BBUS/ AMR 4974

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romanrob

Well-Known Member
Posts
611
Location
SW London
My Defender alarm was playing up in the last couple of weeks... Whilst parked/ locked outside the house did a few blips on the horn for 24 hours before then setting itself off... horn plus siren, and the wife went nuts :). A post mortem reveals that it seems to have been enough to blow the alarm fuse.... I'm guessing that the BBUS is shorting out, it is after all over 25 years old... I have read a few threads of people attempting to replace the embedded battery packs, didn't sound too successful... Official sources seem to be out of stock of the replacement part and its updates... Is there a best practice work around for replacing/over-hauling this unit?

Thanks in advance
 
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Like you I have never read a thread where the bbus has been repaired or replaced with a new one.
Please report back if you have any success.
 
romanrob said:
....horn plus siren, and the wife went nuts
Click to expand...
If the horn and alarm sounder went off i seriously doubt that the BBUS is the problem cos it can't trigger the other two, the BBUS is supposed to sound on it's own speaker if the battery is disconnected... better replace the alarm relay first to make sure it doesnt have a weak spring or the volumetric alarm is making tricks
 
I thought that the BBUS can trigger the alrm if there is a short in the unit of the +12 v line to earth... I stripped the unit now - it was completely toast and could easily have been causing a short. Lots of reports of folks disconnecting these and their problems going away... But you could be right - the BBUS has probably been dead for so long that it was something else. As i pull things apart there are others things to be fixed too... the RH horn needed replacing as it was hardly working at all. Good advice on relay - easy fix...
But as per the title, I'm still looking for a resolution on BBUS though... Or else this means that all the battery back up packs on Defenders have died by now
 
The BBUS on such old vehicles is quite useless anyway IMO, i disabled mine on the D2 with nanocom and got done with it... on defender it shares the earth with the alarm sounder which is triggered with +12V on the other pin so a live short through the BBUS to the alarm sounder would not make it scream

those pointed with red arrow are live and through C3 joint is earth

1730726812837.jpeg
 
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The only time I have had spurious alarms was due to the under bonnet switch on the bulkhead not being straight and triggering the alarm, straightening the switch so the bonnet meets it and disarm's it sorted it out, the rubber gaiter was split allowing the switch arm to go off centre, I used super glue to repair the gaiter and have had no more problems.
 
sierrafery said:
The BBUS on such old vehicles is quite useless anyway IMO, i disabled mine on the D2 with nanocom and got done with it... on defender it shares the earth with the alarm sounder which is triggered with +12V on the other pin so a live short through the BBUS to the alarm sounder would not make it scream

those pointed with red arrow are live and through C3 joint is earth
Click to expand...
Thanks. I don't recognise your diagram, but my 1998 V8 appears to have a different set up. I could be mis-reading it, but the BBUS is spliced into a 12 volt wire running from the Alarm ECU to the alarm relay...As long as that wire remains high the relay will not switch.... But if there is a short to earth via BBUS then the 12v input to the relay drops, the relay is thrown, etc
 

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Sorry, i missed your signature which reveals that your's is a V8 and presumed it's Td5, you are right, for V8 its a different setup and if there are shorts to earth within the BBUS it can trigger the relay or directly the horn and if the fuse which blew was F3 in the interior fusebox then the BBUS becomes very suspect, based on the diagram if you remove it the problem should go away if this was the issue
 
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