OK, so it has taken a while but I now have a functioning alarm klaxon.
I thought a step by step guide might help anyone else in this situation ...........
On my car the siren mounting bracket is welded to the back of the front nearside wheel arch and the siren is covered with a protective metal shield.
It is obscured by the main fuse box and was attached by a tamperproof nut so it is fairly inaccessible but I guess that's the idea.
I tried to undo the nut with a small pair of mole grips without success however BMW helpfully left a small rectangular cut-out in the side of the mounting bracket and by inserting a sharp, flat head screwdriver through this slot I was able to dig the corner of the blade into the nut then, using the cut-out as a fulcrum, lever it round.
Once released the siren can be unplugged and removed.
The unit was marked on the back with the original manufacturer's name 'Gemel Italia SpA' and I spent some time trying to track them down to see if they could supply a new unit however they seem to have gone out of business (coincidentally there is a Gemel
s Italia SpA but they have no link with Gemel).
It is obvious that BMW never intended for the battery in the siren be replaced as the two halves of the shell are glued together with silicone mastic so I needed to find some way to open it - Thankfully I had a couple of spare units to practice on (Thanks again Brian)
After removing the three screws from the back of the shell I tried running round the joint with a single sided razor blade and then prizing them apart with a flat bladed screwdriver but this just chewed up the plastic.
My second attempt was to try and cut along the joint with a Dremel "circular saw" but I found it next to impossible to keep a decent straight line and the plastic tended to melt and splinter.
Having got a unit open I could see that the joint is actually made up of a lip on the bottom shell which slots into a gutter in the top shell. It looks like this gutter is filled with silicone before the two halves are put together and the screws tightened so I was actually trying to cut through three layers of plastic and the silicone.
I wanted to end up with a "as new" looking unit so I decided to use two complete units and to cut through one unit just above the joint (sacrificing the top half) and one unit just below the joint (sacrificing the lower half.
After cutting the first unit above the joint .........
After cutting through the shell the top section can be discarded.
Take care when disconnecting the speaker as the white plug is retained by a thin black tab on the circuit board which is quite fragile - it only needs lifting slightly with a small flat-blade screwdriver while pulling gently on the cable.
With the main part of the top shell out of the way I was then able to remove the remaining "stub" by using a small pair of mole grips and working my way around the case gently flexing the stub to break the silicone seal and then pulling the stub upwards.
Take care around the connection socket as the connection pins are very thin and there is a soft sticky membrane on the circuit board that presumably acts as another seal.
It took a few loops around the rim before the seal finally gave way and I was able to pull the stub out completely leaving the bottom shell intact, complete with the circuit board and rechargeable battery.
In my case the internals were in a pretty poor state - the battery had leaked and the circuit board was corroded so everything had to be binned but at least that explains why it wasn't working
I then just repeated this procedure cutting through the bottom shell just below the joint leaving the top shell intact.
Take care when cutting through the bottom shell on the corner where the loom connection socket is mounted as if you cut too deep into the bottom shell it will damage the side of the connection socket in the top shell
Take care not to cut too deep on this corner .......
It is then just a simple matter of cleaning up the two halves of the shell and removing all the old mastic - I found a very small Torx bit (T7 or T8) worked well for breaking up the silicone in the gutter followed by a light grade Scotchbrite pad pushed into the gutter with a small flat-bladed screwdriver.
The battery cover is held in place with two of the screws that hold the shell together so can just be lifted out of the way and the battery disconnected from the circuit board - again be careful with the plug retaining tab.
The circuit board is held in place using two small screws with insulating spacers but if your circuit board looks clean then then it can probably be left in place. In my case I removed everything and washed and dried the bottom shell to get rid of any battery corrosion residue.
The battery is a Varta 6/250H 7.2 Volt and is basically six NiMH cells wired in series arranged in a clover leaf.
I contacted Varta however, although they still make the 250H cells, they no longer make the 6/250H assembly.
I then tried eBay and after a bit of searching I came across an eBay trader called Xcelplus who appeared to be selling the correct battery although it was only listed as being suitable for E38 and later.
When the battery arrived it was the correct physical size but had a different circuit board connector.
All the connection plugs on the old batteries I had were showing some signs of corrosion but luckily the plugs for the battery and speaker on the circuit board are the same size so I just cut the plug off the new battery and soldered on a speaker connection plug in it's place.
As the speaker cables are both white you need to take care to get the polarity right - I offered up the speaker connection plug to the circuit board and marked the negative connection with a marker pen before soldering.
I have since had some further contact with Xcelplusstore and have supplied them with one of the connectors used on the Z3 siren and I believe they are now going to offer this as an option going forward.
It is then just a case of re-assembling ....
I didn't want to use silicone to seal the two halves of the shell together so I made a simple O ring from a length of 1.5mm O ring material cut to length and then superglued the ends together - maybe not as effective as silicone but at least it is maintainable and the mounting position is in a fairly protected area within the engine bay.
Take care when plugging the loom back into the siren and check that the four pins from the circuit board are all vertical and parallel as they are quite fragile and easily bent when the plug is pushed in.
When I first tried my new siren it didn't work but when I unplugged it again I found one of the pins was bent out of line.
I straightened it with a pair of long nosed pliers and then only pushed the plug in part way and then retracted it again, checked the pins and then pushed it in a bit further and check again until it was fully home.
I now have a working and maintainable siren
Hope this helps
