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Old 06-10-2011, 09:58 PM
Yak Yak is offline
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Location: San Antonio, TX
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I don't like this thread due to the incorrect info sprinkled throughout. I've drawn a schematic and included a labelled photo of what I'll call the fader harness. It starts at the radio with 4 wires, the two black (-) wires are spliced with four more black wires at the two solder blobs. So you've now got one black "input" per blob and two black "outputs" per blob or four total black "outputs". Each of these black outputs goes to a corner connector (white = left, black = right, they're also marked R and L).

The colored wires from the radio go directly to the fader. That's one red (or red/black) and one green (or green/black). At the fader, there's a rheostat and four outputs two each red (or R/BK) and green (or G/BK). The fader basically splits the one red (or green) input into two each outputs. These four outputs go to one each corner red to the left and green to the right. These four colored wires join up with the black wires from the above paragraph.

The front speaker connectors are actually soldered (the long way around) to the black wire at the HU radio connectors, and the front colored wires are connected at the fader.

If you run new wires for the fronts (pretty easy in the W123) and simply unplug the rears DOWNSTREAM of the fader, then the fader harness stays intact. You don't need to run new wires to the rear.

The numbering code between the drawings is:

ONE: four wires at the head unit, red + bk; grn + bk (red and green and red/bk, grn/bk are used interchangeably in the drawing since I didn't have a stripe tool). The black wires from here go to---
TWO: two solder blobs in the sheath. The colored wires go to---
THREE: the fader, where they split again and go to four plugs at ---
FOUR: four speaker connectors, two in the console and two near the speakers in the dash where the wires rejoin their black counterparts last seen dividing at the solder blobs.
Attached Thumbnails
w123: Speaker Wiring Myths Debunked (Diagram) Bypassing the fader-harness.jpg  
Attached Files
File Type: pdf Coupe harness.pdf (6.5 KB, 913 views)
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Old 09-11-2011, 08:04 PM
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Join Date: Sep 2011
Location: Bonney Lake, WA
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great diagram, Yak. BTW, I got signal to all four speakers OK without separating the commons, just tapped into the ones on the 4 wire set that was originally at the head (but still used the 4 coloreds coming out of the fader). Does that affect my sound i.e. do I need to go back in there and separate the four? I've got some static in the system I'm trying to run down, wondering if that has anything to do with it.
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Old 09-12-2011, 11:45 PM
Yak Yak is offline
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If I understand what you've described, you've bypassed the fader by re-using four of the six wires there (the four that go to the speakers + terminals, but you're still using a common ground for the the front and rear channels. That might be causing static, but it's tough to say. You could maybe do a quick diagnosis for static by unplugging one set or speakers and seeing if the static clears up. Is it static, like crackly, or a whine/hum? If the latter, that's probably alternator noise.

The other problem about tying the speakers together has to do with the impedance of the speakers and the power output of the HU. If they're not compatible, then you can burn out the amp in the HU. This is probably not a problem if you're not cranking it up.
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Old 09-19-2011, 03:40 PM
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Location: Bonney Lake, WA
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Thanks. Yeah, I dug down and separated the wires. No static now (other than when i turn it up loud - but that's because of the old speakers). My back right isn't working, although it gives me a crackle with the battery test. When I'm rich enough to replace my speakers I'll worry about that.
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Old 04-24-2012, 10:40 PM
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Location: Ontario, Canada
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Just rearranged the speaker wiring on my 85 300D.

It was arranged as Yak described. In other words, positive wire from each speaker goes to fader. Also, the positive wires from the back of the radio go to the fader.

The negative wires from the speakers terminate about 1ft short of the fader under a plastic sheath. The negatives from the back of the radio terminate at the same point and are soldered to the left or right hand speaker wires making two 3-way joints. Yaks drawing shows this.

What I did, was:
- cut off the fader plug
- pull off the sheath on the fader harness after removing a couple of tie-wraps under console cover. (this exposes the soldered negative joints)
- taped over the soldered joints, then used the smaller sheathed wire that connects to the radio, to pull the wires into the radio opening.
- I then snipped off the solder joints. This leaves the speakers wires in pairs - the long one (colored) is the positive and the short one (plain) is the negative
- It is easy to see which wires are which for the front speakers (see below for my colours)
- For rear speakers use flashlight battery to make speakers click to find out which is which.
- Hook everything up and Bobs yer Uncle
(see Noob needs help replacing old Becker with new stereo for power connections)

For my 85 300D:
Front left speaker - Green/Brown
Front right speaker - Red/Brown
Rear left speaker - Yellow/Black
Rear right speaker - Black/White

The speaker wire colours on this wiring diagram don't seem correct

1985 300D that had Becker Grand Prix 612, now with Alpine CDE-100.
85 300D,72 350SL, 98 E320, Outback 2.5

Last edited by Graham; 04-25-2012 at 04:16 PM.
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Old 11-18-2013, 11:25 PM
JB3 JB3 is offline
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I just want to take the time to tell The Don that I love him for this thread.

Middle of the night, leaving on a trip next morning, installing a working radio, this thread solved all my problems
This post brought to you by Carl's Jr.
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Old 05-14-2015, 11:19 PM
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Location: Inglewood, CA
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Thanks Thos, When I saw only 2 speaker connections on the back of my old Becker it stopped me right in my tracks. With all the helpful info on this thread I was able to continue on with my audio upgrade install. I completed it successfully. I replaced my front speakers with drop-in 3.5s from Crutchfield. The stereo was part of a $90 set from Pioneer (Walmart online) that came with 2 6x9s and 2 6.5s. I put the 6x9 in the rear. I now have much improved car audio. I had planed to put the 6.5s in the doors but, I don't think I need to. Thanks to all who posted on this thread.

1985 300D Turbo CA Fed 223K
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Old 07-10-2017, 05:48 PM
Join Date: Jul 2017
Location: El Salvador
Posts: 15
thanks so much just started on my [new for me] 1993 300D 2.6 Turbo, bad Mercedes radio or something I assume so I have an old DUAL i will put in cut the fader wires Now I will hope to hook up 4 speakers, got to be better than my phone and or old ambulance with 2 speakers, jaaa
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Old 09-10-2017, 11:41 PM
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Join Date: Jul 2009
Location: Langley, BC, Canada
Posts: 43
Wiring Stereo W123

Thanks for the thread

Here’s what worked for my son and me on our 79 300TD.
Cut all wires at existing stereo and at fader.
Non-speaker wires coming from the engine side were:
Yellow (constant battery +)
Red (fused, switched +)
Black (neg)
A quick voltage test from the yellow and red to the black confirmed this.

Then we pulled up the rear speaker wires from under the cover behind the shifter – they are easily identifiable by the little clip-on joints (a black one and a white one).
Cut them off too
Stripped off all wire ends.
Now we had a number of mystery wire ends to identify

Did a continuity test to see which ones paired up, and made a list of them.
On each pair, did the click test with a 1.5v battery to see which ones lead up to a speaker.
To check polarity, we undid the cover of one of the front speakers and noticed that connecting +ve to its solid color wire made the speaker diaphragm rise, and connecting –ve made it fall, so we identified the solid color as +ve. We then assumed that all the other solid colors were the positive and used that as a rule of thumb. (This ended up working perfectly but see note below about the black ground wires).
Soon we identified each pair of speaker wires, and were left with four wires that seemed to serve no purpose. More testing showed that they simply went from the radio to the fader. So we have left them tucked inside.
We used the conventional codes for our stereo found easily on the web and soldered up all the 8 speaker wires, solid colours for + and striped for -.
Now interestingly the black grounds that were soldered to the rear speakers were all soldered to solid colour wires. This contradicted our findings about the polarity of the front speaker. Nevertheless we wired everything consistently and the sound we now have is perfect
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