Im having an issue with my #1 fuse blowing every time i turn my lights on.

What ive done so far to diagnose:
1. replaced all tail light bulbs/ inspected turn signal bulb (also recently replaced)
2. disconnected harness from turn signal and tail lights then tried turning lights on...still blew
3. removed radio face to hopefully not allow it to power on...still blew
4. inspected wires for shorts turn signal harness to ensure no shorts.

I added HID headlights a week ago but it just started this issue yesterday. Everything is properly grounded and wired correctly. I even disconnected the HID lights just to make sure...still blew.

Im stumped on what to do next. ive done all the research and still came up short (no pun intended)

HELP!

First we need to know what car you have. Fuse 1 to me says 123 but let's confirm before we go further.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Since you've added non standard(not to mention illegal) headlights, your first step is to check your wiring.

How about photos of your HID wiring job. Could be competent, but I have seen twisted wires "insulated" with scotch tape.

I noticed on your profile there is a picture of a W126, so I think that's the car you're talking about.

Since it happens only when you turn the headlights on, I'd say you've narrowed it down pretty well. Start at the headlights and go from there back, to the switch and so on...but I suspect, it's wiring as well.

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1983 W123 300TD US spec Turbo engine, with Euro bumpers and manual climate control, and manual transmission.

I'd like a little more info. Typically the headlights are fused separately on these cars. So does #1 not blow UNTIL the headlights are switched on? Or does it blow as soon as the parking lights are switched on?

Happy Motoring, Mark

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DrDKW

You should have a cheap digital meter. It helps run things like this down quicker usually. You have a constant short. Or severe overload present for the fuse size.

Correct, W126...sorry about that. Im going to go back over the wiring today. Its pretty straight forward (at least in my head). a connection to high beam, low beam, grounds, and battery.

Yes, didnt clarify that well. it blows as soon as its switched to the parking lights. like you said, the headlights are fused separately which leads me to think its something else.

luckily i have one. would i simply place the leads across the fuse prongs to get a amperage reading?

Better yet, pop the fuse out and check the non-powered clip to ground, in ohms, with the light switch in each of the standing light settings and park lights. This will tell you if the problem is on one side or the other or if it may be something common to both. This should help narrow it down some.
Also, disconnecting the light switch connector and checking the pins to ground, may at least help to point towards the problem area.
Does the light switch itself seem normal without any looseness?

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Sam

84 300SD 350K+ miles ( Blue Belle )

My DMM doesnt have an amp setting :|...the switch seems fine to me.

this weekend i double/triple checked all wiring with my HID lights and everything looks right to me. All connections are soldered and heat wrapped btw. So im ruling this out since it still functions when the fuse blows.

A wiring diagram of fuse #1 would be a huge help!

Here is the Factory Service Manual which includes the electrical schematics:
Mercedes-Benz 126 Service Manual Library

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Sam

84 300SD 350K+ miles ( Blue Belle )

THANK YOU. ive been needing this resource for awhile now.

I didnt realize fuse #1 had any association with the anti theft system. I recently dug it up from the passenger foot well only to find a TON of corrosion. I removed the black box to deal with another day but i guess that day may be sooner than expected. Im hoping this will lead me to a solution to my electrical gremlins!

thanks again SD Blue

So my issues continue...

I cracked open the so called "security system" to find this...



I simply deleted the plug and the other components in hopes to solve my fuse issues...and potentially future electrical issues.

The fuse lasted a quick trip to the store and on my way to work...it blew.

Im completely stumped right now and dont know what else to check!