Factory procedure to reduce draw on electrical system to disconnect batteries - Mercedes-Benz Forum
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post #1 of 11 (permalink) Old 04-30-2019, 07:52 AM Thread Starter
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Factory procedure to reduce draw on electrical system to disconnect batteries

If you own a 2007 or 2008 W221 and have had to disconnect, specially the rear battery, you may have noticed a significant arc while attempting to get the terminal loose.

There is actually a factory procedure titled "End internal on-board electrical system functions". Although it is only a one page document, and some of the suggestions it recommends likely contradicts what you have learned over several years about leaving the key in the ignition when making changes to the charging system; whether it be disconnecting a battery, jump starting , etc.

Anyway, I have attached a link to the document below so t may be of help to some 2007 and 2008 owners with the Starter Battery under the hood and the large Consumer Battery behind the rear seat.

https://resources.mbdx.us/files/pdf/..._functions.pdf

I hope others benefit from this information.

Thanks!
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post #2 of 11 (permalink) Old 04-30-2019, 08:51 AM
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The link is not working. It says " page not found "
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post #3 of 11 (permalink) Old 04-30-2019, 09:26 AM
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I get the same error message. Sounds like valuable information, though, so I'm interested in reading it.
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post #4 of 11 (permalink) Old 04-30-2019, 06:35 PM Thread Starter
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My apologies for the permissions error. Please attempt access again. I am sure it is frustrating to see a topic you wish to read up about and it is broken!

https://resources.mbdx.us/files/pdf/..._functions.pdf

Beware it is one of the shorter WIS documents you will find - it is not truncated accidentally - it is intentionally short.

Please update this thread if you are able to access the PDF.

Thank you!
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post #5 of 11 (permalink) Old 04-30-2019, 06:39 PM
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I have disconnected both my batteries many times without doing this and nothing ever happened.
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post #6 of 11 (permalink) Old 04-30-2019, 08:45 PM
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I can see the information now. Thank you for fixing it.

Like 2008CL6, I have never done this either, and I haven't had any problems. But, it's a simple enough process so I'll start doing it just to be safe.
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post #7 of 11 (permalink) Old 04-30-2019, 10:36 PM Thread Starter
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Nothing that appeared to have caused a long term effect happened on mine, but when I connect the ground in the trunk, the arc that is created indicates there is a fairly substantial quiescent draw on the entire system. Due to the fact I dont thing it is ever good to disconnect nd reconnect the battery, I have not put it through any kind of test. When disconnecting the rear battery, I generally remove the 15mm bolt first. The arc that happens when doing this indicates to me pretty substantial draw. I hope this procedure addresses this.
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post #8 of 11 (permalink) Old 05-01-2019, 02:09 AM
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Quote:
Originally Posted by TiffanyTwisted View Post
Nothing that appeared to have caused a long term effect happened on mine, but when I connect the ground in the trunk, the arc that is created indicates there is a fairly substantial quiescent draw on the entire system. Due to the fact I dont thing it is ever good to disconnect nd reconnect the battery, I have not put it through any kind of test. When disconnecting the rear battery, I generally remove the 15mm bolt first. The arc that happens when doing this indicates to me pretty substantial draw. I hope this procedure addresses this.
This is quite helpful! AND that there is a procedure for something that would otherwise seem straightforward means (German thinking hat on) there must be a reason why! Thank you!
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post #9 of 11 (permalink) Old 05-01-2019, 06:50 AM
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Note 2007-2009 221

https://workshop-manuals.com/mercede...ir/procedures/
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post #10 of 11 (permalink) Old 05-01-2019, 05:29 PM
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Thanks, TT! Added to the DIY & FAQs Electrical section.

Warren

2013 S550

Formerly:
2010 S550
2005 S500 4-Matic
2000 S500
1978 450SL
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