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Old 05-08-2009, 01:09 AM
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Join Date: Nov 2008
Posts: 104
Rewiring Alternator/Starter/Battery cables

So, I'm upgrading the alternator on my 81 300SD (617.951 126.120) to 120amps and am using this as a guideline, but I've got some questions, and I don't want to get my wires crossed (I know... I'm sorry). My reason for the upgrade is better charging for short trips and better starting in Winter.

I removed the battery to starter, starter to junction, and alt to junction cables today. In the junction the right 2 spots have a band connecting them. I can't tell but am guessing that this is conductive and connects those two terminals because I can't remember if there was one or two wire on the term where the alt connected to the junction. If there was one, then that would make sense to me. If there were two, then I've lost something in the garage. What is that smaller wire from junction to starter? A ground or something to do with the dash light? That'll get replaced too.

Here's my main question. In the previously mentioned post, they wire the alt directly to the battery. Would that be in addition to all of the other connections going back where they originally were, or is it in place of a connection somewhere?

I guess I'm wondering if there is any reason to bypass the junction and why/how. I'm getting the feeling it's more of a swap job than a rewire, which I thought it was going to be.

btw, I am using 2ga for everything except for the body ground to engine ground which is 4ga cause that's all I could find today.

I hope that was clear enough. Thanks for reading!


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Old 05-08-2009, 02:57 PM
Diesel911's Avatar
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Join Date: Sep 2007
Location: Long Beach,CA
Posts: 36,754
I can only be of minor help.
The wires at the Old Alternator connector: The circuit connected to the 2 Fat Red ones end up going to the Battery +, I do not know why there is 2 of them.
The siknny Purple wire ends up going to the Charging light in the Dash Board.

There is no Ground Wire of anysort down by the starter; I was also told by some Members that Ground wires on Mercedes are Brown or could be Brown with another color strip on them.
On mine in the image below the White and Purple Wire on the terminal by themselves. The white wire goes down the the Starter Solenoid and when the Key is in the start position carries the + current to that Purple wire that is under it. The Purple wire goes to the Glow Plug Ralay so that your Glow Plugs get + current while the starter is cranking.

84 300D, 82 Volvo 244Gl Diesel
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Old 05-08-2009, 06:21 PM
toomany MBZ's Avatar
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Join Date: Nov 2006
Location: central Va
Posts: 7,820
This is my '83 SD junction box.
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Rewiring Alternator/Starter/Battery cables-img_1680.jpg  
83 SD

84 CD
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Old 05-09-2009, 03:38 AM
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Join Date: Nov 2008
Posts: 104
a picture is worth...

a thousand curse words.

Ok, so I wasn't far off from memory as to where things go in the junction, and if that band all the way to the right is conductive then I wouldn't have made any functional errors. Now I just need to stop confusing myself and keep the cable upgrade simple. I'm hoping to wrap this up tomorrow before work.

Thanks for your help. The author of the post I referenced is helping me with the rest. You guys are the best!!


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