e46 M3 battery disconnection

[Baz Tench]

Hi. As above, I need to disconnect the battery to do a little bit of work to my car. Will it trigger the alarm?

The alarm on my old Boxster used to go off when it detected a voltage drop, and I'm wondering if the factory standard one on my M3 will do the same.

Many thanks in advance for any advice/opinions.

[Frances The Mute]

Voltage drop is one thing. Complete voltage loss is another.

You'll be fine to disconnect it.

[Baz Tench]

Cheers, my Boxster alarm must have been extremely sensitive then I suppose. I've just been out and done it anyway, and it was fine.

The battery compartment was flooded with water though, about an inch and a half deep! I've dried it out now and reconnected the battery. The next job is to find where the water is getting in. Could be the rear-light cluster at fault?

[steve singh]

Baz Tench said:

Cheers, my Boxster alarm must have been extremely sensitive then I suppose. I've just been out and done it anyway, and it was fine.

The battery compartment was flooded with water though, about an inch and a half deep! I've dried it out now and reconnected the battery. The next job is to find where the water is getting in. Could be the rear-light cluster at fault?

Could be the cluster...also assume the sub-frame is fine?

PS Re Boxster - if you turn the ignition to number 1 (when the dashboard lights up) and then disconnect the battery the alarm doesn't go off...I know it's late now!

[Frances The Mute]

The light cluster is the usual supect.

They are quite poor quality to be honest. My E46 items were shot, as were both of my E38's. I've seen plenty of the E39's with the same issue, too.

[waxaholic]

Frances The Mute said:

Voltage drop is one thing. Complete voltage loss is another.

You'll be fine to disconnect it.

If you disconnect the battery without unlocking the doors first it will trigger the alarm.
hth

[E38Ross]

waxaholic said:

Frances The Mute said:

Voltage drop is one thing. Complete voltage loss is another.

You'll be fine to disconnect it.

If you disconnect the battery without unlocking the doors first it will trigger the alarm.
hth

yup, as i found out in my E38

[blondini]

Just remember that the battery needs to be connected for the glass to 'drop' when opening the doors, and more importantly the glass needs be 'dropped' when trying to close them.

[Baz Tench]

Helpful tips there, cheers.

The water is definitely getting in from above the battery tray, so I suspect it is the cluster. It turns out that I needn't have disconnected the battery anyway (removed both door mirrors for respray). It was initially down to worrying about the door mounted airbags possibly going off when the door cards were disturbed.

I'm glad it brought to light that leakage problem though! I probably wouldn't have found it otherwise.

[Frances The Mute]

waxaholic said:

Frances The Mute said:

Voltage drop is one thing. Complete voltage loss is another.

You'll be fine to disconnect it.

If you disconnect the battery without unlocking the doors first it will trigger the alarm.
hth

I took it as read that the car would be unlocked prior to doing it. I didn't think of someone just opening the boot.