Trouble shooting cause of blown fuse - PeachParts Mercedes-Benz Forum

95E320cab · #1 12-23-2003, 11:42 PM

It's been a while. Glad you're all still here.

I put the top down here in Dallas today. It was a little chilly but with the heater and seat heater it was real nice. Around 5:00 I turned on the headlights and the blown exterior light indicator came on. I stopped at the next red light and could see that the passenger (right) side parking light was out. Thought it was a bulb. As it got a little darker, noticed the instrument panel was not lighted either. Got home and checked the fuse box. An 8 amp fuse was blown in the #3 position. Replaced it and it blew again. And again. No more wasting fuses.

The fuse box cover says the following for fuse #3:

"parking/tail light right, headlamp cleaning unit, illumination: license plate, instrument, control elements"

All that stuff is out. (Notice no mention of the convertible top controls at this fuse.) How do I track this little bugger down? I hope there is some common failure that will be a first place to look.

Thanks a bunch and Merry Christmas / Happy Chanukah!

haasman · #2 12-24-2003, 02:30 AM

95E320cab

if it were my car, I'd start with the easiest to check: Go to the trunk and twist the holders on the right tail light assembly and check each of the bulbs. Twist them out; look for corrosion and blown bulbs.

Next I would check the right front turn signal marker assembly. Do the same process above.

Then I would check the license plate illumination bulbs.

If these do not yield anything then I would recall whether I used the headlamp cleaning unit at all during this time .... if not, I would by-pass it for now.

The remaining items can start to get more complicated ....

If you need to replace bulb(s), only use quality bulbs. Auto parts stores often have them on racks under the heading of OEM replacement lamps and bulbs. They are almost always made of silver looking metal. Do not waste your time or money buying cheap bulbs.

Try the above and let us know.

Haasman

95E320cab · #3 12-25-2003, 11:02 AM

Thanks Haas. I did just as you suggested. Rear lights no problem. Seem okay. The front side markers are a bit of a bear to get at, so I just unplugged and tested. Fuse blew. On to the license plate lights. Unscrewed the assembly and one of the connectors looked slightly burned, so I unplugged it and tested. Did not blow the fuse! I gave myself a high-five and pulled a blue connector out of the tool box and connected and tried. Blew the fuse. Called the dealer and ordered the assembly for $30. Unplugged the connector and taped it with electrical tape so it wouldn't ground out and replaced the assembly. Alas, last night before church when I turned on the lights, POP. So I'm not done yet. Any tips? Have I found the problem but just not correctly handled it? Thanks again.

95E320cab · #4 12-27-2003, 12:47 AM

This is not my first electrical quirk. Sometimes my combination switch malfunctions and the wipers run without being turned on. I guess any car can have its gremlins, but I am suspicious of the wiring on this car because of the wiring harness issues. Isn't the same bio-degradable wire used throughout the car? Do I have a subterranean short, buried a thousand dollars deep in the anus of my Benz?

Pulled the trunk liner and can't see see an obvious short. How does one go about testing the continuity of the license plate light wiring? Is that the route to take?

Thanks again.

haasman · #5 12-27-2003, 02:40 AM

95E320cab

Sounds like you are making progress in the process of elimination. One thing you might want to try is the "Old rotating of the fuses in their holders" routine.

Often the fuses get corrosion at the ends where they make contact. The fuses with the aluminum links are very prone to this.

Ideally replace all the aluminum ones with copper ones. In the short-term check each fuse. At a minimum rotate each one.

This is another thing I would do to eliminate any fuse related items.

Some people spray a bit of WD40 on them or use it on a rag to clean the contacts and the fuse ends. WD40 does allow electrical conduction.

I do not believe the wiring harness issue was centered around the overall system, but only related to the engine harness.

Haasman

KenP · #6 12-27-2003, 07:01 AM

An old mechanics trick that may be helpful here is to rig a 12v light bulb with some short leads and use it to bypass the fuse.
If the bulb lights brightly you have a short... then start disconnecting things on the circuit... when the bulb goes out or dims significantly, you've found your short.
Hope this helps.
KenP

95E320cab · #7 12-27-2003, 01:34 PM

Thanks for your help guys. The latest: It appears there is power at the license plate lights, but the bulbs won't light and are not blown. (I tried replacing with new bulbs.) The test light lights-up if I attach the clip to the inside of the trunk frame and probe the connection where the bulb attaches. Does this mean that I have a bad ground? Can I just rig a ground wire and make my own rather than trace the bad one? Is there a place to look where that bad ground might be?

95E320cab · #8 12-27-2003, 04:51 PM

Jeez. Lots of machination to find the root of the problem. Finally pulled out the lower half of the trunk liner and pulled the wires out of the hinge. Stripped down far enough to see that one of the license plate light wires and one ground wire had totally separated from the years of bending in the same spot. (I use my trunk a lot since I have a 2-door.) So I spliced and fashioned a plastic stint to keep the wire from bending at the splice, wraped her back up nice and pretty with electrical tape and she works good. The only trick I can pass on was that where I spliced and stinted there was a big bulge in the bundle of wires and it had to have a piece of rubber conduit pass over it. So I wiped a little silicone on the bulge and the conduit was able to stretch over.

I spent about $10 bucks on fuses and bulbs. Probably took me five or six hours, stopping and starting from the first day of misdiagnosis through today. I hate to think what it would have cost me if I had to take to the shop and have them track the short. Thanks for all your help Haasman and KenP

KenP · #9 12-28-2003, 07:17 AM

Outstanding! Congrats on finding your short... a good feeling when you find an electrical problem and know its fixed...
As for the shop time, many shops won't touch electrical problems on an older car... they can't be sure of finding it at a reasonable cost (shop rate x hours) and even if they do, most folks (not understanding the expense of running a business) would be outraged to pay 500-600 bucks to connect two little wires back together...
Its even worse if its intermittent... and you give it back to the customer thinking its fixed and its not... then, when it comes back, you get to spend lots more time (for free this time) chasing one of these gremlins...
KenP

haasman · #10 12-29-2003, 01:28 AM

Good job!

Nice to not only diagnosis the problem but to also repair it.

I think most will agree that electrical problems can often times be very hard to figure out. By you following the obvious and employing a step-by-step process of elimination you saved your selftime and money, not to mention the bonus of the satisfaction of doing it yourself.

Thanks for keeping us updated.

Haasman

Southern · #11 12-29-2003, 12:07 PM

Congrats on fixing your problem.

My approach to finding electrical shorts is the same approach you took - through process of elimination.

Your recent experience reminds me of an electrical short I had in my 79 Toronado many years ago. The cause of the short was a penny that my young son had dropped into the cigarette lighter. Back then it was common for a car to have a cigarette lighter in each passengers armrest.