V70 Alarm going off - Again!
 

When my 2001 V70 petrol was 9 yrs old (in 2010) I had the "alarm keeps going off" problem. I had the siren unit replaced and all was well.
Lately the problem has returned. I've also had to charge the battery overnight a couple of times. I have even had the alarm go off within 48 hrs of recharging the car's battery. So I have just ordered a new Bosch battery. This will replace the genuine Volvo one that was fitted in 2009 (when car was 8 yrs old). Will be interesting to see if this cures the alarm problem. I hope it does. Otherwise it looks like the 2nd siren unit has only lasted 4 years. Anyone else experienced this?

If it is the main battery, then it looks like the first one lasted 8 years (when I was doing higher mileage) but the second only 5 years (when I'm using the car far less). I suppose that makes sense.

Well that didn't take long! Just a few hours after fitting the new car battery (fully charged), the siren went off again.
Does anyone know how difficult it is to dismantle the siren unit and fit a new rechargeable battery?

It's easy enough to pull apart - assuming you know how to get the unit out, cut it open with a dremel/hacksaw between the siren unit and the body (black/grey join). Then do the necessary and glue it back together with epoxy/silicone etc.

There are loads of threads on here with more detail if you search.

Before you start chopping the siren up it may be worth getting the rear window sensors checked as these can cause problems.

Well this was the problem! The rear windows sensor contacts needed cleaning. Exact same symptoms as a defective siren battery - but much cheaper to fix!
Thank you!
Alec

What are the rear window sensors ? Are they micro switches to sense window opening or is it the heater type things on the glass?

The rear window sensors are the push on clips at the bottom of the window glass. They look lie connectors for the aerial.

Your lucky,
When that was happening on my S80 (ALWAYS at 0300hrs approx) Itried everything, battery, sensors, anything anyone suggested.
I eventually sat up and watched my car until the alarm sounded to see if there was any outside interference.
It turned out to be a Feral Cat that roamed the area. It found out that when it jumped off the fence and bounced off my bonnet it could launch itself onto the roof of my neighbours shed.
A few well placed spikes along my fence stopped him getting to his launching point and silence was once more resumed. it was bloody lucky I don't own a gun, after all the sleepless nights, and searched and changes we had to endure.
joe