Charging warning light - Mercedes Benz SLK Forum

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#1 Old 11-21-2012
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Charging warning light

1998 SLK 230 automatic with 117,000 miles.
All warning lights come on when the key is turned to position 1. A week ago, the charging warning light came on. While the car was running, my voltmeter showed only battery voltage. I pulled the voltage regulator; brushes were short but almost the same length. I replaced the voltage regulator. The light went out. Voltage from the positive cable to chassis ground was 13.88v. Two days later, the warning light became intermittent. However, it is always off right after the car starts. My voltage meter still shows 13.88v. Auto Zone tested it and said that the alternator was fine and was charging at 13.99v. I cleaned as many grounds as I could find without making a difference. A rebuilder near me said that he didn’t recall intermittent faults with alternators, but that I could bring it to him and he could check the diodes on his oscilloscope.
Does anyone have any thoughts? I hate to take it out if I don’t need to.
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#2 Old 11-21-2012
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The light has been out for a while. Has anyone experienced brushes taking a while to wear in?
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#3 Old 11-22-2012
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Battery

I am sure you have checked the battery and it seems from the figures to be fine. However, how old is it ?
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#4 Old 11-22-2012
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The battery is about six months old. Auto Zone tested it with a load on it and said that it was fine. The connections are both clean and tight. While it seems fine at the moment, it would be nice to know what might have caused the warning light issue. It has me stumped.

This is our first Mercedes so I have a steep learning curve ahead of me. Does the ECU trigger the light?
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#5 Old 11-27-2012
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Normally the charge warning light is powered via the position 11 then earths out via the rotor windings and voltage regulator.
If the light is glowing when the alternator is charging it indicates a high resistance between the alternator output and battery. Worth checking the maxi fuses in the engine bay. I will have a look at the wiring diagram and get back.

Bob
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#6 Old 11-28-2012
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Thanks Bob. The light has been out since my last post. I hate intermittent issues. I will clean the fuse connections just in case they were the problem.
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#7 Old 11-29-2012
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You might check for volt drop between the alternator output and battery and the earth return between the alternator and battery. Any volt drop on either side above 0.5 V will cause problems.
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#8 Old 12-01-2012
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I've seen this before when replacing brushes. Sometimes is takes a little while for them to wear into the commutator which may be grooved. Kind of like new brake pad on old rotors. While the brushes are out I will usually spin the alternator and scuff up the commutator with fine sandpaper, then clean it with contact cleaner.

Cheers!
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#9 Old 12-01-2012
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hi did you clean old carbon of commutator as old carbon can and does cause problems
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#10 Old 12-01-2012
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Quote:
Originally Posted by outonmybike1 View Post
hi did you clean old carbon of commutator as old carbon can and does cause problems
I did clean the commutator. I first used throttle body cleaner on fine fabric wrapped over a pencil eraser and followed that with a spray on contact cleaner.

Unless someone has a better thought, I am going with the idea that the brushes took a short while to wear in. I have done quite a few voltage regulator changes on Bosch alternators and never did have this issue. Always a first time, I guess.
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