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Alarm in an older car (2005 Volvo V70) goes off almost every time a mobile modem and WiFi hotspot is left in it (Huawei Mobile WiFi E5377).

This happens even if:

  • the modem is switched off
  • the battery is removed
  • the battery is removed while the modem is switched on to drain any internal capacitors

If the modem is left in the car, it usually takes from a few to a few dozen minutes for the alarm to go off. If it goes off, unlocking and locking the car does not guarantee that the alarm won't go off again.

What could cause this and how to prevent it?

Update 2019-05: The problem mostly went away on its own, although it still occurs every few months or so. Car service tried to figure it out, but couldn't find anything wrong. They suspected a possible gap on the trunk door, but fixing that did not change anything at the time.

Update 2023-09: Coming back a "few" years later - the car is still in use, and the spurious alarms haven't returned. The modem is still available, will give it a go just for the curiosity.

Richlv
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2 Answers2

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The Volvo V70 factory alarm siren has a battery that would probably be bad by now. It's sealed in. When the voltage drops it will cause the alarm to go off for seemingly no reason. Check that first, its a common issue in several Volvos.

Prevention would be by replacing the battery prematurely every few years although it's not particularly easy or simple and the battery lasts a while.

finleyarcher
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from wikipedia:

Certain car manufacturers use the 2.4 GHz frequency for their car alarm internal movement sensors. These devices transmit on 2.45 GHz (between channels 8 and 9) at a strength of 500 mW. Because of channel overlap, this will cause problems for channels 6 and 11, which are commonly used default channels for Wi-Fi connections. Because the signal is transmitted as a continuous tone, it causes particular problems for Wi-Fi traffic.

this may explain why the alarm goes off when the modem is powered on;

when you switch it off you will need to wait for some time before all the echoes of the signal wear off, I have never tested how much you need inside such a small environment, to give you the idea, inside an apartment with doors and windows open, all the echoes disappear in 25 / 35 seconds, depending on the quantity of iron that is in the walls. Inside a car that is completely closed and made of metal i wouldn't be surprised if you have to wait for a couple of minutes before the signal disappear completely. please try again taking this into consideration

Edoardo
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