S205 Key Repair by Re-soldering

[mattula]

I just want to report a successful "repair" of the key.

I have had the S205 since November 2015 but it is now a 2016 model. It is not a keyless entry system, it is similar to the classic two button keys and is used to turn the ignition on by inserting it into the ignition and turning it.
According to my data, this key is FBS4, which no one but MB can fix. And MB does not fix it, they sell you a new key for a donation of about $600 or more.

So the symptoms were as follows:

The key did not open the doors except 1 in 100 attempts.
The key mostly locked the doors.
The key could always start the engine.

But the red control LED and IR light (seen by the smartphone camera) always blinked when I pressed the buttons, and they looked exactly like my working key, so I thought the problem was not with the button itself.

And no, it wasn't the battery, and even training the key by inserting it into the ignition and waiting for an hour or so didn't help.

So I opened the key case and pulled out the PCB. At first glance it looked fine, but maybe some of the solder points looked a little dull.
Therefore, I re-soldered all the contacts, especially from the buttons, and additionally heated the entire board with a heat gun.
And all this together can become a solution.

I approach the car... one press, the doors open, one press, the doors close even from a distance of 10 meters. It worked many times. Yes.

 

[idan586]

I do not know how relevant this additional information will be but I have fixed multiple keys in the past using almost this exact same method.

One of these that was nearly identical was for a '96 W140 S420. Key would light up and blink with each button press, matching the other working key but would not lock or unlock any doors. Engine starting worked but it was a physical key without EIS at that time so this was not a relevant test. The PCB was removed from the casing and the entire board heat soaked using a heat gun, allowing the solder (and any cold solder joints) to reflow. Following this procedure the key worked and would lock and unlock properly.

I have also found multiple W210-era keys to have similar cold soldering issues and have re-soldered quite a few over the years to save the owners the cost of going to the dealer for a new key. This can be done by individually re-soldering the bad joints or the "Heat Soak" method with a heat gun or dedicated PCB Heating Oven. These keys tend to break solder around one of the legs for the buttons but I have seen breaks elsewhere along the PCB as well.

I am not sure what process MB is using to produce these boards or if this is just a common failure point but it appears as though this has been an issue for years at this point. I have since made it a habit to open any key that has an operational concern as a precaution to ensure that this is not the issue before diving further into my diagnosis.

 

[esteta]

I had several keys from different OEMs with the same problem. Usually a good flux + careful heating of the board to the solder melting temperature helps, but sometimes you need to re-solder BGA chips.