Fixing a broken remote IR key fob with single button for Merrcedes Benz W140

Ever experienced that the remote IR key fob for the W140 have stopped working even after the batteries are replaced? This can generally one of two issues:

1. The key fob needs to be synchronised again with the car. This happens due to loss of voltage on the chip when batteries are completely dead for a while.
2. One component fall off due to old solder and mechanical stress over time.

Checking functionality and proper voltage

This is a very simple task. Point the IR towards your face and hold the button for one second at least. A red light will blink if the key fob works and the battery is OK. If not change batteries and look for the red blink. The battery type is 2x of CR2025 or CR2032 with 3V. If there is no blink, you need to open the key fob and maybe do some soldering. When the blink is OK, try to see if it can open the doors. If not, then it needs to be synchronised.

Synchronising the key fob

This is a very easy job, make sure you have good batteries and the key will give a blink if you hold the button for at least one second. In order to synchronise it, go inside the car. Point the key towards the mirror and press twice at the button in rapid succession. Then within 30 seconds put the key in the ignition and turn it to position 2 without starting the engine. After you do this operation the key should be synchronised with the car, then you can check again if the IR key will close and open the doors. Retry this procedure if your are not successful.

Fixing loose components inside key fob

This is a bit more tedious and require you to do some electronics fixing.

Preparation

In order to do this job you will need a solder iron and some solder wire to fix it. Make sure you have flux cleaner after soldering to clean up the mess. To get the case open you need some small flat head screw drivers and a sharp and narrow knife to cut the glued case. For securing the case at assembly, you will need some glue. I can recommend Loctite 406 rapid super glue.

Procedure

Now you have figured out that the key fob is not blinking a red light after you swapped batteries. So you need to open the key to inspect the electronics. The plastic key case is glued together and it needs a bit of fiddling to get it open. Follow the pictures below.

Now your key fob should be working again and not come apart.

Cheers, Robs out!