Hi,

Sorry, this a bit long ... please help !

I have a late 2006 UK S500. I think its a W221 ? (Its got a little wheel / puck to control the Comand ?)

I've only had it since Xmas. Recently, I've had a number of symptoms:

• The alarm goes off spontaneously. When it goes off the indicator lights are not flashing.
• I get a red battery warning symbol on the dash randomly as I'm driving.

I did some Googling and IIUIC there are sometimes two issues; a) the alarm siren has a built in battery that sometimes leaks and damages the PCB, causing the battery to drain and the alarm to malfunction. b) If the battery is faulty and the voltage drops too low, the siren thinks its been disconnected to it sounds, using the built in battery.

So, I thought it could either be the battery or the siren unit.

I decided to use an OBD2 bluetooth adaptor to monitor battery voltage. This just confused me more. As I was driving, the voltage is normally 14.4v, however when the battery warning symbol appears, the voltage drops to 11.5v, sometimes as low as 10.9v. It stays there for a while, if I rev the engine, sometimes it recovers to 14.4v.

IMHO that cant be the battery ? If the engine is running, even if the battery is faulty the voltage wouldn't drop below 12v unless the alternator is faulty ?

So, I took the car to Halfords, and they tested the battery. They tested the battery in the front engine compartment (The starter battery ?) and said its fine. They also tested the charging circuit, and said that's faulty, and told me I need a new alternator. They then quoted me the price of a new car to replace the alternator :-) £850 (apparently £550 for the part and £300 to fit it)

So, here I am.

If its really the alternator, then if I put a battery conditioner on the car overnight, that should charge / hold the battery above 12v and the alarm shouldn't go off. But tonight I did this, with the battery in the engine compartment, and the alarm is still going off every 10 minutes !!

HELP ! :-)

So, some questions :

• Is it true that if the battery goes flat the siren will sound ? As it thinks its been disconnected.
• Does my car have two batteries ? I cant find details in the manual, but Google seems to think I have a main battery behind the back seat ? and the front battery is just a starter battery ? Which battery does the alarm need connected, I assume the rear battery ?
• Should I connect the battery conditioner to the battery in the rear ? will this stop the alarm going off ? that's more complex, as its really tucked away.
• Is there a way to easily connect a charger to this rear battery, without dismantling the entire boot ? Can it be charged from the front engine compartment somehow ?
• Does all this sound like an alternator fault ? Am I right that the voltage shouldn't drop below 12v while driving, unless the alternator is faulty ?
• How much (roughly) is a new alternator ? Can someone point me to how to find out what the exact model number is ?

THANKS !!!

I really need to stop pissing my neighbours off. My alarm has been going off all night for the past week ! :-)

Jon.

 

Unplug your siren so it will not go off any more while you work out whats wrong my Audi A8 did that to me so i unplugged it then once sorted just remember to plug it back in!! Good luck!!

 

If you have a 2016 I would take it to the dealer. It should still be under warranty.

 

Sorry. Typo. 2006 !! - fixed.
:-)