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.
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.