It just happened; car just got towed in. The car wouldn't start but it would click. I ran to the auto parts store and got what I thought was the correct relay for the starter as a quick "let's check this before going in deeper". Turns out the relay was bad: you MUST use a GREEN relay - it has a diode to limit current.

Installing the wrong relay and trying to start the car led to a bunch of smoke/burning out of the front right fuse box area (I yanked the relay out - got burned lol), and now the ground wire's insulation is completely cooked, but more worryingly: turning the key doesn't do **** anymore. Everything seems to be working as functioned, but turning the key to position 2 doesn't turn on the car or anything. Something's cooked, and I'm suspicious of the right SAM, but I'm hoping it's only that and not something more major.

No fuses in the front right are blown, and the SAM is getting power. The ground wire also seems to have continuity. I'm trying now to decipher Mercedes wiring diagram in WIS, but it's a nightmare to use.

I only see the fried ground wire, but certainly whatever positive fed the relay (that shorted upon insertion of the relay) likely fried too. I'm hoping and praying the ECU isn't fried, because that will truly be a nightmare to fix. I'm not sure how to test, and if anyone has advice, I'm all ears.

For now I'm going to get another SAM module. Do these need programming? Can I simply swap the PCB over that's attached to the SAM? If not, can someone lend advice on getting a used SAM that roughly matches the programming?

 

OP start with plugging it in to Xentry and see what is missing.

Buy electrical parts from Mercedes. They look everything up based on VIN number. Not saving any money when something like this happens.

 

With a symptom of clicking, but no engine crank, I would have checked the battery as a first diagnostic step. The diode with the relay isn't for limiting current, but for clamping reverse current that results when a relay is de-energized. Without a diode, the transistor in the SAM that drives the relay gets reverse polarized, and can go poof. I'm surprised there was wire damage as the purpose of fusing circuits is to protect the wiring when some component goes haywire and overloads the circuit. This needs to be repaired even though only the insulation is damaged. My guess is you need a new SAM, although it's likely only a transistor is blown (but with a fried wire, SAM damage could be greater). I don't know if the SAM requires coding. You need to let the dealer or your mechanic consult on this. Good luck -- I hope it's not extensive or expensive to repair.

 

Yes I think I figured out how/where the electricity flowed. I think the positive lead inside the actual SAM unit is fried, as it's the ground lead coming out from the relay that was toasted.
I'll replace the SAM and see if I can simply swap the PCB: the flow of electricity doesn't look like it will have caused damage to the ECU or anything. Fingers crossed.

Once that's done I can check the starter to see if it's the starter that's bad, or some stupid EIS being problematic.
The battery is fine.

 

which sam

i have a rear one if you want for free

 

Front right, the one that controls the engine ECU and starter

 

OP it is very easy to bypass the relay and ask the starter to operate for testing.

The signal wire is the small one that connects to the starter. Supply power to it. It cranks? The starter works.

You still have burned wiring to solve and likely a SAM.

 

Replacing the SAM module did not bring back Position II activation on the key. Either the SAM I got is bad, or there's something else wrong.

 

Back to basics -- are you certain your starter battery is good?

 

Yes, but the issue right now is turning the key to position II doesn't do anything. Position II is the activation of the ECU? Perhaps the ECU isn't getting any power.

 

Checked prefuse box under passenger feet; 60A EIS Fuse is blown. Hallelujah.

 

Fuses seldom blow for no reason. There could still be wiring or a starter problem to repair. If a replacement fuse blows, you'll know for sure.

 

It didn't.

The fuse blew from the wrong relay that was installed, but like I said, now turning the key to position 2 turns the fan onto high.

 

Congrats ! Looks like you're back in business.

 

Unfortunately not. Whereas before the car wouldn't start, it now still doesn't start, and the fan kicks into high.

I finally got a code reader across all the modules and don't see any explanation on why the fan is suddenly running on high. Someone told me this is because of no comms between the Engine ECU and the car, but the code reader reads data off the engine ECU ("Motor Electronics 2.8").

The only indicator I'm getting from the FR SAM that there might be an issue is: 029 Terminal 61 "off" when reading live data.

Re: the car then not starting:

Maybe:
"604 Drive Authorization Routine in the control module 'Electronic Selector Module (ESM/EWM)'" is active is set to "No".
"614 Version of control module 'Electronic Selector Module (ESM/EWM)'" has a value of "Replacement"


@tusables brings up a damaged engine ECU here:
https://mbworld.org/forums/s-class-w...ml#post6778327

I might open it up and take a look

 

Not sure why it took me so long to try this: I bypassed the starter relay and the starter did not turn on despite obvious arcing when bypassing the relay. The starter/solenoid are likely bad. Unfortunately, I still have as mess I have to deal with on why the cooling fan is kicking fully on when the key is put into position II. I suspect perhaps the burning ground lead caused a short in another wire in the wiring harness.

 

Back in the day, fans on high speed with ignition on was considered to be a prime clue to a damaged ECU.

 

Could well be, but nothing in the failure (shorted wire) suggested the ECU should be affected. That said, a damaged sensor wire would explain this as well. I'm going to fix the wiring harness and see if I find something as I go in. The computer was able to read the ECU off the car, so it's getting power. I have, as yet, not found any blown fuse other than the big 60A EIS fuse. I'll see what I find once I dig into the wiring harness tomorrow (not a fun job). The starter should get here on Wednesday.

 

Digging in, I suspect everything is working except the fried ground wire took out other wires with it in the wiring harness for the front right SAM.

Unfortunately I don't know how to access the wiring harness;
I thought it was somehow tucked behind the fender liner, but it's all inside the body. This looks like it's going to be a pretty miserable process but I'm about to begin and undertake it.

 

If you need to pull the wiring harness out to repair it, try to find the far end and tie a parachute cord on it so that after you do the repair, you can use the cord to pull it back in. Much easier than trying to push on it !

 

Very nice.

Things are coming apart, and I am getting a little bit more optimistic.
I've found the termination of the fried ground wire; and I've extracted the affected wiring harness from the SAM box to the passenger footwell.

 

Followed the fried ground wire home (under the passenger footwell), for better or worse found no destroyed signal wire, just put everything back together and am about to test it once the battery charger finishes its battery repair cycle (while the battery is disconnected)

 

Have the fixed ground wire in. Have the new starter installed. Car fired right up as if nothing had ever happened.



I'm not sure what caused the fan to start running when the key was in position II earlier. Maybe I didn't install a fuse or connector correctly.

The top connector board of the front right SAM had melted somewhat, I replaced that but stuck the original front-right SAM PCB back into the car. It showed no signs of an electrical short, neither physically, nor from following the wiring diagrams. The new connector plug (with the ground lead that melted) is coming in Saturday so I can repair it properly.

The new starter is a made in China Bosch I got from Rockauto for ~$200 shipped. Bosch SR0502N. Quality-wise it looks excellent - even better than the genuine Mercedes/Bosch starter I removed (the new one has copper binding posts, the old one doesn't). What's unusual: the old starter is dated 2021, but I don't remember ever having replaced it. Maybe when I did the rear main seal??

HAD I known the location of the starter, I probably could have tried harder to tap it with a stick to get it started and driven home. The old starter kicked on intermittently on the bench. I should have tried, but an EIS issue was in the back of my mind.

Notes on replacing the starter:
The 4matic cars need to remove the exhaust, my ABC equipped car did not. There's a heat shield you have to remove, and a 10mm nut to remove the ABC supply hose retainer, and then you'll have enough room to maneuver the E14 socket in position to remove the lower starter bolt. For the upper starter bolt I had to pull out my E14 wrench, remove the air filter housing, and climb on top of the engine to get leverage to loosen it from the top.

To remove the solenoid connector, I had to first remove the starter most of the way from the bottom, then remove the 10mm nut from the top. Then I could disconnect the main +12v line from the bottom and remove the starter. (Re-install the solenoid connector from the top).

[url=https://youtube.com/watch?v=ISVn4VSMmgo]Here is a helpful video that shows the procedure on a 4matic S430.

While I haven't thrown the interior back together, overall I am very impressed with how well Mercedes cars come apart and go back together again. VERY impressed. You do NOT see this from the likes of Audi and BMW of that era (Audis give me nightmares and I don't even own one).

Final Warning:
DO NOT INSTALL A YELLOW RELAY IN PLACE OF THE GREEN STARTER RELAY! Whatever relay you use MUST have a diode in line with the 87A pin!
O'Reilly listed this as a compatible part, and having spoken with them on the phone, they put the blame on Bosch. I've called them twice now to make sure they change their website. Maybe this relay works for other M113 vehicles (that might explain the mixup), but NOT on the W220 and W215 chassis!

 

I admire anyone willing to roll themselves underneath a car and get their hands greasy. Glad to hear you got it done.

 

I appreciate that, although this was less willing and more coerced. Goddamn starter.