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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3720
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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Fuse 29 is blowing at key on

Customer Question

Fuse 29 is blowing at key on
Submitted: 4 months ago.
Category: Mercedes
Expert:  Lou P. master tech replied 4 months ago.

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!

Expert:  Lou P. master tech replied 4 months ago.

so this happened directly after replacing the starter/alternator?

Expert:  Lou P. master tech replied 4 months ago.

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

Expert:  Lou P. master tech replied 4 months ago.

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

Expert:  Lou P. master tech replied 4 months 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

Customer: replied 4 months 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?
Expert:  Lou P. master tech replied 4 months 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 ?
Expert:  Lou P. master tech replied 4 months 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.
Expert:  Lou P. master tech replied 4 months 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.
Customer: replied 4 months 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: replied 4 months 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.
Expert:  Lou P. master tech replied 4 months 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.

Expert:  Lou P. master tech replied 4 months 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.

Expert:  Lou P. master tech replied 4 months ago.

ok so if you follow 85 you sill see there are 2 internal splices and 3 wires that come out of 2 connections underneath the fuse box, i have screen shot the areas so follow the diagram i sent you to locate what i found , i will get you the connections as well .

Expert:  Lou P. master tech replied 4 months 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.

Customer: replied 4 months 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.
Expert:  Lou P. master tech replied 4 months 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.
Expert:  Lou P. master tech replied 4 months ago.
It's rare to have that Happen. My vote is still for the wiring.
Expert:  Lou P. master tech replied 4 months 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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