Okay I have a situation. 2002 S500 304K miles. Went to the gas station. went to start the car. Starter continued running after engine was started (just replaced the starter 200 miles ago) I disconnected the battery. Went to work just fine the next day, I went to leave work and boom same thing happens. Starter continues to run after engine started. I disconnected the battery again to stop the starter. But when I went to crank it again, nothing happened. I just now replaced the starter and starter relay and the starter still doesn't want to work. I don't hear any clicks or anything when i try to start the car. Any suggestions?

 

you have a dead starter that melted itself. defects on this stuff are higher than your usual car part which is just cast and pressed together because it has to be handbuilt and Q/A'ed

 

I just replaced the starter with a new one about 2 hours ago. I've replaced the starter now two times lol. Once when the original failed, a second after it got stuck on. When I replaced the starter 2 hours ago I replaced the relay as well. I had the new starter tested at the parts store before I left just to be extra sure.

 

i missed that, is your battery good?

 

Yes battery is good.

New discovery though. I checked the passenger side floor board fuse block and found the 60 amp fuse blown. I replaced it with jumpers and another 60 amp fuse and attempted to start. Fried the fuse instantly. There's a short to ground somewhere but I don't have wire diagrams for anything. Any knowledge on where that 60 amp fuse connects to? See attached photos

 

Have an update. Had one of my mechanic friends who's worked at various shops (including MB) come and take a look at it. We weren't able to verify the cause of the short in that time, but we were able to elimate the new starter (just in case it was the problem) and the alternator. We disconnected the alternator pigtail and battery wire to test. Upon turning the key to the ON position with the alternator and starter fully disconnected, the 60 amp fuse still blows after a second or so. If anybody has any info regarding wire diagrams or where the short could be coming from I'm all ears. Thanks!

 

Hey, take a look here: https://w220.wiki/Fuses

 

Yayyyyy, dealer part and install only. Is there a way to test to see if it's the ignition switch itself or the entire module?

 

Before buying anything, would suggest to check wires thoroughly.

 

Will do. Ill report back with any findings!

 

So I fixed the issue!

After throughly checking the EIS wiring and module and not seeing any obvious signs of damage, I decided to retrace my steps to try and see if anything was staring me in the face.

I checked the new starter, all terminals were free of corrosion and tight. I checked the alternator and again, all terminals and connectors free of corrosion and properly secured.

I checked the fuse boxes, all terminals free of corrosion and secure. (Minus the fuse block but I sanded the terminals to get rid of surface rust right after.)

This brought me to think about the original issue. The starter continued to run after the motor was cranked and running on its own power. Top three candidates for causing this issue are A: Starter Relay. B: Starter Solenoid and C: EIS. Of course there could be other issues such as the SAM, but I still had dash lights, and the fuse wouldn't immediately blow, so I assumed it was still good.

The only thing I had changed after the incident was the starter and the starter relay. Those were two out of the three common reasons for the issue.

I thought it was highly unlikely that I got a SECOND bad starter (if I were to assume the first replacement was bad). The only other thing I've done is swap the starter relay. It's possible I got a bad relay, I worked at an autoparts store and I've given my fair share of bad off the shelf parts before. I've seen it happen. So I decided to dig into the relay.

I pulled the relay, hooked it up to 12V and tested it. I heard the click, everything seemed good. But I didn't want to stop there, I really wanted to see if it was in fact the relay before I dove into EIS.

I did some digging and found a forum discussion with a person who had almost the exact same symptoms as me: no preignition lights, no AC, no windows, consistently blowing 60 amp fuse. He ALSO changed the relay. I then saw some comments saying that he needed to make sure the relay was a GREEN relay.

I had ordered a yellow Bosch relay, as Rockauto.com currently had that part number listed as a starter relay. So I decided, what the heck, let me go down to the local parts store and grab a relay from them. It's possible a listing was incorrect online.

I got the relay, it was green, like the discussion post suggested. Apparently the green relays have a diode included in the relay assembly. I've never seen a relay with a diode before, but this is also a Mercedes, so nothing out of the question.

In fact the autoparts store had the Mercedes branded relay as their in-house brand relay, so I ended up getting an OEM relay with a lifetime warranty (because whitebox products can sometimes be great).

Anyways, I took out the Bosch yellow relay, and compared that wire diagram on the case to the wire diagram on the green relay. Sure enough, the yellow relay was missing the diode (for those who don't know, a diode is represented on a wire diagram as a triangle with a line at the tip that is perpendicular to the wire, see attached photos) So I plugged in the green relay, swapped the fuse, and turned the key. I'll be darned, the fuse DID NOT blow. I had fixed the problem.

So, in the event anyone else runs into this problem. If you've replaced the relay for the starter, make sure its a green housing. Better yet, make sure its got a diagram for the individual relay on it, and make sure it has a diode in line with the relay. It WILL blow your 60 amp floorboard fuse.

Thanks to everyone for the help they provided! It really was invaluable in helping me narrow this down

I edited the first photo to make it easier to see the diagram on the relay.
Notice the missing diode on the yellow relay diagram.