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Old 01-24-2003, 11:59 AM
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Join Date: Oct 2002
Posts: 188
W140 Alternator Load

Hi there:

I read from many posts that the S500 uses a 143 Amps alternator. I have an early model 300 (104) engine which uses a 120 Amps alternator. I am sure its 120 Amps because I just had to repace it and was given with the stock configuration.

My question is:

1) How do we know how much is our car drawing with standard stuff turned on and how much amps "spare" we may have left; and

2) I have the "Cool Harness" installed and understood that the fans draws 35 amps (I think so) when turned on at high. Is this going to cause any problems to my 120Amps setup? I hate to drain my battery, again, just like the failed alternator I have to replace.

Your help will be appreciated.


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Old 01-24-2003, 02:00 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
The later S500's use a 150A alternator & that is also the rating of the replacement for the 143A as well.
Most of the V-8 S-class need more power than the 6-cyl cars require.
OF course on ANY car more power can be consumed than the alternator can put out.

How long did the alternator last?
Did they try a replacement voltage regulator?
Most of the time the brushes are worn out on the regulator & the whole alternator will get replaced!!
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Old 01-25-2003, 02:34 PM
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Join Date: Oct 2002
Posts: 188

Thanks for your reply.

My old alternator lasted 64000 km. Sorry, my car is on km rather than miles as I am on the other half of the world! It was factroy fitted in 1992.

They did not test the regulator nor did they rip apart my alternator. Out here where I am, they don't tend to have any re-conditioned parts and thus everything is new. Mine was a brand new Bosch 120 amps.

Bottom line, was I dumb in replacing with a 120 amps and should I have gone with a 150 amps? Also just again, anyway to test if there are any spare capacity on the existing alternator output? I am hestitate to use the "cool harness" as the fan will draw 35 amps which can be a bit harsh on the electrical system.

Your advice is very much appreciated. Thanks again.
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Old 01-26-2003, 12:07 PM
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'94 S500: only 793 sold!
Join Date: Mar 2001
Location: San Diego, CA
Posts: 1,933
I wouldn't worry about . . .

the current draw. You are being abit paranoid here re the CH usage.

They almost NEVER come on high! During normal CH operation, the current draw is about 10 - 12 amps. Your headlights draw more current than that!
Regards . . . . JimF
'94 S500 Cpe

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Old 01-26-2003, 12:29 PM
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Join Date: Oct 2002
Posts: 188
Many thanks Jim...

In that case, I will go ahead and leave them in the car. I just didn't have too many luck recently with the alternator breaking down and some hard-to-start problems. My battery does give me 12.75 volts but anyway, thanks again for your kind reply. I did have an independent tech look at my car.

By the way, I think my W140 with the M104 engine uses three stage fan operation rather than two stage like most 140s. I am sure because without the CH, my fans come on at 14 bars, rather than 16 bars, which is a three stage setting.

Thanks to MB DOC too for his help.


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