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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.

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  • Are you certain it is the modem? One forum post I came across the owner said he found a large bug that was setting the alarm off once the vehicle was empty and locked...
    – Solar Mike
    Commented Aug 8, 2017 at 14:55
  • is it aftermarket or the oem alarm?
    – finleyarcher
    Commented Aug 8, 2017 at 15:19
  • It is very consistent - if the modem is in the car, alarm goes off. Take the modem away, does not. It is likely OEM alarm, the car has almost no aftermarket modifications at all. This has been happening for more than a year now, so if it's a bug, it is a long-living one :)
    – Richlv
    Commented Aug 8, 2017 at 18:35

2 Answers 2

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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.

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  • This is a good point about the battery, relevant for many cases not only volvos - ford & jaguar are also candidates...
    – Solar Mike
    Commented Aug 8, 2017 at 15:33
  • also Saabs :') I think the swedes like hard to replace batteries
    – finleyarcher
    Commented Aug 8, 2017 at 15:37
  • Ah interesting! I bet that's why my MINI's alarm goes off randomly sometimes.
    – Spivonious
    Commented Aug 8, 2017 at 16:47
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    Thanks, will try to get it checked. Why would a switched-off modem affect it, though?
    – Richlv
    Commented Aug 8, 2017 at 18:36
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    @Richlv Coincidence, probably. Unless you have a very strong magnetic field inside your car somehow, a modem/hotspot that has no battery and isn't on shouldn't have any affect.
    – JAB
    Commented Aug 8, 2017 at 18:59
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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

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  • 500mW may be enough to backfeed power through the hotspot just enough to get it to echo back and set it off. Especially if inside the car and multipath potentially increases the "gain" of the "system"
    – Brydon Gibson
    Commented May 22, 2019 at 14:44
  • The mystery is still not fully resolved, added an update. This is a very interesting guess - as the modem is still available, will try placing it in the car just to see what happens.
    – Richlv
    Commented Sep 5, 2023 at 17:28

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