'98 528i - Alarm goes off after battery charged

[rickpthomas]

Head scratcher...helping a friend get this beast ready for sale. Battery was drained due to lack of use in storage (assuming). Interior panel lights would still come on but not crank. Pulled battery to charge it and then after reinstalling, the engine cranks fine and car starts but the alarm now goes off every time we start it. Not even sure where to start looking. Any ideas appreciated!

[VormDK]

So the alarm is sounding while engine is running?

Is it OE or aftermarket alarm?

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[TheAngryBear]

If you are sure the battery is good (deep discharges can kill so it is probably worth it to take the battery somewhere for testing) then try the following:
1) after an overnight charge, let it go to sleep, disconnect the negative and positive, wait 10 mins, reconnect, and try. Anecdotal stories say this works and it is also the alarm/code reset procedure.
2) check your siren battery voltage. Likely the siren battery is also dead from the discharge. One of the alarm triggers is the siren sensing a change in voltage. If it senses that, the alarm sounds. IIRC the only way to fix is to replace the unit as the siren battery is sealed in the unit.


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[Qsilver7]

Ditto TAB's comments above as to why the alarm siren may be sounding off.

See info below about the car's alarm siren and it's OWN internal battery. It is charged by the vehicle's battery. If the car's battery was deeply discharged...then it will eventually take out the alarm siren's battery or at least cause it to be deeply discharged. And in your attempts to get the car's battery up to snuff...the weakened alarm siren's internal battery may not be fully charged and is sounding off (as TAB suggested) due to it being in a state of discharge:

[rickpthomas]

Yes, while the engine is running. OE alarm.

- - - Updated - - -

If you are sure the battery is good (deep discharges can kill so it is probably worth it to take the battery somewhere for testing) then try the following:
1) after an overnight charge, let it go to sleep, disconnect the negative and positive, wait 10 mins, reconnect, and try. Anecdotal stories say this works and it is also the alarm/code reset procedure.
2) check your siren battery voltage. Likely the siren battery is also dead from the discharge. One of the alarm triggers is the siren sensing a change in voltage. If it senses that, the alarm sounds. IIRC the only way to fix is to replace the unit as the siren battery is sealed in the unit.


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Great info. Will give it a try and report back. Thx!

[rickpthomas]

Ditto TAB's comments above as to why the alarm siren may be sounding off.

See info below about the car's alarm siren and it's OWN internal battery. It is charged by the vehicle's battery. If the car's battery was deeply discharged...then it will eventually take out the alarm siren's battery or at least cause it to be deeply discharged. And in your attempts to get the car's battery up to snuff...the weakened alarm siren's internal battery may not be fully charged and is sounding off (as TAB suggested) due to it being in a state of discharge:

Awesome info, thank you!

[rickpthomas]

So the alarm is sounding while engine is running?

Is it OE or aftermarket alarm?

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Turns out it is an aftermarket Clifford alarm. How does that change the potential cause? Thx!

[VormDK]

The problem could be similar, but it could also be something like the alarm unit forgetting its settings. E.g. a trigger that should respond to ground now reacts to 12v - and your car puts 12v on that trigger when turned on.

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[Dustyleek]

If you are sure the battery is good (deep discharges can kill so it is probably worth it to take the battery somewhere for testing) then try the following:
1) after an overnight charge, let it go to sleep, disconnect the negative and positive, wait 10 mins, reconnect, and try. Anecdotal stories say this works and it is also the alarm/code reset procedure.
2) check your siren battery voltage. Likely the siren battery is also dead from the discharge. One of the alarm triggers is the siren sensing a change in voltage. If it senses that, the alarm sounds. IIRC the only way to fix is to replace the unit as the siren battery is sealed in the unit.


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Old wives tail is if you touch both positive and negative cabling together (disconnected batt.) That it will complete circuit and ensure static power is not retained in the system.

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