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2005 Volvo S40 with about 100k kms. Every once in a while, usually after I've just parked the car, exited, and locked it, the alarm will go off.

I understand there is a common issue with these vehicles. The siren module has a backup battery, so the alarm can sound even if the main battery is disconnected. I've pulled the module out and cut it open, expecting to find an old, dead, leaky battery that's destroyed the circuit board inside. It turned out to be in great shape.

While I had the siren module out, I've continued to drive the car. I've noticed after parking that the alarm continues to be triggered. While there is no sound with the siren out, the lights flash and I get a notification on the dash the next time I get in that the alarm was triggered. This doesn't happen every time, only occasionally. This suggests to me the siren module is unrelated.

I tried a second test where I put the alarm into a "reduced" mode with a button on the ceiling. This disables the motion detection for when animals are left in the car, or on ferry crossings. I've noticed at least once where the alarm was still triggered, which suggests the motion detection component is also unrelated.

I'm now thinking this has something to do with the door, hood, or trunk sensors. I've armed the car and walked around trying to wiggle the doors, hood, and trunk to see if I could cause the alarm to sound, without result.

Does anyone have an tricks or suggestions to track down the source of the alarm triggering? Is it possible to get any information about this problem using a standard OBDII scan, or do I need to use the VIDA tool?

  • shot in the dark - but owned a few Volvos and worked on way to many with the same type of issues... replace the key fob battery. It's cheap enough to do to eliminate this as a factor. There are codes that are exchanged betwixt the security module and the fob, and these can get confused - even rendering the car unable to start. Then suddenly everything is fine. Your issue doesn't exactly match what I have seen, but it's worth a try and not a huge investment. I think it has something to do with the car responding to a weak code, and then thinking that code was issued by an improper key. – SteveRacer Sep 13 '17 at 1:43
  • Oh - and a really good diagnostic tool (SnapOn, Volvo - NOT a generic) can read out the alarm "security" issue and report on what caused a trigger. In fact, isn't there a security led in the cluster or somewhere that blinks out the reason for the alarm? – SteveRacer Sep 13 '17 at 1:46
  • Good idea, thanks. There is a light on the dash that blinks slowly when the alarm is armed, and quickly when the alarm has been tripped. I'm not sure about any sort of pattern though. I'll see if I can find out more about that. – raydowe Sep 13 '17 at 7:34
  • I think there's a meaning in the pattern. Check your owner's manual if you have it. – SteveRacer Sep 14 '17 at 5:14
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I was mistaken when I said the module was in great shape. Although the battery had a good charge and the circuit board was intact, I later noticed that the connection between the two had a significant amount of corrosion. On cutting the battery wires, I found the metal strands of the ground (which was suffering from the corrosion) appeared black as if they had been burnt or melted into the protective sheathing.

I've replaced the battery with a similar battery of the same voltage and cell type, and the problems with the alarm have yet to return.

I believe the alarm triggering when the module was removed was simply the car panicking that a module was missing.

  • Good work and well done for the update! – Solar Mike Oct 4 '17 at 11:47
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Does the alarm trigger only when the vehicle is locked or also when driving? It doesn't seem clear from your question.

You need a good diagnostic scanner, a generic OBDII will not be good enough. You need a scan tool that will communicate with the body and alarm system and give you live data.

As you say the alarm is still triggered in reduced mode it points towards the doors, tailgate or bonnet.

I would put it in this reduced mode then lock the vehicle whilst sat in it monitoring the status' of the doors and catches. Usually you will see the culprit door flicking between open and closed. You can help to find an intermittent fault by wiggling wires, pushing/pulling at the doors etc.

  • Once I've exited and locked it. I've updated the question. Thanks for the suggestions, will give that a try. – raydowe Sep 11 '17 at 13:33

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