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-   -   '87 300e right tail light blows fuse every time (http://www.peachparts.com/shopforum/showthread.php?t=128616)

tino 07-19-2005 01:39 PM
No cigar
 
Removed ashtray unit - all seem ok but just in case unpluged and inspected and retaped the light> Fuse #3 blew again.

Moneypit SEL 07-19-2005 02:39 PM
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.

tino 07-19-2005 02:49 PM
Can a relay cause a short?
 
Thought I should start pulling out relays in a hit and miss attemp - what do you think?

Moneypit SEL 07-19-2005 03:16 PM
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.

tino 07-19-2005 03:37 PM
Quote:
Originally Posted by Moneypit SEL
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?

tino 07-19-2005 04:22 PM
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.

Moneypit SEL 07-19-2005 04:34 PM
Quote:
Originally Posted by tino
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?
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.

tino 07-19-2005 04:40 PM
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?

Moneypit SEL 07-19-2005 09:24 PM
Quote:
Originally Posted by tino
The problem is that the wiring is so well consealed - I have wiring diagrams but have trouble locating the wires.
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>
Quote:
Originally Posted by tino
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?
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.

tino 07-19-2005 09:56 PM
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.

tino 07-20-2005 03:46 PM
Quote:
Originally Posted by MikeNJ
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.
;) 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???
__________________

MikeNJ 07-20-2005 06:27 PM
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...

MikeNJ 07-20-2005 07:21 PM
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...

tino 07-20-2005 10:32 PM
Only brown wires are ground.
 
Quote:
Originally Posted by MikeNJ
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...
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.

MikeNJ 08-06-2005 12:56 AM
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..


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