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Discussion Starter #1
hi all hope everyone had a good xmas.. right now to the problem, went to use car tonight and a few miutes after setting off it started to squeel really loud , nothing i done stopped it i tried turning off and then on again , locking and unlocking , disconnected the battery and it stopped so left a min or so and reconnected and squeeling started instantly..thought back and the only thing i had done to car in the last few weeks was refit my hids, so ripped them out to suprisingly still have the squeeling car lol..i have just took headlight out and unplugged the alarm siren .. and hey presto the squeeling now does not exist but my question is what could be the fault ?? is it a case of siren ramdomly messed up and stuck on or could my hids of affected it ?? will replacing siren with another likely solve this ??? any help would be great thanks

3 Posts
Z3/E36 Alarm Siren failure

Yep, I have had exactly the same thing, although the siren would just go off by itself, usually a minute or so after I started the car. Not a good look - driving away with the alarm screaming. A week or so ago it started just going off continuously, and the only way to stop it was to pull the battery. Reconnecting the battery just resulted in the stupid thing just squealing continuously (i.e not "beep beep", just SQUEEEEEEEEEEEAAAALLLL!)

Anyway, I found BillyWhizz1984's excellent write-up on BMW alarm issues (, and disconnected the siren. Then I thought I'd have a look at replacing the batteries. Unfortunately not an easy thing to do as the siren is a sealed unit - the only option is to replace it. Undoing the three screws in the back won't get you anywhere because the unit is glued together.

So, I figured "what the hell. It's stuffed, I'm just going to cut it open and see what I can see". This post is the result.

Firstly, this is the unit I'm talking about:


I ran a slitting saw (in my mill) around the seam to cut the two halves apart, but a hacksaw would have also done the job:

Siren cut open.jpg

Now we're getting there. Fortunately there are no critical components near the seam. The seam is about 6mm thick, so if you don't cut any deeper than that you won't damage anything. The top half has the speaker element in it, and the bottom half contains the electronics:

Siren halves.jpg

The reason you can't just pop these things open is that they have a labyrinth seal around the edge that's glued together:

Siren edge detail.jpg

Now we discover why this is a sealed unit. There appear to be 3 button cells inthe bottom half, but they are soldered into the PCB and the PCB is smothered with epoxy. Replacing these batteries is not just a matter of popping the dead ones out and popping in some fresh ones:

Siren batteries.jpg

The batteries are Varta V170R which are a 1.2v, 170mAh NiCd with solder tabs on them. upon prising them out with a screwdriver (hey, the unit is stuffed anyway), I discovered that there are actually 6 cells, not 3:

Siren battery close-up.jpg

Siren battery stack.jpg

Upon seeing this, the electronic engineer in me said "Aha! it's a split rail" (i.e +/- 3.6v), but that isn't the case. It looks like there is a 4.8v supply and a 2.4v supply. What each is for would require furter destruction, and I didn't really want to go that far. Anyway, there is a common terminal with what could be a +2.4v supply and a -4.8v supply, although it could also be +4.8v for the electronics and +7.2v to run the speaker:

Siren Battery Voltages.jpg

So now I can replace the batteries. Haven't done it yet, but if I do I'll use some solder tabbed 1.2v Nimh cells and just solder them back in the same way the dead ones came out. No need to use V170R's, anything with the same voltage, the same or greater capacity and the same basic size will do.
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