I have a W203 Coupe that had dodgy central locking. The central locking operates in 2 ways on this Mercedes. Infrared on the drivers door handle and radio frequency through the antenna amplifier.
Its common for the antenna amplifiers to fail, and on the coupe W203, requires removal of the rear window to replace. Its also common for the IR receivers to be absolute rubbish and barely work.
When your in that situation, you are stuck with a car that has very intermittent central locking. Here is a cheap fix for the IR side at least.
First thing to do is remove the door lock. One Torx bolt through from the inside of the door and it comes off.
You will then have to snip the metal tabs to get the sensor removed:
Once removed, you will have to remove the silicon conformal coating. Its best to start in the opposite corner that I did, and try and pull it off in one piece. IF you do as I did, you will have to pick bits out with a sharp knife, without damaging the PCB.
Once I had it off, it was obvious that there where 2 dry solder joints on the sensor pins:
Re-solder the 2 pins with some nice LEADED solder (Lead free solder is the reason there are cold solder joints):
Once dry, I used a bit of metallic epoxy to secure it in place:
I can now use the IR locking from afar without any issues, even in direct sunlight!
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