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Old 10-22-2006, 05:01 AM
ThosDoran's Avatar
Aspiring Wrench
Join Date: Jul 2006
Location: PTKT, RI
Posts: 151
w123: Speaker Wiring Myths Debunked (Diagram) Bypassing the fader

Myth 1: The w123 uses only one set of wires going to the front and back set of speakers, instead of the standard two.

Myth 2: To use all four channels of a newer head unit = must run all new speaker wires.

Yes, there are only two sets at the radio, but. . .
it is not necessary to run all new wire.

You do have a +/- for each speaker. It's true. Stay with me.

I just finished putting in a Sony head unit and 10 disc changer into my '82 240D. After much head scratching, alot of searching to glean from other folks' experience and a trip to the junk yard, I made it happen; My first stereo install.

The w123 is not a common ground system (which, I've read, is bad juju). Each speaker has both +/-. What's tricky is the fader and what's been done to tap two stock channels to drive four speakers.

As for the fader, I knew I had to bypass it (apparently it can't handle the power output of new stereos) so I just opened up the console, pulled the switch and snipped.

I snaked those six wires back up past the shifter and pulled them out through the radio cave(DIN opening).

(Keep in mind that the original harness plugs had been disappeared long ago (thankfully).)

Curiously, I now had 10 (ten!) spkr wires to contend with; 6 from the fader and 4 that were hooked to the radio outputs.
The 6 are your friends. The 4 are imposters and have hidden the grounds that you need in the sheath that ran to the fader.

Have a look at the diagram below - cut open the sheath that you worked so hard to snake back up and find the spliced ground wires.

You should soon have 8 wires (4+ and 4-). The fronts are still paired. For the the rear spkrs, find which ground is which by using a 9v battery (the speaker will crackle when you've got a match).

Good Luck. I know it's a long post but I hope it helps someone out.

I couldn't have done it without this forum. T
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w123: Speaker Wiring Myths Debunked (Diagram) Bypassing the fader-speaker-wiring.gif  
'82 240D, 215,000 +
Classic White, Regenbogen

Last edited by ThosDoran; 12-13-2007 at 12:31 AM.
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Old 10-22-2006, 05:20 AM
ThosDoran's Avatar
Aspiring Wrench
Join Date: Jul 2006
Location: PTKT, RI
Posts: 151
If it helps. . .

The harness that came with my stereo was a GM connector. I found it's car side mate pretty easily. It's only 12 - pin and it's got everything I need.

Metra 70-1677-1
Metra 71-1677-1
'82 240D, 215,000 +
Classic White, Regenbogen
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Old 10-25-2006, 11:27 AM
Freybird's Avatar
1995 E300D 124
Join Date: Jul 2006
Location: Central Maryland
Posts: 18
Question Clarification

Your post is inspiring! However, could you please elaborate on "Cutting the spliced grounds"? MY car is an '87 and I'm not sure of the extent at which my stereo differs from yours. I have 8 wires snipped from the fader (you had 6) which I have pulled through the cave, and the 4 wires (imposters) which came from the radio output. Are you saying that if I stip the sheath from the fader bundle, some spliced wires (grounded) will be found and I should use them for the negative lines of the front speakers? Or are these to be simply unspliced?
Thank you and for more information on my situation, please see my post here in Car Audio forum.

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Old 10-25-2006, 03:54 PM
ThosDoran's Avatar
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Location: PTKT, RI
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Originally Posted by Freybird View Post
if I strip the sheath from the fader bundle, some spliced wires (grounded) will be found

That's what I found. The two grounds from the Front and the two grounds from the Rear speakers are spliced together up inside the sheath. A third pair of grounds is spliced in there also to ground all four speakers to two channels of the stock radio. (see diagram)

But. . .this is the w123. Do you have six speakers stock instead of four?

For me it was really helpful to have found a junked car with zero interior that let me trace everything easily.

I did see and read your post, but the 8 wires at the fader throw me.

One thing I did notice is that ALL of the grounds on my car were solid black whether they were front or rear.

Whatever you do, cut that sheath off, just so you have all of the wires free to work with. I imagine you'll discover something similar.

Good luck. Maybe post some pics of your set up too. If I can I'll add pics down the road from some other w123.

'82 240D, 215,000 +
Classic White, Regenbogen
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Old 05-06-2007, 09:13 PM
1985 300D-T
Join Date: Mar 2007
Posts: 8
Wow. This actually worked. My '85 300D-T was slightly different as far as wiring (I discovered two sets of soldered grounds when I opened up the sheath), but the process was similar. Very useful post.

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Old 05-31-2007, 05:54 PM
My hood can go higher?
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Location: Santa Rosa, CA
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Thanks for this post

I just was confronted with this same problem and was looking for a solution
1983 240D AT, 163K running on VO, "The Patience Tester"
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Old 06-03-2007, 02:36 PM
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Nice Admin...,,
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Old 06-18-2007, 07:24 PM
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Does this wiring diagram hold true for the 300 SDL as well?

Also, are the colors shown on your diagram accurate?
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Old 12-10-2007, 12:17 PM
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Join Date: Nov 2007
Location: Sac, CA
Posts: 17
I just tried this and If u have the fader intact, you can just use the wires into the radio. Cut the connectors there split the 2 wires the black connector is the left side, the white is right. You will have 4 wires, 1 each for + and - for left and right coming from the dash now. Splice the + & - together from your deck (e.g. rear right and front right) and connect the corresponding side to the wires coming out the dash. What you should have when u are done are a pair of wires from the deck to each wire in the dash. this will give you fader control in the center still.
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Old 12-10-2007, 08:46 PM
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An old trick used in car audio biz is to connect a 1.5 volt AA battery and touch to the speaker leads temporarily to hear the click in the speaker and that will tell you which speaker goes to which wire.....
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Old 12-13-2007, 08:55 PM
Join Date: Apr 2003
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Originally Posted by Ramblin View Post
An old trick used in car audio biz is to connect a 1.5 volt AA battery and touch to the speaker leads temporarily to hear the click in the speaker and that will tell you which speaker goes to which wire.....
And polarity too, so you watch the cone move out or in when the + and - poles of the battery are touched to the wires.
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Old 12-14-2007, 02:27 PM
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Location: Albuquerque, NM USA
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Your post mostly reflects confusion.

Of course each speaker has two wires. That's how speakers work.

However, the OE rear speaker wires BEGIN at the fader, i.e. there is nothing near the radio.

The old radios are 2-channel, meaning there are only +/- connectors on the radio for left and one pair for right.

Therefore the left/right pairs extend from the radio directly to the fader. Then two pairs (four total) extend from the fader to the front and rear speakers respectively.

All new radios are 4-channel, meaning there are 4 sets of connectors at the radio. Left/right for front and left/right for rear.

"Common ground" is completely irrelevant as both the OE radio and any new radio features +/- pairs of connectors for every channel.

The wires from the existing rear speakers have to be, at minimum, EXTENDED to reach the radio. (Because they BEGIN at the fader, which is separate from the radio) On a W124, at least, they have to be cut from the fader to accomplish this--I bet it's the same on a W123.

There aren't any myths, there's just your confusion.
Kent Christensen
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Old 04-17-2010, 12:27 AM
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Which sheath contains the missing grounds? I just snipped my fader connection and snaked the 6 wire sheath back up behind the radio. I can't really see where the 4 wires coming from the radio meet up with the 6 from the fader though. The wires back there are pretty tight so I can't move them around too much to check.

"Gerwalt" 1984 - 300TD
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Old 04-17-2010, 12:56 AM
JWJ JWJ is offline
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The 6 wire sheath you snaked back up to radio area contains the grounds.

After I snaked this sheath to the radio area I had to cut the wires due to their over length. But, your wiring might have been modified previously.
1983 MB 300D - > 430,000 miles - Deep Blue - Bilstein Comforts - 0-60 in 24 seconds - 27MPG
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Old 06-28-2010, 09:21 PM
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Join Date: Jun 2010
Posts: 3
Hey Thos, great post! This is exactly what I was looking for!

Quick question, when I cut the sheath that was holding the 6 wires, I found three (!!) black ground wires connected together. Does anyone have any idea as to what I should do?

Thanks for the help! I really appreciate it. I took some pictures that I can upload on the center console removal process if anyone has any questions.
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