I keep blowing fuse #5!!!?? - PeachParts Mercedes-Benz Forum

abracadabra · #1 08-29-2009, 04:32 PM

Hey folks-

My trusty '76 240D has developed quite a problem with blowing fuse #5, the 8A fuse for the instrument cluster, brake lights etc.

It first happened a few weeks back, noticed the temp gauge wasn't working and also the rear brake lights, checked the fuse, it was shot. Put a new fuse in and no problems for a few weeks.

Then a day or so ago it blew again.
This time the new fuse only lasted a few hours, and now, I can put a new fuse in and it will blow almost immediately!

I'm a real putz with electrical stuff, I don't even know where to start!

I do know that I also have "fast blinker" syndrome, but I checked all my bulbs for the signals and they're all good......

Any ideas? Where to start? I hate driving without the brake lights!!!!

mark in Portland

ashedd · #2 08-29-2009, 06:35 PM

I have never messed with a fuse box in a w123 so I dont know how easy this will be.

You need to gain access to the rear of the box. On the 126/124's there are screws that can be removed to allow you to pull the actual fuse panel out. On the back of this panel are all the wires coming in to the fuse block. Find the group of wires that go to fuse 5. Remove the phillips screw holding them in and remember where that screw came from.. meaning what side of fuse 5.

Each one of those wires represents a part of the wiring harness that is getting it's juice from fuse 5. One, or more, of those wires has a short. At this point you would get your multimeter out and figure out which one. But if your a little lacking in the electrical department you can do this:

Put a new fuse 5 in the box and attach each wire one at a time. The bad one will blow the fuse. Leave that wire off and attach the others, replace the fuse. Go have a look and figure out what was on that wire, what doesn't work? You will have to decide if it's worth fixing or to just leave it alone. Tape the end of the wire if you leave it that way.

If it's brake lights or something important then you will need to figure out where it's shorting out. A whole different can of worms.

Enjoy

tankowner · #3 08-30-2009, 12:06 AM

I gotta tell ya, I like working on my cars . . . . except for electrical things. When somethings like this pops up you can spend days tracking it down. However, you will be doing yourself a big favor if you trackdown a wiring diagram and use a multimeter to test things our. Just be glad you have this site and its knowledgable people to answer questions along the way.

Sounds like you're shorting out somewhere. I had a very similar problem in a car I used to own (not MB). Every time I touched the brakes the fuse blew. I spent forever chasing that one. Check all the brake lights/sockets/wire, etc. Finally tracked it down to something buried in the dash between the brake pedal and the fuse box. Finding the exact point would have involved tearing apart the dash and cutting into bundles of wires. I opted to cut the wires and run a new one.

Yak · #4 08-30-2009, 09:45 AM

Check the bulbs and the bulb sockets. The wrong bulb (or a bad socket) can cause fast blinker syndrome. Long term effects of the wrong bulb may mean a melted socket.

I'd start looking on the side with the fast blinker. Make sure you don't have a two-filament heavy duty brake light in there - same size/same shape as a standard bulb, but too much heat.

LNGfish · #5 08-30-2009, 11:45 AM

Yak is right on!

Make sure all the bulbs are correct. Fast blinker sounds like a short in this circuit and the heat from the near-short is causing the blinker to act faster. Most likely nothing major it sounds. Take your time.

engatwork · #6 08-30-2009, 07:53 PM

I had an electrical issue recently on a different brand that resulted in not having tailights. Got pulled over at 5:30 am one morning on the way to work. Found a wire behind the radio that was on the circuit for "low level" lights providing 12 volts to ground causing the problem. Wrapped it up and no problems since.