Repeatedly blowing starter fuse
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2008 Mercedes C300
Repeatedly blowing starter fuse
2009 C300 has been sitting for months as I have intermittently been trying to diagnose the problem as to why the car won't crank / start. Diagnostic scanner started off being able to only read error codes from the ESP and SRS modules but no longer communicates with any of the modules. The front Sam has been removed and checked for corrosion as well as all the plugs as they were reconnected back into the Sam every fuse was checked before it was put back in as well as every relay. The starting motor was removed from the car and bench tested. I get the five volts total for the CAN H and CAN L at the Chassis distributor block. Pins have been checked at the ECU connector battery is less than 6 months old voltage reading of 12.8. After doing some of the testing and have repairs on some of the ground wires I tried to start the car and it is still doing nothing. Dash lights come on and steering wheel unlocks. However, 3 different attempts at starting car have caused starter fuse to blow every time. Any guidance?
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From: Northern Indiana
2014 c300 w204.080 3.5 v6
You should check the starter relay circuit for evidence of damage or water. Also you can switch the starter relay with one of the other relays to rule that out.
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W212 MY'14 M276-3.5NA @75kMi
CONFIRMED LIVE ISSUE
This is a real short circuit.
The starter is a simple circuit with very few items to disconnect for testing.
The starter is a simple circuit with very few items to disconnect for testing.
Last edited by CaliBenzDriver; Feb 25, 2026 at 04:26 PM.
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2008 Mercedes C300
Well I was told to disconnect the starter signal wire and turn key to position 2 and see if it still blows which it did. Next was diconnecting all the plugs from the SAM and seeing if it blows. It did not which was supposedly to suggest that the SAM was bad. The SAM was replaced but yet the fuse blew when key was turned. Next step was to unlug the 2 connectors to the ECU and see if the fuse blows which it did not which tells me there is a short in the wiring harness. I am using WIS to try and locate the wiring diagram for the ECU and fuse 7 branch so that I can measure resistanc from each wire to ground. I havent exactly gotten the hang of using WIS and thought I would check on here to see if anybody could guide me to where I need to look.......please
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From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
SHORTED STARTER SIGNAL...
Well I was told to disconnect the starter signal wire and turn key to position 2 and see if it still blows which it did.
Next was diconnecting all the plugs from the SAM and seeing if it blows. It did not which was supposedly to suggest that the SAM was bad. The SAM was replaced but yet the fuse blew when key was turned. Next step was to unlug the 2 connectors to the ECU and see if the fuse blows which it did not which tells me there is a short in the wiring harness. I am using WIS to try and locate the wiring diagram for the ECU and fuse 7 branch so that I can measure resistanc from each wire to ground. I havent exactly gotten the hang of using WIS and thought I would check on here to see if anybody could guide me to where I need to look.......please
Next was diconnecting all the plugs from the SAM and seeing if it blows. It did not which was supposedly to suggest that the SAM was bad. The SAM was replaced but yet the fuse blew when key was turned. Next step was to unlug the 2 connectors to the ECU and see if the fuse blows which it did not which tells me there is a short in the wiring harness. I am using WIS to try and locate the wiring diagram for the ECU and fuse 7 branch so that I can measure resistanc from each wire to ground. I havent exactly gotten the hang of using WIS and thought I would check on here to see if anybody could guide me to where I need to look.......please
In WIS, locate the starter diagram that's actually near the main battery + Alternator.
The circuit you're dealing with "starter signal" has very few connections.
It will soon be working - No more guessing with a diagram.
Done under 30mn, likely free fix, no parts .
1- Post the diagram here...
2- Disconnect both end of "starter signal" circuit
3- test if it is shorted to GND ...( likely yes) ?
use a test-light or a DVM.
(Do not run down your battery low)
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2008 Mercedes C300
Hopefully this is the diagram you are referring to. I know that M1 is the starter, G1 - battery, and G2 is the alternator. I am surprised to see that the red power wire goes from the starter to the alternator. Anyhow you are saying that I need to test the starter signal wire at both ends, correct? I know one thing that has been tested as good already was continuity from the end of the signal wire to the pin in the plug that connects to the front SAM box.
I have been keeping a battery maintainer on so that battery doesn't go low.
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From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
short... almost there
is this your shorted starter command line ?
if yes disconnect from X26 junction and from starter then test for short circuit to GND.
This is a "Violet-White stripes" single wire from F-SAM direct to stater relay, not shared.
direct SAM output, not shared.
I think we can ignore the power side here... correct?
Starter battery power cable comes straight from the battery. the starter post also has a cable that goes to Alternator to collect battery charging power during run time.
What Starter fuse is blowing... inside the SAM ??
SAM has a small built-in relay that controls the line we are looking at...
Physically check that harness is not kissing the exhaust line on the way to starter.
Last edited by CaliBenzDriver; Mar 17, 2026 at 01:25 AM.
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Thread Starter
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2008 Mercedes C300
is this your shorted starter command line ?
if yes disconnect from X26 junction and from starter then test for short circuit to GND.
This is a "Violet-White stripes" single wire from F-SAM direct to stater relay, not shared.
direct SAM output, not shared.
I think we can ignore the power side here... correct?
Starter battery power cable comes straight from the battery. the starter post also has a cable that goes to Alternator to collect battery charging power during run time.
What Starter fuse is blowing... inside the SAM ??
SAM has a small built-in relay that controls the line we are looking at...
Physically check that harness is not kissing the exhaust line on the way to starter.
Then went ahead and tested end of starter wire to the pin in the connector plug those goes into the SAM, result was the same.
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From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
HUNTING THE SHORT....
based on your DVM test: where is the Fuse7 downstream short ??
- Outside/After the SAM/X26 ?
- Inside/Before X26 ?
Last edited by CaliBenzDriver; Mar 18, 2026 at 11:48 PM.
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2008 Mercedes C300
Bigger issue
I am not understanding exactly how what I need to check or look for in order to answer your question about it being inside or outside.
However, I just realized that I am not getting the constant 12V at the pin 30 under starter relay in the SAM. I checked that 12V is showing on all the studs of the Prefuse box. 3 in total. Should I only have 3 of those to check is my first question?
However, I just realized that I am not getting the constant 12V at the pin 30 under starter relay in the SAM. I checked that 12V is showing on all the studs of the Prefuse box. 3 in total. Should I only have 3 of those to check is my first question?
Out Of Control!!
Joined: Apr 2019
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From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
searching for short
I am not understanding exactly how what I need to check or look for in order to answer your question about it being inside or outside.
However, I just realized that I am not getting the constant 12V at the pin 30 under starter relay in the SAM. I checked that 12V is showing on all the studs of the Prefuse box. 3 in total. Should I only have 3 of those to check is my first question?
However, I just realized that I am not getting the constant 12V at the pin 30 under starter relay in the SAM. I checked that 12V is showing on all the studs of the Prefuse box. 3 in total. Should I only have 3 of those to check is my first question?
Disconnect X26 to find out if your short is before or after X26 junction.
Then go from there.
