'87 300e right tail light blows fuse every time
 

this just started about a week and a half ago. first the fuse that controls the left front and rear right windows blew (fuse #8 i think). with that fixed, later that day the right tail light and the backlight to the instrument cluster went out. i replaced the fuse (#3, the circuit also has the cruise control, tachometer, and probably some others i'm forgetting) and it blew again. i am without a multi-meter, though i suspect i'll be buying one very soon having spent six months with this car now without one. i understand the longer of the boxes mounted under the fusebox cover is the one that controls the lights; the questions i have are relating to getting at the thing and testing it, cleaning it and possibly repairing or replacing it. i am still quite a novice at this type of thing.. does the part pull right out or is it more delicate than that? is there a way to test the unit? what method should be used to open and clean the part? also, i have never soldered anything in my life, so i wonder what advice anyone might have as to whether i should even bother trying to resolder connections as i have often heard here is a good way to fix these old components as they break down. if so, any advice before taking the plunge and buying a soldering kit (and a multi-meter while i'm at it)..? any advice would be greatly appreciated.

perhaps if i reply to my own thread...
 

...someone might respond? please? i just know i would rather not replace this silly thing, and someone out there must have seen this before.

There aren't any silver bullets. You need, as a minimum, the owner's manual, a multimeter, and some test lead extensions are always handy.

Then you start looking for shorts. Go to http://mb.braingears.com, download the electrical troubleshooting manual and schematics, so you can print out the schematics you need for the suspect circuits.

Your car is 18 years old. Who knows what it's been through. I just spent several hours over the last few days trying to figure out why my backup lights didn't work (results in a nearby thread).

Electrical troubleshooting can be a massive amount of grunt work, and even if you are experienced, bizzare results can sometimes take you down blind alleys.

Duke

Same mystery problem
 

I have the exact same problem with fuse #3 that controls the tail lights, instrument lights and head light washer units. When I plug in a new fuse it instantly blows when I turn on the lights. I tried to find the short by unpluging both tail lights and the wiper motors but the fuse still blows. If you solve your problem please let me know. Ohh on my instrument cluster the replace lightbulb warning is also on when I turn on the headlamps - is this also in your case?

ha. now we have two threads
 

yes, when the headlights are on the bulb indicator goes on. i'm going to buy a multi-meter today and start following the trail of wires wherever it leads me.. hope it's not the control unit. faulty wires i can handle, fancy electronic boxes are as yet over my head.

water
 

water in the rear light may cause that..

i lied about buying a voltmeter today. instead i disconnected each of these items individuall, each of them running on circuit number 3: the right taillight assembly: fuse blew; the right front parking light: fuse blew; the lisence plate lights: fuse blew. i'm out of fuses. unless it's something obscure (or something in the wiring i missed) i believe the short exists somewhere behind the instrument cluster. i'm sure someone here has taken this apart before.... any ideas? am i on the right track?

fuses
 

before this had you attempted to modify the wiring at all? we had a car like this come into our shop with the similar issue, the customer had tried to install a trailer light for a tow trailer, he shorted something and damaged the bulb control relay, replacing the bulb control relay fixed the problem

WOW Thanks for the manual...I had bought this 2 cd set on ebay for $10...but its GREAT to have on my computer..

Hi,
I also have the exact same problem with the #3 fuse shorting out. I have disconnected the tail light, headlight wipers, front right bulb, license plate lights, inspected all visible wiring and still the fuse blows when I turn on the parking lights. I have just installed a new radio/cd player which works fine - do you think I may have disturbed something in the radio cavity area? Also can you please tell me where the bulb control relay is located.
Spending countless hours and going crazy in the heat,
Tino

You might check the cigarette lighter wiring at the lighter/ashtray. Mine would short out and take out a fuse and/or tail light bulb, or leave a bulb on or not working. The wiring insulation stretched loose (exposing a small section of wire) when I pulled the radio, and would short when the ash tray assembly was bolted back in place.

this cigarette lighter causing a short sounds intriguing, because i have disconnected the wires from the lighter to run aux wires from the stereo through the hole where the cigarette lighter once was, and i only capped them off with electrical tape. now, if this is causing a short back there, would this indeed fry the bulb control relay? where is this relay and how much does it cost to replace? how can i tell if this is my culprit or does one simply guess and check at these things? this is not the same as the light control unit located under the fusebox cover, which i pray to the mercedes deities is not the problem..

What does the light control unit look like? Is it in back of the fuse box with the other relays?

The lighter is not on the same #3 circuit so how can it cause this problem?

Isn't the ashtray light on the same circuit as the instrument cluster though? It's easy enough to look.

No cigar
 

Removed ashtray unit - all seem ok but just in case unpluged and inspected and retaped the light> Fuse #3 blew again.

Well, then...it's either in the instrument cluster, or somewhere in the wiring between the fuse and *everything* that draws power from it. My guess is it's in the wiring somewhere, and I don't have an easy way for you to find it. I'd pull the instrument cluster and disconnect the wiring to isolate whether or not it's the cause. Assuming that the problem shows as in the wiring, I'd then try to isolate sections of the harness. When I worked for Cadillac, their service manuals showed the location of every splice, connector, and ground. I've not yet found that information in my MB service information, but that does not mean it doesn't exist somewhere.

It maybe wasn't a lot of fun tearing the interior out of a car to fix a short circuit, but it was better than tearing the interior out and then finding the problem was somewhere else.

Can a relay cause a short?
 

Thought I should start pulling out relays in a hit and miss attemp - what do you think?

I think you'd be better-served by getting a wiring diagram and looking at the items fuse 3 deals with. Further up in the thread, somebody posted the url to a website where you may well find the needed diagrams.

Thank you for advise - I realy do appreciate - I have downloaded looked at the wiring diagrams however the relay layout shown is not the same as is in my 1986 300E. Anyways I removed the headlamp washer relay and the #3 fuse still blew so the short could not be in the headlight washer/wipers or the lines to them. Whats left are the right parking lights, plate lights and instrument and interior switch lights. All the wires I can see seem AOK. Perhaps I should lift the fuse box and try to isolate from there?

What if I just wait for the smoke and smell ?
 

Hi,
In desperation I am considering on bridging the #3 fuse and look and smell for something burning so as to locate the problem. Is this just too dumb - or is it worth a try? :confused:
I did momentarily hold a solid wire to the fuse posts and the lights did come on but then I chickened out ( the wire did get very hot )
*** When I bridge a light bulb to the fuse contacts the light works with no problems.

I would say DON'T DO THAT! :eek: While you might end up finding your short, you almost certainly will also cause damage to the fuse box and all the wiring between your fuse and the short circuit. I think the chance that you'll find something to fix without causing much more damage to areas that otherwise would not need fixing to be almost nil.

If you don't have the patience to start following the problem circuit from the fuse box to wherever it may lead you, it may well be time to take it to a pro. Perhaps somebody with a more-intimate knowledge of your model could guide you to locations on the car where you can separate parts of the wiring harness, and thus narrow the search down a bit.

It was a foolish thing I considered.
 

The problem is that the wiring is so well consealed - I have wiring diagrams but have trouble locating the wires. What I can see looks good. If a switch light in one of the doors came loose and is shorting out would it cause this problem ? Would like to isolate the wires from the headlight switch to the lighting for the instrument panel, doors and console switches but I can't seem to get at the harness behind--is there a special procedure?

That's why I said it may be time to take it to a pro. Every place the wires go hiding is a potential place to hide your problem. If you are not willing (or able) to start uncovering the wiring harness, it's time to go pay somebody to find it. <dirtyharry>A man's got to know his limitations</dirtyharry>
Anything that gets power from fuse 3 is a potential source for a problem. Disconnecting various lamps and such only eliminates those items from the problem. It does not eliminate the wiring harness right up to said item, though. I've never worked on a car like yours, so I can't tell you how to go about taking things apart. You've got a source for service information, and you have the car right in front of you. You can either begin to learn about your car, or you can find somebody who already knows.

Still at it.
 

Well I am pulling up carpets, checking the wires under the fuse box and it all looks like brand new. I even cut the grey/Violet wire from the fuse box to the dash lights and still the fuse blows ( I spliced the wire back again ).
The only thing I can think of is my radio installation. The existing radio had a seperate brown wire (ground ) for the constant 12V and another ground to the 12V when the ignition is on. My new radio just had one ground for both and I just joined the two grounds from the old radio and spliced to the single ground of the new radio. I don't see how that would short anything but when I get some free time I am removing and reinstalling my new radio.
I also called a Mercedes dealership and was told that my car is too old and none of their mechanics have experience with older cars and could not do much more then me and could turn out to be many hours and much $$$$ with no results. I will fix it sooner or later - even if I have to make new circuits.

;) Fuse #3 problem solved.(BUT brown/blue wire -was does it do?)
After countless hours of not finding the # 3 circuit short (but I at least I am now more familiar with my car) I decided to unwrap and check all my new radio connections - and there it was - I had mistakenly joined what I thought was a ground which looked like a brown wire but was in fact a brown striped wire. I cut this wire and NO MORE SHORT.
I now have a mystery wire which is BROWN with a BLUE stripe - this wire went to the centre 5-pin plug at the rear of the old Becker radio. Has it anything to d with the alarm system - what should I do with it???
__________________

i wonder
 

i'm going to check that immediately, because i grounded that wire on the advice of a previous post on the stereo forum on this site...

well, that didn't work either. next i'm going to have to start ripping apart the interior looking for shorts behind the control elements and instrument cluster. anyone know why that brown and blue striped wire could have caused tino's short? is it possible i fried a relay by wiring it that way for all of this time? the saga continues...

Only brown wires are ground.
 

Make certain that you carefuly check your work - some wires are brown with a stripe and at first glance they can be mistaken for ground.

problem solved
 

i finally found the short. after much trial and error i realized i hadn't checked the headlamp wiper unit. disconnected and all is well. i've changed all fuses and it was a good call, many were corroded at the connectors to the point where they came apart in my hands. persistence pays off again..

Wow, I'm have this exact same problem with curcuit #2 on my '92 (it has the headlamp wipers). I just bought a bucket of 8a fuses. I'll try those first. I've already started to look at he wires on the trunk lid but have not cut open the harnes and extracted them from the trunk to verify there is no short.

I thought I had a good test by checking resistance with an ohm meter from the body side of the fuse to ground, but the curicts the work normally, have the same nearly zero reistance... :p

So.. I'll try to locate the plugs for the wiper units to see if I can unplug them, pop in a new fuse and pray my lights come on. Just bought it, and neglected to verify the dashlights worked (dumb*** me - slapping forehead). She runs great though. My '90 with nearly twice the milage has never had these sort of issues. At least there does not seem to be any common circuits to the engine wiring harness which it sounds like the '91-'95 model years have issues with.

At least I can fully enjoy the car during the daytime still!!!! :cool: