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  #1  
Old 04-24-2010, 09:55 PM
JamesDean's Avatar
Electrical Engineer
 
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,038
w126 headunit replacement keeping factory amps

Hello everyone,

I just picked up a 420SEL and the radio isnt working. I figured I'd replace it with a newer mp3/usb capable unit.

I want to keep the factory amps in tact for the time being as I have other things that require the money at the moment.

After looking at the FSM for wiring it looks like i should be able to tap the signal wires (B and C) and connect it to the RCA output of an after market head unit...



Has anyone done this before?

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
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  #2  
Old 04-25-2010, 01:39 PM
Bio Brewer
 
Join Date: Apr 2010
Location: Napa, CA
Posts: 301
While that probably is possible, I would wager that the new deck will have a better "amp" built in than the stock amps that came in that car in 1989.

If you could find the power output ratings for the stock amps you would be able to tell what would be better. Most decks these days are anywhere from 18-24 watts RMS (continuous power output). The amp that came in the car would probably put out less than that but I don't know for certain. I'm new to Mercedes but from what I know the aftermarket decks should have better sound than your stock setup.

If all you want it MP3 capability I have heard that Becker can install an aux input to the stock radio so that might be something to look into if your deck is a Becker unit. Hope this helps!

-David
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  #3  
Old 04-26-2010, 03:49 AM
mak mak is offline
mark
 
Join Date: Jul 2006
Location: Westfeld .
Posts: 687
a new amp will give better sound , i have a orignal head unit with a alpine amp ,speaker still to be updated.
mak
300se
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  #4  
Old 04-26-2010, 12:53 PM
JamesDean's Avatar
Electrical Engineer
 
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,038
I agree that replacing the head unit with a new one and replacing the amplifiers with an after market one will yield much better sound. I've done this in my one 300SD. My 190 has three separate amps powering Boston Acoustics all around.

However this issue with the 420 is that I dont want to buy an amplifier on top of a deck right now. I will down the line but, for now, I'd rather spend the money on other things for the car.

I think straight up wiring the "line out" harness to an RCA style connection would work. This would leave the center console fader in tact. I might be inclined to bypass that if it proves troublesome. At that point it will be the same procedure, just RCA-itize the connections post fader. I'd rather not rewire for headunit power of speakers as eventually I will get an external amplifier.

When I do put an amplifier in i can just install it in the trunk and tap the wires @ the factor amps. The only things that I'll have to figure out is what wattage and impedance.
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
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(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k
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  #5  
Old 05-02-2010, 01:33 PM
JamesDean's Avatar
Electrical Engineer
 
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,038
First off, the images i posted above are the incorrect ones for my car.

However I am pleased to report that one can certainly hookup an after market head unit to the factory amps.

At the same time, one can also hook up an auxiliary input.

The wiring is as follows:

From the connector on the "line-out" module of the Becker:

Brown = Ground (Separate from the 3-pin connector)
Blue = Left Side
Red = Right Side
White/Yellow = Ground.

It is important that you connect up the White/Yellow to ground otherwise you will lose a lot of signal, mostly bass.

If you are just going to hook this up to an after market headunit, I would recommend picking up some RCA terminals from RadioShack
(something like this:
http://www.radioshack.com/product/index.jsp?productId=2104066)

and soldering the wires to the RCA terminal.

Alternatively, if you wish to have line in with your factory (or after market) deck you could simple install a switch...

Something like this:

http://www.newark.com/multicomp/1m33t1b5m1qe/toggle-switch/dp/95M4616

You would hook up the factory wiring (Left/Right/Amp turn on) to one side of the switch and the end of your line-in wire to the other side (Left/Right/Amp turn on wire)

As for the grounds you could simply tie all of them together. As the system isnt going to care if they are on all the time or not.


For bypassing the fader The procedure is somewhat similar, however wiring for auxiliary input is a bit more complicated. For after market headunit wiring one would simply grab the wires that come OUT of the fader, and connect the wires to RCA style jacks. If you need help feel free to PM me.


Hopefully this helps some of you out there.

-Kris
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k
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  #6  
Old 06-11-2013, 06:49 PM
macdoe
 
Join Date: Sep 2007
Location: Canada
Posts: 759
I would like to hook up an after market deck to replace the non-functioning becker in the 1988 420sel. I would rather not disturb the crossovers that these speakers have. Is that why you choose this way of installation rather than powering the speakers with the new h/u? If I have only four speaker outputs on the new h/u and the car has six speakers how will that work. I am really bad when it comes to electrical. Thanks
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  #7  
Old 07-18-2013, 09:11 AM
Registered User
 
Join Date: Jul 2013
Location: Charlotte
Posts: 7
Turn Off The The New Radio Head Amp.

Just installed a new Alpine radio head with Bluetooth in a 90 420 SEL. Used 2 RCA plugs from the head line outs to the fader (use front left and right only). She sounded good but really thought it should be better. Read the radio manual and in the setups it recommended turning off the new heads built in amp if using line outs to hook up to the factory amp. Works perfect, no rewiring speakers direct and have full control right to left with great quality sound as it should be. Fader fully functions and can be fine tuned bass, treble, and balance from the new head. All you are doing is providing new sound input source to the factory system. If you need wire color coding specifics let me know and hope this helps.
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  #8  
Old 07-18-2013, 10:17 AM
compu_85's Avatar
Cruisin on Electric Ave.
 
Join Date: May 2008
Location: La Conner, WA
Posts: 5,234
You'll need to use the preamp that's on the back of the headunit now, or use something with a high level output as the factory amps expect. It's somewhat above line level.

Then again, the factory amps are kind of junky... I'd just replace everything.

-J

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