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Fuse 29 is blowing at key on

Customer Question
Fuse 29 is blowing at key on
Submitted: 1 year ago.Category: Mercedes
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Answered in 5 minutes by:
7/17/2016
Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago
Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 5,331
Experience: 15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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Hello my name is ***** ***** i will do my best to help you.I am looking over your info now and will reply back soon with some info , thanks!

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

so this happened directly after replacing the starter/alternator?

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

also do you have the wiring diagram? how do you know what is in the circuit. ?

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

fuse 29 runs through relay K, so unplug that relay and see if the fuse doesnt blow

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

ok i have to leave for a bit, i see where fuse 29 connects to the starter relay. we will have to trace the wiring from the fuse box to the other fuse box, i would suspect a damaged or mouse chewed wire . i have to leave for now but i will check back later

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Customer reply replied 1 year ago
It ran great for several months after putting the starter and alternator on. The diagram I have in front of me from Alldata shows the the circuit after the fuse goes to pin 85, to the ECM, and a 3rd leg go towards 54.15-u-2102SB. Is that the other fuse box?
Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago
It branches to left and rear fuse boxes as well. We need to access the bottom of the fuse boxes and see what wire is shorted to ground. Then trace or overlay that wire. So with the starter relay removed does the fuse blow ?
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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago
Ok so looking at the diagram lets shy away from the starter relay. The fuel injectors and cylinder shut off valve all get power from this fuse as well.
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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago
Ok so I'm not sure how much of this you are doing today since you are not responding. I have a few things I need to get done so I won't be able to get back until later today or tomorrow am. But I can help you figure this out. If you cannot wait on me then feel free to opt me out no worries. And hopefully someone else will help you.
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Customer reply replied 1 year ago
Measured resisance to ground at pin 85 of the starter relay again, reads 1 million ohms. I can push the fuse block towards the engine and get less than 1 ohm. Lifted fuse block out again and install new fuse, does not blow with key on and relay out. Put relay in, will not blow now until attempting to crank engine.
Customer reply replied 1 year ago
The diagram I have in front of me seems incomplete. I have attached the portion of it that shows the fuse. The leg heading off to the left goes to pin 85 of the relay. My information is also telling me relay L, not K, is the starter relay. The leg of the fuse output that goes to the right to what I assume its saying is 54.15-u-2102SB is what I don't have a diagram of.
Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

ok your diagram is not correct. its close but not correct, i will send you over mine since you seem to know how to read and interpret diagrams, there are 2 different types of v12 engines and also a year break in the diagrams so the once you are looking at is close not complete. the relay you are talking about is the starter relay, the other relay i am talking about is a relay that activates to allow power to go to the fuel injectors off the same circuit, since you can manipulate the readings inside the fuse box then we will start there by checking the underside of the fuse box i will get you the wires to look for and the connections as well. so please refer to my diagrams so we can be on the same page.

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

ok here is the right front fuse box diagram and legend. i will get you the connectors so we can locate the wiring underneath from circuit 85 of the starter relay.

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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

ok here are the connectors, so it can be a little confusing but there are multiple connections that look the same, so connectors

  • I3 and C1 are the same as the top left icon
  • M2 and MR2 are the same as the top right icon
  • and I1 and MR1 are the same, i think the last MR2 is a typo.

so we need connector I3 pin 4 with the red wire green tracer and red wire blue tracer that are coming out of it, also connector M2 pin 3 with the green wire with red tracer, unplug those connections and the short should go away,, then we check for the short from those wires to ground and trace those wires. let me know if this helps, with this you should find the issue since you can flex the fuse box and the short comes and goes, you either have melted wires or stripped wires or chewed wires. let me know what you find and i am here if you need me.

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Customer reply replied 1 year ago
The pin with the 2 wires coming out of it and the other 2 pins in M2 are all open to ground. I was suspecting the problem was internal to the SAM/fuse block earlier today but this seems to confirm it.
Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago
Ok just make sure you still have continuity to ground then from terminal 85 of the relay to ground then with those connection removed. That would verify it for me.
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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago
It's rare to have that Happen. My vote is still for the wiring.
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Mercedes Mechanic: Lou P. master tech, Mercedes Technician replied 1 year ago

I have taken my time to answer your question the best i could and i will continue to help if you need follow up information , all i ask is that you PLEASE take a second to rate my service so i may receive my credit for assisting you. Thank you.

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