www.jec.org.uk

This might be of use to others!!
went shopping in my x300 3.2 sport 97 rplate. transponder knackered so locked with key.
On return opened car audible constant beep, turned on ignition started car, nothing! started again car started presumed alarm std function.
Drove home charging gauge read 11 volts only noted it as checking fuel but thought strange carry on wipers going headlamps in rain tried washers after big lorry went by no spray. Now i knew it was full filled it up before journey so thought ok carry on.
slowely everything died dash lights came up no power coasted to stop in layby.
called help got jump car started no charging thought aha alternator has gone that explains it all ! no it dont.
Got home with power pack assist checked fuses etc belts one fuse gone, right hand engine comp f10 ignition diagnost 5amp. Thought ok start there turn on ignition washers back then ping heard it go with a click checked fuse gone again thought ok try a 10amp dangerous i know but
fed up by now anyway turn ignition on smoke under fuse by washer bottle turn off check area after cover removed audible alarm sounder wire clearly warm at plug. been here before with alarms shorting out thought unplug that.
new 5 amp fuse fitted got charging back washers back so for now leave it unplugged but how these weird electrical things are connected in modern cars never ceases to surprise me .so went from almost buying £200 alternator to 25p fuse in a car now only worth £750 these days .
had similar with the x type unplugged the alarm back up under front pasenger wing to get gearbox out of limp mode !!!!????
I still think the worst thing we ever did was connect computers to cars.
Lance

The backup cells go duff / age in alarm systems.
Usually located around the alarm sounder in a sealed box.

Not had one go myself so can't give benefit of experience (I'm X308 '01MY two off)
Taking down other components is a function of lowering of system volts due
perhaps to the cell not accepting charge... my best guess

Lance
Thank you for your post. It helped me solve the problem I had with failed windscreen washer. Symptoms were exactly as you described - charging at 11 volts and failed windscreen washer. The security sounder is now disconnected and everything works. As well as your post, page 55 of the X300 vehicle care book - engine bay fuse box - Right hand side - Fuse F10 nailed it down. Thanks
JT

Interesting.
Dead alternators are so common on x300, I probably would have assumed the same.
So is there a battery inside that security siren? Surely the battery in there could be renewed to maintain all original functions,