Key fob 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.

Here you actually see the IR diode becuase the camera picks it up. However with your eyes you will only see the red light blinking once at the lower edge of the IR diode.

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.


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.

Loctite 406
Loctite 406 rapid super glue


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.

top side key fob
Top side of key fob
bottom side key fob
Bottom side of key fob, here is the battery lid located.
Flip open the key
Flip open the key
Take off the battery cover
Take off the battery cover.
Take out the batteries
Remove the batteries.
Use a flat scredriver to slightly pry open this side
Use a flat screwdriver to slightly pry open this side. stop when you feel a resistance. The other side is glued as well and do not try to pry the whole case open. It will break.
Use a knife to cut the opening
Use a knife to cut the opening this side since it is glued quite well.
Cut is open and free from glue
Cut is made and is now free from glue. It can now be pried open. Good luck!
Pry open the two sides of the fob
Pry open the two sides of the fob. Notice the push button component is totally loose from the PCB. This is our issue and why the key is not working. And probably the issue 95% of the cases.
Make solder pads on the PCB
Make some new solder pads on the PCB for the button on the PCB. Make sure not to get too much heat on the PCB and and clean the flux off with flux cleaner.
Solder the button back in place. Make sure the button body is firmly pressed to the PCB backing otherwise the button might crack loose when it is pushed next time. Make sure not to use too much heat for a long time that can damage PCB and other components. 380-400 Degrees °C is good enough.
Solder in place the button component
Assemble the PCB again in the lower housing. Make sure all other components have good solder.
Place spring in groove of the lower plastic case
Place spring in groove of the lower plastic case. Notice the angle of the spring.
Place button in key grooves
Place plastic tab in key grooves
Put the key in the upper plastic case
Put the key in the upper plastic case and align it with spring, then turn it around a full turn to tighten the spring so the key will flip out. This might take a few tries.
Key fob assembled
Key fob assembled. It will come loose without gluing, so you need to glue the case now.
Put a little strip of glue here behind the key
Put a little dab of glue here behind the key
Finish with some glue here one the other side
Finish with some glue here one the other side as well.

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

Cheers, Robs out!

By Robs

Passionate about vintage Mercedes Benz where driving, maintaining and learning new things and teaching others on the black arts of car maintenance.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.