SL320 Alarm

[william thomson]

Has anyone experienced an alarm which has not been set going off anyhow? My car is laid -up in the garage and although the car is locked the alarm is not set. Nevertheless after a few weeks(3-4) the alarm suddenly goes off of its own accord. This has happened twice now both times when I was away and it goes on until battery flat.
Is this normal? Does it set itself automatically?
William Thomson

 

[Arnie]

Hello,

When did you last change your battery?

Most alarms are very sensitive to battery voltage and can go-off if the car is left for a long time and the battery goes a little flat.

Even if you have not set the alarm, most cars, especially the SL will draw a few mA (120ma peak in the SL, I believe) from the battery and this will, over a few weeks, cause an old battery to lose charge quickly.

 

[william thomson]

Arnie thanks for your reply. The battery was replaced in 2000 and seems to be sound but it could be faulty. Can an alarm which is not set go off anyway? Would the change in voltage make the alarm go off even when it is not set?
I asked locally about it and someone suggested it might be triggered by low-flying aircraft. We have had a lot of military aeroplanes flying over our house this winter. Some are very low and they come in waves of two or three. Is it possible an aircraft could switch the alarm on and also then trip it so it goes off?

 

[Arnie]

Well,
The radar from such aircraft could be resposible (My uncle once got stuck on a ferry when the system on his volvo locked him out and wouldn't respond to the keyfob because of the interference from the ship's radar).

However, I think that the central locking on the later SL's is activated by infra-red through a sensor on the driver's door handle (MBSS1 alarm).

Do you know which type of alarm you have? I presume that you are locking the door with the key rather than the remote-fob? .. and if so the alarm LED is probably not flashing?

Ultimately, it could be a faulty alarm unit, but this will need more investigation.

(The thing about the battery is that most car electronics will operate quite well between, say 9v and 16v. If your battery is bordering on being flat then its voltage may drop to the point where the logic/microprocessor circuits in the alarm start behaving erratically and lose their set logic states, hence setting off the alarm siren.)

If you don't use your car, it's best to disconnect the battery since, the SL especially, because of it's permanetly-active roll-over sensors etc, constantly draws a small current from the battery and even a new battery will be discharged within 2 or 3 months