Alarm Siren Fix
This Technical FAQ (How to...) has been carefully compiled by John Handford
The alarm siren on the TT has a limited battery life and has given quite a few people trouble. The first symptoms of failure are a series of beeps from the rear - usually on a cold morning or after not running the car for a while. This means the rechargeable Ni-Cd (or more correctly Ni-MH) battery is getting old and can no longer hold it's charge and maintain it's voltage under load.
Replacement or fix?
You can either get the dealer to replace it for £140, buy the part yourself (P/N: 8L0 951 605 A) for £70 or buy two replacement batteries from Maplin or the like for £10 total and fix it yourself (Maplin Order Code: BN24B [2 required]) . Your choice.
Tools
If you decide to fix it you'll need:
10mm ring spanner or similar
Dremel saw or junior hacksaw
Soldering iron and solder
Hot melt adhesive gun or similar
Some wire and insulating tape.
Procedure
The unit is relatively easy to remove as you can see in Wak's guide:
http://www.wak-tt.com/ttalarm/ttalarm.htm which involves taking the rear light cluster off too.
I found you can get the alarm unit out easy enough through the inspection flap without removing the rear light cluster. You can see it here.
You just need to use a small 10mm ring spanner or similar. Reach round the back and loosen the single nut in the middle of the back of the unit. Once it's loose you can remove the nut with your fingers - be careful not to drop the nut! The Alarm can then be unplugged and removed.
Once you have the unit out you need to perform some surgery. The case is made from thick plastic and is welded and sealed extremely well. Basically you need to cut into the plastic case along the weld seam. You can see the line here 20mm from the edge of the speaker opening. For this you can use a Dremel type tool. I used the 25mm diameter circular saw attachment. You could also use a junior hacksaw.
You need to cut about 4mm deep all the way round. I left a little under the connector to act as a hinge - not strictly necessary but the prongs in the connector, which come from the circuit board, need to swing clear of the connector aperture, so it helps.
Here you can see how thick the plastic is. Don't cut deeper than necessary or you will cut through the circuit board support pillars.
The circuit board has two spring contacts which make contact with the battery you can see at the back of the unit. Unfortunately in this case the battery had leaked and corroded the contact and actually unsoldered it from the circuit board. This alarm siren had been dead for some time. If you catch your siren early enough (when it's still beeping) you shouldn't have this problem as the battery will not have leaked.
Watch you don't zap the electronics with static electricity from walking across a nylon carpet or similar. Best to earth yourself by touching a tap or earthed item before handling.
Here you can see the circuit board unplugged and unclipped.
Here you can see typical damage to one of the PCB battery connectors, caused by acid leaking from the battery.
The leaking battery can detach the connector from the PCB if it has been leaking for long. Here you can see that the corrosion has penetrated through to the other side of the PCB. This might just respond to cleaning with IPA and a toothbrush but in some cases the solder joints may need repair.
If the acid has eaten away tracks and components, as in this example after cleaning, the circuit board is beyond easy repair.
Here's a close up of the Ni-MH battery. There are two of these 3.6V batteries wired in series to give 7.2V. If you can find a readily available source you could just replace them.
Unfortunately I've been informed that the only manufacturer Emmerich, a German company, has ceased trading. Their battery was a 160mAh capacity but in it's later production form had been superseded by a slightly increased capacity 170mAh version. The Full specification was:
Ni-MH three cell stack
3.6 V nominal voltage
160 or 170 mAh capacity
320 mA maximum discharge load current
22.1 mm diameter
11.6 mm height
15.5 gms weight
There is an alternative. These "3V" described nominal 3.6V Ni-MH batteries from Maplin are rated at the same 160mAh and are a suitable equivalent.
As you can see they are a similar size but a different shape with PCB contact pins. If you use them, you need to remove the old batteries and cut away the plastic that held them, in order to make enough room for the two new batteries.
Make sure you observe the original polarity of the old battery. You need to solder the new batteries in series and solder wires between the new batteries and the two spring connectors on the circuit board. The spring connectors can be unsoldered and removed or cut back to make attaching the wires easier and neater.
The new batteries should be wired in series so that they make one big (twice voltage) battery.
Here's another view: > 
Then the overall 7.2V battery needs connecting to the circuit board with the same polarity as the original batteries. Look at the way they were connected. You can see here that the original +ve connection from the old battery is in the lower part of the picture. This must also be the +ve connection from your new battery assembly. The -ve connection must go to the upper of the two PCB battery connections in the picture.
Remember the new batteries will be charged, so it is important not to short them out or allow them to make unintentional contact with the circuit board components. Construct the battery assembly separately and insulate well before gluing to the case and soldering it to the circuit board.
You can glue the new battery assembly in place with hot melt adhesive or similar. It should go in the same position as the original batteries so it clears the protruding transformer on the circuit board. Make sure none of the battery contacts are exposed and likely to touch the circuit board. Adding some insulating material to protect it is a good idea.
Finally the case can be closed up and held together with some hot melt and some tape. As good as new.
