This is a very well researched post on a common R170 problem. I've seen a lot of people over the years trying to source the components you've provided links to.

It might be a good idea to either repair or buy new a second K40 and carry it as a spare. It's the only R170 spare part I carry around in my boot as if it fails it will stop the car dead and emergency towing repair garages don't carry them.

 

it was a tricky one to diagnose, especially if you're one person with no helper. It started off intermittent leaving me stranded with no idea what was wrong, and got progressively worse. Intermim fixes were disconnecting the battery or relieving the fuel pressure at the injector rail ... but I think thats more of a coincidence, but it got me home twice. People told me it was a flat spot on the fuel pump armature or worn out brushes, hence being intermittent .. there were some relatively cheap new k40 boxes on ebay from poland or one of the baltic states, but they were still us$150 or so from memory, and some truely awful ones on aliexpress. Ultimately I just think the windings overheat over time, being on all the time, but finding naked - uncovered - versions of the relays isnt easy, but as they are all made to a standard form, buying a few relays with cases, then removing the 2 piece plastic case to expose the naked relay is a very cheap fix if you can desolder the non-functioning OEM relay, so thats what it did . Make sure you look for the vesions with a EURO (not US) terminal footprint as that is what fits the k40 PrintedCircuitBoard .. in the end the k40 box is just a plastic box with a few fuses, and, 5 relays made to a particular standard. choose FORM A or FORM C - doesnt matter as change over function isnt required and there is the an unused pad is on the PCB for the unused terminal. If you're in the US probably easier to source the American Zettler relay which doesnt have a plastic outer cover, but being a Kiwi the US Seller wanted to sell me dozens of relays for us$12 each plus expensive shipping, so i bought 4 Tyco/TE's from RSComponents instead. Mouser or Octoparts in the US will also have these

 

Thanks for the informative video and diagnosis. I reached the same conclusion after having to replace my K-40. They get too hot. I didn't replace the single relay like you did but bought a new one altogether, $189. But now that I know the problem I still have the old one and I will repair it for a spare. I had seen other videos on YouTube taking the same approach and repairing the old one.

 

well each relay dissipates 1.6w, so like a dash light bulb. If you cant get the coverless Zettlers, just take your time removing the 2 piece cover. My Tyco V23076 was ips67 sealed meaning the 2 pieces were glued together so i cut the top off with some wire cutters, then pulled off the bottom plastic - but look at the spec sheet and start cutting away from any critical internals or try prying it off from the bottom .. youll see what i mean by comparing the terminal footprint with a picture of the naked cage. if its not glued (ips54) then the 2 pieces will just be connected by pryable tabs accessable from the bottom. Youll need decent equipment to desolder the OEM relays as the solder isnt lead (Pb), and the cage soaks up alot of heat

 

MEYLE Multifunctional Relay For MERCEDES S210 W210 W463 95-04 2105400072 | eBay new k40 box u$106+$10 shipping to NZ ex Latvia

attached image shows EURO relay footprint on k40 PCB scan

 

Hi, I have a 2000 SLK 230 Kompressor that won't start. The mechanic says I probably need a new relay box but he can't do it because he doesn't have the computer equipment to program the new relay box. I would have to have Mercedes "flash" it or "reprogram" it, or a shop that has the equipment, so the relay will pick up the signal from my keyfob. Is this true? Does the whole car have to go to a shop with this equipment, or can I just bring them the relay box and my keyfob?: I may have a friend who knows how to solder try first. The mechanic did mention some people do this. But, for now I have to tow my car back home. I also have to get another keyfob because I recently misplaced one and I have only one now! Thanks.

 

The mechanic says I probably need a new relay box but he can't do it because he doesn't have the computer equipment to program the new relay box. I would have to have Mercedes "flash" it or "reprogram" it, or a shop that has the

If he is talking about the K40 he is confused/inexperienced on Mercedes. It's plug and play. Basically just a set of fuses and relays with a diode thrown in. It's not too difficult to do some diagnosis if he has a circuit diagram. Maybe find a different mechanic?

 

If your engines cranking but not starting there's a fair chance you have a problem with the K40. Are your symptoms cranking without starting?
The K40 does not interface with the keyfob- it's more like a giant, sophisticated fuse.
You can repair a K40 - they generally fail due to dry solder joints and there are posts on this repair procedure on this and other forums.
The K40 is the one component who's failure can stop a R170 dead. I personally preferred to buy a replacement relay (buy genuine or Meyle), swap it over before it failed and keep the old relay in the boot as a spare.

 

Thank You for the quick response. Yes that was the symptom... cranking nonstop with no combustion. When the issue started happening a few months ago, it stalled once or twice when starting. And then it almost started. But then just cranking. Can a relay do this? Start to malfunction before completely burning out? I replaced Camshaft and Crankshaft sensors with no luck. Then I hade AAA tow it to this mechanic 5 miles away. He said it may be soldered, but he doesn't seem to be interested in doing it... He said I really need to buy the whole box and then it needs to be reprogrammed which he cannot do. He did kind of diagnose it for me though which was a big help, but now I am stuck paying to tow it again (AAA won't tow twice for same thing). Now the 12V battery is dead from sitting there a month and I just want to have it towed back home instead of towing it to a Mercedes dealer 20 miles away or another shop (a European car specialist he recommended) that may or may not be able to repair it I am trying to avoid spending lots of money on a car I rarely use.
So... I can repair this myself and not have to deal with any reprogramming nonsense? But what if I do purchase a whole new relay box? Thank You again.

 

K40 is a very common issue. Think of it as a power distribution block - it sends power to various modules, fuel pump etc. If it fails then things don't work.

Honestly it's not worth fixing as yes you might re-solder a few points but some relays might be on their way out. The cost of a replacement unit is a lot less then a towing fee for your car if you get stuck somewhere.

This module has no "intelligence" and no programming is required all it does is provide power, to replace it, its just plug and play.

 

Thank You. I wish the mechanic knew this and just fixed my car.

 

If you decided to diy, post back and members here can assist. Its not hard to swap over and you don't need any special tools and takes about 15min