A common (ish) fault on these cars is the alarm siren backup battery leaking or suffering corrosion over time and damaging the unit. Symptoms include random, fast flashing of indicator lights when the car is locked or the alarm siren randomly going off or making strange noises.
The fix is to replace the alarm siren. Mercedes actually updated the part number and the replacement is A2198203226. Cost from a Mercedes dealer at time of writing was £99.
It’s a fairly straightforward job – I managed it in about 1 hour with basic tools. You need the following:
1x A2198203226 Replacement Alarm Siren (See Mercedes Dealer or other parts supplier)
I’ve linked to example products/searches for the bits required.
1….Remove the Driver Side (UK, RHS) Wheel
Use the 17mm deep socket and breaker bar to break all wheel bolts and wheel locking nut (if applicable) of the front UK Driver Side (RHS) wheel. Jack up the car on the front sill jacking point, remove the wheel nuts completely and take the wheel off the hub. Use an axle stand to support the car while working!
2. Remove Wheel Arch Liner
a. Remove the two plastic pop rivets at the base of the rear side of the wheel arch
b. Use the 8mm socket to remove the two metal bolts on the underside of the same rear portion of the sill/arch
c. Use the 10mm socket to remove the 5 plastic nuts on the inside of the wheel arch liner. I’ve pictured two below, the other three are visible in the top picture with two located just next to the axle.
d. Remove the wheel arch liner. It’ll take a bit of wiggling about and bending to unhook it from the chassis screw mounts. Once off, you can see the alarm siren housing (the silver enclosure with the wire going over it).
3. Remove the Alarm Siren
Use the 8mm socket to undo the three nuts fixing the silver housing to the car body. The enclosure can then be removed. There is a single plug connector to detach from the alarm siren itself. The siren is fixed to the enclosure with a T35 Security Torx screw. Remove this and remove the old siren.
Old on the left, new on the right. The new design does not reuse the T35 Security Torx screw. You need an M8 nut. I used an M8 flange nut which I put on with a 13mm socket.
Reattach the electrical plug and the enclosure; reassemble the wheel arch (the liner is a bit of a pain to push back into place but make sure it is correctly inserted or it could pop out and rub against the wheel or let water/dirt in). Reattach the wheel and you’re done!
It’s possible, if you have access to STAR, to code an audible lock/unlock beep/chirp. This is not standard for the UK spec but is available for other markets. I’ll not go into those details here though!
Hope this helps solve your issue.