Alarm keeps going off regularly -quick fix to silence it needed

[HPsauce]

Just noticed this starting today, the alarm goes off for a short time, maybe 30 seconds then stops. A bit later it goes off again!
A VCDS scan doesn't show much exciting EXCEPT low internal voltage (dying battery inside as ever) and (probably more relevant) 49-10 No Communications - Intermittent

Any thoughts on what might be causing it and is there a quick way of silencing it, short of dismantling the boot liner and pulling its connector off! It's a bit of a pig to get to so I'd rather avoid that for now if possible.

It's not the original bulky square alarm module, it's the slightly later but compatible rectangular version that might even have been from another Audi model.

[HPsauce]

Gave up in the end, took the dam thing out, all done in under half an hour.
Most difficult step was actually taking the connector off, it's not at all obvious how and the lead is extremely short. Prodding with a small flat screwdriver released the catch in the end.

It's an 8L0 951 605A made by Bosch in Australia of all places!
Might take it apart and see if there's any corrosion inside from failing batteries, if not I'll try to replace them.

[HPsauce]

Opened it up, batteries not good but PCB looks like it's probably OK.
Batteries are of a geometry that's NLA, though you could probably bodge a pair of 3-cell Varta ones to fit by making your own tabs. The battery is a 2*3.6v 140mAh Ni-Mh unit with 8 (yes eight!) soldered mounting pins, a bit weird!

It's not obvious if the batteries supply just a 7.2v feed or if an intermediate 3.6v is also used.
If I do try to repair it I'll take a 3-core lead (0, 3.6v, 7.2v) through the casing and fix suitable battery packs externally. 3.6v Ni-Mh are readily available.

[steamship]

Originally Posted by HPsauce

Gave up in the end, took the dam thing out, all done in under half an hour.

That's pretty quick. Crazy price for new ones, which is why I never bothered replacing mine after the second one.

[HPsauce]

Originally Posted by steamship

That's pretty quick.

Been in there a few times so I know what I can get away with.
Remove top trim strip around boot lid ( 2 clips with central plugs). Remove side panel cover over non-existent TV and CD (that's under the floor now). Remove net loop and next inboard clip on sill cover enough to free boot side panel. Wiggle side panel out at rear and push it away to get at the alarm siren fixings.
The fact that I have nothing behind the boot side panel now really helps a lot - no big metal brackets and electronics boxes and cables to remove.

[The_Laird]

I took mine out years ago after a couple of attempts at fixing/replacing it. No-one pays any attention to a car alarm anyway!

[HPsauce]

Originally Posted by The_Laird

No-one pays any attention to a car alarm anyway!

Except insurance companies! Any excuse to limit or refuse a claim, so I'm going to see what I can do!

I've taken the old batteries out by gently prising the mounting pins away from their faces, so I still have the pins soldered in place to help fixing replacements which are on order.
Cleaned up the circuit board too - note to others - DON'T use isopropyl alcohol, it dissolves the green lacquer on the PCB! A little water via a small brush seems to work better.

[HPsauce]

Interesting to note that, now I have the old batteries off I can see that the PCB is designed for more than the original pair of 3.6v units.
There are 2 pairs of holes (unused) for apparently mounting a single 7.2 v battery. These are marked + at one pair and - at the other and tracing the circuit board tracks correspond in all respects to the combined output of the two original units.
So, assuming there's no interim 3.6v required there's an alternative that may be easier to mount. Or just for connecting an external battery pack.

The pairs are separated by about 23mm to 24mm and the +ve pair are about 8mm apart, the -ve about 10mm. I don't think that corresponds to any current 7.2v NiMh battery but I'll have a search round as it might be easier to use these "clean" connections.

[HPsauce]

Oh well, I fitted a pair of replacement batteries and all looked good, reinstalled the alarm and it was making suitable satisfying chirps.
But a few minutes later the alarm went off, just like before, but MUCH louder. Again for about 30 seconds then quiet for a while - rinse and repeat!

VCDS had basically the same errors as before, low voltage and intermittent no communications.
So I think I must assume that the electronics have been damaged by the leaking batteries, so it's toast.

[HPsauce]

Originally Posted by HPsauce

I must assume that the electronics have been damaged by the leaking batteries, so it's toast.

And to prove that I dug out my original factory-fit alarm, the old cube style. I cut that open years ago but couldn't (then) fit replacement batteries easily, even though they aren't soldered, just in a holder with pressure contacts.
As luck would have it the batteries I bought WILL (just) fit it now! So I cut them off and swapped them over and fitted the original unit.
No sound from it, no idea why, but no errors whatsoever on VCDS, so it's been seen and is communicating happily. Unlike the other one, now proven to be faulty.
I might look at swapping the sounders over to see if that's the problem on the 25yo original.