!!!! I always assumed that I had inadvertantly activated it by unknowingly pressing the fob button whilst it was in my pocket

Unfortunately I dont understand electrics at all, so your post may as well be in a foreign language to me, but at least I can print it out and discuss it with an auto electrician one day, so many thanks

 

[SimonHobson]

Unfortunately I dont understand electrics at all, so your post may as well be in a foreign language to me

Translation :
The alarm system has a battery backed sounder in addition to the vehicle horn, the sounder was dead and the fault was making the horn sound by "back feeding" into the main vehicle loom though a wire that should only have been sensing a signal from the vehicle loom.

A battery backed sounder is essential on an alarm - otherwise you only have to disconnect the battery and the sounder is killed

 

[thebear843]

Translation :
The alarm system has a battery backed sounder in addition to the vehicle horn, the sounder was dead and the fault was making the horn sound by "back feeding" into the main vehicle loom though a wire that should only have been sensing a signal from the vehicle loom.

A battery backed sounder is essential on an alarm - otherwise you only have to disconnect the battery and the sounder is killed

Thanks Simon So are you saying that ideally a faulty 'battery backed sounder' should be replaced instead of just removed? And does it have its own battery source, or just powered from the car battery?

 

[SimonHobson]

Thanks Simon So are you saying that ideally a faulty 'battery backed sounder' should be replaced instead of just removed? And does it have its own battery source, or just powered from the car battery?

Yes. The idea of the battery backed sounder is that it sounds if it's triggered, and more importantly it sounds if you disconnect the power. It does this by having it's own internal battery that's kept charged up all the time.

Without this battery backup, all someone has to do is disconnect the battery and the alarm system is 100% defeated as it won't have any power to make a racket and attract attention.

 

[thebear843]

Thanks Simon

 

[Richie_asg1]

Ahh. If it is anything like house alarms that have a battery back-up, then if that back up battery fails then it causes all sorts of false alarms. Try and locate the back-up battery & replace it (may be a weird one with tags though).

I bought a shed alarm for mine (don't laugh... although it is a bit of a shed). That had 12V and battery back-up, passive infra-red and a remote keyfob. Worked great untill one set of batteries failed. Will rig it up to main battery when I'm running about in it.

 

[thebear843]

Thanks Richie, think Ill get the back up battery replaced, any ideas where its located on a 300tdi Defender?

 

[DWPilbeam]

It's right at the front at the top under the left wing...I needed to remove the power steering reservoir to get at the two securing bolts...it's not something you can do in a hurry if the thing is sounding off...which is probably why they put it here....!

 

[SimonHobson]

It's right at the front at the top under the left wing...I needed to remove the power steering reservoir to get at the two securing bolts...it's not something you can do in a hurry if the thing is sounding off...which is probably why they put it here....!

Ahh, but if you are a tea leaf, then you rip the headlight out to get at it without triggering the alarm. You can then disable it before moving on to breaking in to the vehicle

 

[thebear843]

Thanks. Think Ill have a try at stealing my own vehicle then, and get a new battery fitted at the same time

 

[300landy1]

At last....I've now solved the riddle of the horn sounding at random intervals ...mostly in the middle of the night...

I have a 1998 300tdi Defender 90...which I bought last year. It has the standard Lucas 10as alarm ecu and spider.

I wasn't aware that I had a BBUS - Alarm Sounder with battery backup as well as the normal horn. I worked on my Def fitting tow hitch electrics etc and such like..had to disconnect the battery and it behaved perfectly. I wasn't aware the BBUS should have gone off...but it didn't anyway so I was blissfully unaware of it's existence until recently.

A few months ago the horn (not the BBUS - just the normal horn) started sounding in the middle of the night..a long very loud continuous horn sound that didn't endear me to the neighbours in any way at all. I hadn't set the alarm. It happened twice. Each time I disconnected the main battery and when I reconnected it everything was fine...After the last time I removed the alarm sounder fuse which was the only way to stop the noise permanently apart from disconnecting the battery. I started looking more seriously at the problem when it was impossible to reconnect the battery without the horn sounding continuously.

Long story short...The horn is controlled by the horn switch BUT can also be turned on by the alarm relay. The alarm relay is connected to the 10as alarm ecu by an Orange / White wire. Some way along that wire there is a black wire spliced in which goes to the BBUS. This wire is supposed to be continuously held at +12v. If the alarm detects an alarm event it switches this connection to low, which energises the horn relay AND the BBUS. The problem is that the alarm was never turned on yet this wire was always now -ive. As the splicing of the black wire into the relay wire looked a tad amateurish (crimped wires covered with bright blue insulating tape - I've since been told that this is standard LR practice for splicing) I cut it. Low and behold the OW wire became +12v and the horn didn't go off. everything else worked fine.

If I reconnected the black wire (which goes to No. 3 terminal on the BBUS) to the O/W wire then the horn sounded continuously.

I now suspect that the BBUS as well as being non-functioning over time started to further degrade and intermittently initially at least started to short the black wire to earth thereby causing the alarm relay to energise and the horn to sound. When it finally died or corroded the earthing was permanent.

I've removed the BBUS and everything is fine now. As the BBUS will never be refitted I haven't re-spliced the black wire to the O/W..

So..if you have a problem with the horn going off in the middle of the night for no reason at all..pull the connectors off the BBUS. It might just stop the horn going off.

Hi..
All that gives me less hope for my intermittent fob working ..
Just a question as you seem well switched on to the alarm front..I have a 95 300tdi that has already had the immobiliser by passed. Just wondering if you would know if I removed the alarm fuse , would the engine still work as normal ?
Apart from the back up alarm that is, looking at your in depth above ..
If so would it be possible to disarm it all and just put a new alarm on ?

Shot in the dark as it looks like taking the alarm off is impossible. .

Thanks for your time..Stuart

 

[mjfuk10]

You've got to love the internet. A post from 13years ago just solved my problem Alarm going off all the time even when not armed. Unplugged the bbus. sorted. Will investigate more later.