C220d W203 Fuse 52 blowing sometimes

DopeyDave92

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Hi all, I'm working on a friend's 05 plate C220d. I've repaired a number of faults and it's been running like new for weeks with no issues whatsoever. My mate then rings me the other day to say he's had to get recovered home as it "doesn't do anything when you try to start the car".

Green flag guy said starter tests fine and I later notice fuse 52 (15amp) is blown. I read online that Mercedes recommend changing to 20amp for the C220d and so thats what I did. Car was starting like it's brand new again (barely ever cranks more than twice before firing) and starter does seem healthy. I thought randomly blown fuse and called it a day, but the car has other ideas as next day I get a call from my friend because it's doing again only now it's blowing fuses straight away.

I came over today and I've been feeling round pretty much everything with a multimeter but I can't seem to find anything loose or shorted. Doesn't make a difference whether we use 15, 20 or 25 amp fuse, sometimes it blows straight away but others we can start it 20 times on any size fuse with no issues, can't seem to find a pattern. One time it seemed to be caused by the blowers, another it seemed to be caused by one of the cables in the ECU/SAM housing, and once it even seemed to be caused by a faulty relay but checking these things shows no fault.

I did think starter for a while but it seems very strong and following a good service a few weeks back it's been starting like a brand new car, at least until this issue. When the fuse doesn't blow it's probably the best starting >3year old car I've ever seen. At this point I'm leaning towards faulty SAM, which does make sense as his ECU needed replacing a couple of years ago but I have no idea whether that was due to liquid damage or broken pins...

I don't have a diagnostics tool that can test or reprogram a SAM so short of going over the PCB with a multimeter I can't think of any way to test it. As far as we can tell there are no other electrical issues other than 1 of the indicators on the mirror blinks fast and theres a clicking noise as the blower starts up.

What I'm wondering is if anyone can shed some light on what may be happening here, or direct me to any common faults I may be able to check. Any help will be appreciated. Thank you.
 

Jobsworth

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One or two...
In my experience it’s most likely to be a bad starter motor, even though it sounds fine. What resistance to you have between the solenoid terminal and earth?
 

Trickythemerc

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You could have a couple of bad segments on the armature or a carbon build up between the comutator segments. When the starter motor comes to rest with the brushes on the bad spot it could blow the fuse as it is effectively a short circuit.
 
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DopeyDave92

DopeyDave92

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Yea I'm not remotely convinced it's the starter... when it's not blowing fuses the starter only ever cranks the engine once or twice before it fires, and if you power it directly from the battery it cranks very strong, never misses a beat, over and over again without fail... Surely if it was a carbon buildup I'd be able to replicate the fault whether I'm engaging the starter from signal wire or by direct connection to the battery? Also whacking the solenoid with a bit of wood should rectify that for at least a couple of tries, but again that has arbitrary results (sometimes it will blow the fuse straight after, others we get 20 plus starts 1 after the other with no issues) so I don't think it's actually making a difference...

Anyone know of a way I can diagnose faulty SAM without removing it or using OBD Scanner?
 
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DopeyDave92

DopeyDave92

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I went to take the starter off today and let's just say screw that for a joke! Unless theres only 1 bolt attaching it to the engine/transmission I can't even see or feel how to detatch the bloody thing without the vehicle being on ramps.

I did get a video of whats going on though, posted to youtube as it was too large for the forum, but if you watch I'm pretty sure you'll understand why I'm highly doubting a failing starter.

 
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DopeyDave92

DopeyDave92

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I know it's been a while but in case anyone was wondering we took it to an independant specialist who has STAR diagnostics and it turns out the Front SAM is faulty. Mercedes quoted around £570 just for the part, which is obviously a no go after the bodge job they did when they apparently fitted a reconditioned ECU to the car a couple of years ago. When I get a bit more time I'm going to try see if I can figure out what is wrong with it and if it's repairable. Going to be pretty fun I think.
 

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