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2008 C200K (W204) & 1998 SL500 (R129) Imperial Red
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3,373 Posts
Discussion Starter · #1 ·
Hey comrades..

I know that the very fist batch of W140 models that were built from 1991 - 1994 had a manufacturing flaw of the infamous "Biodegradable engine wire harness"

Due to high heat in the engine bay and age, the insulation on the wires dries up and crumbles off, leaving there wires bare to short-circuit with the other wires running parallel.

Most common is that the main, upper engine wire harness is the one which inevitably fails. It causes all kinds of major problems when failed.

MY QUESTION IS:

Are all the wires in car insulated with the same form of biodegradable insulation?

Are there any other wire harnesses that also fail due to this manufacturing flaw? Eg: Lower engine harness, electric mirror wires, interior electrics, etc?
 

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1995 S600 (sold)
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1,127 Posts
Yes, there have been many failures of other harnesses in the car. But as you point out, the ones closest to extreme heat are the ones that tend to go.
 

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CLS350 C219 2006
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582 Posts
Hey comrades..

I know that the very fist batch of W140 models that were built from 1991 - 1994 had a manufacturing flaw of the infamous "Biodegradable engine wire harness"

Due to high heat in the engine bay and age, the insulation on the wires dries up and crumbles off, leaving there wires bare to short-circuit with the other wires running parallel.

Most common is that the main, upper engine wire harness is the one which inevitably fails. It causes all kinds of major problems when failed.

MY QUESTION IS:

Are all the wires in car insulated with the same form of biodegradable insulation?

Are there any other wire harnesses that also fail due to this manufacturing flaw? Eg: Lower engine harness, electric mirror wires, interior electrics, etc?
Also not to forget:
- ETA (Electronic Throttle Actuator
- Side mirrors (wires move a lot, break, cause shorts and smoke)

Cheers
 

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1985 300CD 2005 E320-CDI 2004 Dodge 3500 Diesel Dually 1993 300SD-Sold
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15 Posts
93 300SD Already replaced main engine wiring harness. Glow plug harness on order. Now the aux fans don't shut off after the key is turned off and will kill the battery. Seems like the electrical gremlins might get the best of me!!!!!
 

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1995 Mercedes-Benz S420 (W140), 1997 BMW 740iL (E38), 2011 C300 Luxury (W204)
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1,314 Posts
As others have said, its mainly the heat that kills the insulators. I have found that the wiring throughout the car has been great, but my fuel injection one I can see the crumbling off at the plugs. I already ordered a new harness which is coming in today (although I have no issues with the engine performance, preventative maintenance). I don't think the wiring from the mirrors die due to constant moving in and out. If that is the case, your whole door harness will suffer the same issues because that is constantly being opened and closed.
 

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Premium Member
1999 S600 Sedan, 1999 S600 w/ 4-pl seating, 1995 S600 Coupe, 2-1992 600SEL's, 2002 ML55, 1998 SL600
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2,442 Posts
For the W140, 1996 was the model year the switch from soybean insulation was made. 1995's and earlier had it, 1996 and later have the conventional wiring insulation.

Since MB spec'd out the production of the engine harness (upper as some some call it) and the body harness (lower), those were the ones with the soybean coating. The harnesses plug into components made by others (Bosch, etc) and those components did NOT have the soybean wires, so they are fine ONCE YOU REPLACE THE HARNESSES. Recent discussion on later models having wiring issues with the side mirrors I can't confirm as I have not seen evidence of that, but that may be an issue with some cars.

The electrical gremlins are a direct result of either NOT replacing the harnesses that are failing at this point, or worse: A PO tried 'fixing' the failing wiring harness by extensive use of electrical tape or some other 'home-made' concoction that got the car to the point he sold it to YOU.

At this point, 16-years into production for the latest models, it is safe to say that ALL soybean-based wiring harnesses either have been replaced or are in dire need to be replaced.
 

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2008 C200K (W204) & 1998 SL500 (R129) Imperial Red
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3,373 Posts
Discussion Starter · #8 ·
thanks for all the feedback guys :)

Yes I also saw the side mirror insulation problem on my S500 while I was attempting to repair the mirrors. (Still waiting for parts from W210 mirrors.) There was even slight evidence or burning caused by the wires. but I insulated them properly. Will consider replacing them at a later stage.

I am also going to be looking at some engine wire harnesses for my S320, because the one that is fitted on right now crumbles even from just looking at it lol.. If I find a good condition one I will buy it on the spot :)

What I found interesting is that the wire harness on my 1995 S500 seems like it is made with proper wire insulation, not the soybean one. Maybe it was replaced if the change only happened in 1996?

Or did the change happen a little bit earlier? When is the exact date that Mercedes decided to use proper wire insulation on their engine wire harnesses?
 

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1994 S420
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310 Posts
Wiring Harness from Junkyard

thanks for all the feedback guys :)

I am also going to be looking at some engine wire harnesses for my S320, because the one that is fitted on right now crumbles even from just looking at it lol.. If I find a good condition one I will buy it on the spot :)
If I had known last week, I could have pulled one from an S320 in Nashville, salvage yard that was in excellent condition. I go back in about six weeks if you can wait and the car is still there. Yard Cost is $15. I will sell to you for $40 plus shipping if you are interested...total should be under $75 US

Would it make sense for us to set up a parts swap want/need thread ?
 

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Premium Member
1999 S600 Sedan, 1999 S600 w/ 4-pl seating, 1995 S600 Coupe, 2-1992 600SEL's, 2002 ML55, 1998 SL600
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2,442 Posts
The wiring harnesses started to go bad within 3 or 4 years after production, so it's possible MafiaDon bought his car and it already had the switch. Replacement harnesses, like replacement evaporators, are much more long-lived than the originals because they were re-engineered to last, basically the life of the car.
 

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Banned
2008 C200K (W204) & 1998 SL500 (R129) Imperial Red
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3,373 Posts
Discussion Starter · #11 ·
If I had known last week, I could have pulled one from an S320 in Nashville, salvage yard that was in excellent condition. I go back in about six weeks if you can wait and the car is still there. Yard Cost is $15. I will sell to you for $40 plus shipping if you are interested...total should be under $75 US

Would it make sense for us to set up a parts swap want/need thread ?
For real?

Yes I definately would be interested.. It doesn't matter if the car is RHD or LHD when it comes to the engine wire harness. $75 in total sounds very fair to me.

Let me know if you get your hands on that one. I need one from 1996 - 97.. (Has the long black main plug) I think in 1998 they changed the plug? When was the main plug format changed?
 

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2003 Mercedes E500
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1,194 Posts
Ok so the harness wires to the engine are the ones that tend to go bad? I see all this talk about early models and i own a 93' that im planning on keeping. So i should replace the engine wire harness or is the the WHOOLLLEEEEE harnest that goes thru the whole car?
 

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Premium Member
1999 S600 Sedan, 1999 S600 w/ 4-pl seating, 1995 S600 Coupe, 2-1992 600SEL's, 2002 ML55, 1998 SL600
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2,442 Posts
Hellah Fresh:

Since your 1993 probably HAD the harnesses replaced (assuming you're not having any troubles NOW), probably somewhere around 1998, you're good!

Remember, the fix for these harnesses is replace the soybean with conventional wires and you're done. Probably for the life of the car, barring an accident or something like that.

Cars that have been sitting for long periods of time typically haven't been run regularly, so the soybean-coated wires not only crack/come apart, they are also rodent food, so you usually have to clean out mouse nests under the hood. Mouse nests = biodegradeable wiring harnesses present. Most times...
 

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2003 Mercedes E500
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1,194 Posts
^^^ Whewww that makes me feel a little bit better now. I probably had this car for about 3-4 years now and just the regular minor stuff here and there that falls apart.
 

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1993 S600 coupe (RHD)
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199 Posts
Stryker I reckon you are spot on about rodents. i got my vehicle last year and it had been sitting for a while (couple of years) the top harness was degrading and there was actually a mouse nest in the vehicle engine bay area. The beasties had eaten into the bonnet heat pad lining and into the rubber of two of the HT leads. Found the nest tucked away just on front of the bulkhead - no occupants though!
nest gone & i rebuilt the harness and its all good now
 

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1994 S420
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310 Posts
Salvage Yard Wiring Harness

For real?

Yes I definately would be interested.. It doesn't matter if the car is RHD or LHD when it comes to the engine wire harness. $75 in total sounds very fair to me.

Let me know if you get your hands on that one. I need one from 1996 - 97.. (Has the long black main plug) I think in 1998 they changed the plug? When was the main plug format changed?
Don't know when the plug changed. According to my parts cross reference from another provider, the harness for the W140 8cyl was good for S420/S500 from 94-99 and 94-97 on the S500coupe

Bottom line, send me your part # and I will verify when I see at the yard.

I'll send you a message a couple of weeks before I go...I can also see the salvage yard inventory online and let you know what the odds are. Sadly, I've been to a couple yards and people have sliced the harness while pulling out other items.
 

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1995 Mercedes-Benz S420 (W140), 1997 BMW 740iL (E38), 2011 C300 Luxury (W204)
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1,314 Posts
^^^ Whewww that makes me feel a little bit better now. I probably had this car for about 3-4 years now and just the regular minor stuff here and there that falls apart.
Why don't you just go under the hood and check? I noticed on mine even though the car ran fine, the fuel injector plugs exposed some insulation that was crumbs...all cracked and exposing the wires.
 

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2008 C200K (W204) & 1998 SL500 (R129) Imperial Red
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3,373 Posts
Discussion Starter · #18 ·
Don't know when the plug changed. According to my parts cross reference from another provider, the harness for the W140 8cyl was good for S420/S500 from 94-99 and 94-97 on the S500coupe

Bottom line, send me your part # and I will verify when I see at the yard.

I'll send you a message a couple of weeks before I go...I can also see the salvage yard inventory online and let you know what the odds are. Sadly, I've been to a couple yards and people have sliced the harness while pulling out other items.
I will pm you the part number and we can take it from there.. It is the biggest part on my shopping list
 

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1995 Mercedes-Benz S420 (W140), 1997 BMW 740iL (E38), 2011 C300 Luxury (W204)
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1,314 Posts
Replaced mine for the V8 today. Wow, sounds and looks like a simple task. Boy was I wrong. The hardest part in this task is the clips for the fuel injectors... particularly for cylinder 1. That one took me forever to remove and also replace. The thing is you have to squeeze the clamps really hard and yank it out. To plug it in, squeeze really hard and plug it in. The hard part is doing in the very tight spaces. I imagine it can be easier if you remove a lot of hoses/tubes/fuel rails on top (a lot of work in itself). Took me about 2 and half hours! My old part was manufactured in July '93. I got a used part from eBay excellent condition, manufactured in '01. Did I notice a difference after the replacement? Sort of, seems to be smoother.

To find out when your harness is manufactured, there should be a tag on the wire that goes up along the firewall (under the plastic cover). It should state the production date.
 

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88 Mercedes 300SEL/15 Hyundai Sonata/91 Cadillac DeVille/72 Caprice Estate/73 Pontiac Grand Ville
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540 Posts
"Fresh" asked a question, but not sure if it was directly answered about the entire wiring harness of a 92-95 S-Class having the bio wires.

I am considering a '93 400SEL and the engine wiring harness was replaced in 2001, but someone on another Mercedes site told me that the wiring harness throughout the entire car had bio-degradable wires. I had been under the impression that it was only the engine wiring that they used was bio-degradable. I was wondering about this, because I have never heard of anyone changing out the harness inside of the car before. This car I am considering has been in a climate controlled garage since new and the interior is like new, so I am hoping that since its not been exposed to alot of extreme heat that the interior wiring is OK.

Is there any way to check to see if the interior wiring had been replaced already?
 
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