Mercedes S550 Sedan no crank no start

Freddie Owner/Technician Houma, Louisiana Posted   Latest   Edited  
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Electrical
2009 Mercedes-Benz S550 5.5L (273.961) 7-spd (722.9)

Towed in, noticed fuse F9 by battery was blow. Replaced fuse and it immediately blows with key on. Looked at schematics on ALLDATA and they are very vague. Can anyone help me with some schematics on this car. Looks like fuse F9 feeds front SAM module for starter circuit but I need some better schematics. Thank you ahead of time

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Florin Technician
Niagara Falls, Ontario
Florin Default
 

As a simple and easy way to test this circuit and the integrity of the starter, I would disconnect the starter wire vio/wht at the starter if easy accessible and try again to see if the fuse is still blowing. I am assuming the fuse is blowing when attempt to crank. If not I would remove the relays and check for short before and after the relays. If needed I could help with a Mitchel diagram or…

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Taylor Technician
Los Angeles, California
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F9 feeds relay in front sam remove relay and proceed basic relay fault diagnosis, more than likely however the starter is culprit but testing amp draw across relay should show definitive answer hth

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Freddie Owner/Technician
Houma, Louisiana
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looks like power is coming into the starter relay directly into pin 30 Relay and power is going to 87a and shorting to ground causing fuse F9 20 amp fuse to blow. Fuse 9 is good until you turn on the ignition switch to the On position. According to the diagram where does 87a go?

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Randy Diagnostician
Glendora, California
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Pull starter circuit 15 relay and see what happens

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Aaron Owner/Technician
Mount Cook, New Zealand
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Check to see if the start relay had a diode in it. May be shorted out from jump started or other testing.

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Freddie Owner/Technician
Houma, Louisiana
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Here’s what I have so far drawn on schematics when starter relay gets plugged in with ignition switch in on position fuse F9 immediately blows. There’s no chance to try and start because it blows .

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Randy Diagnostician
Glendora, California
Randy Default
 

Disconnect starter 50 relay

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Freddie Owner/Technician
Houma, Louisiana
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I did pull starter relay and that middle terminal 87a is shorted to ground, the schematics don’t show where it goes. I am thinking the SAM module is shorted out to ground..

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Randy Diagnostician
Glendora, California
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Are you sure it is right relay .when you pull relay does it set a code? use a jumper relay and see what happens. 87a might be logic circuirt to see if relay turned on

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Aaron Owner/Technician
Mount Cook, New Zealand
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Thought, is the start relay correct. Has not been swapped out for a change over relay.

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Freddie Owner/Technician
Houma, Louisiana
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It’s the only five prong relay in the box and terminal 87a shorts to ground.

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Randy Diagnostician
Glendora, California
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Ok . try using jumper relay to see if you can get it to crank

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Freddie Owner/Technician
Houma, Louisiana
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Update the starter had to be replaced to go any further because I could not get it to engage by injecting power to terminal 87 on the starter relay. After changing the starter I am able to start the car by turning the ignition switch to on position and injecting 12v on terminal 87. I replaced the 20 amp fuse on F9 and with the starter relay back in as soon as ignition is turned on it pops fuse…

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