The K40 is involved with various engine features including fuel injection and EGR/resonance flap on my W210/606. I don't have the complete list of what it does, but I do know that they fail frequently and cost significant dollars.
Mine "failed" shortly after I got the car, and the local indy shop said whatever the car was doing was classic for that. THe K40 is also implicated in the bucking issue, but it's down the list.
Anyway, after I bought a new K40 I had them save the old one and took it to the bench. Carefully I broke the three tabs and eased it apart. (It snaps back together just fine). Inside you'll find simply three relays and a few diodes. A very simple circuit with 5 bucks worth of parts. I found some cracked solder joints (hard to see) and repaired the piece with about 2 minutes of soldering. If you can't see the cracks in the solder, just resolder all of them and it only takes 10 minutes. Taking a hot solder gun and a little thin solder it's quite easy to make sure that the joints are good. You may also "burnish" the relay contacts with a thin file or tool meant for this purpose. Now I have a spare K40- but perhaps if you need one you'll be able to fix yours! I think it's a recurring problem due to heat, heat cycling, and vibration affecting the solder joints. I suppose someone is making some money on this part.
Cheers to all- Joe