Alternator Readings
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Joined: Oct 2024
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From: Pennsylvania, US
2008 C300 4Matic
Alternator Readings
I have a 2008 C300 4Matic and am looking to replace my alternator because the pulley is broken causing some squealing and it’s original with over 180k miles on it now anyways. I also suspect the voltage regulator is on its way out, and today I used the IC to get some readings while driving around.
At its absolute lowest, it only hit this value once, it was 13.3v. The rest of the time it never dropped below 13.5v.
At its highest, it was 14.9v, and it hit that a few times as well as between 14.5-14.7v fairly often.
It’s most common reading was between 14.1-14.3v and stayed there for most of the ride.
Thoughts on these voltages and if it’s a sign of the alternator failing mechanically as well?
At its absolute lowest, it only hit this value once, it was 13.3v. The rest of the time it never dropped below 13.5v.
At its highest, it was 14.9v, and it hit that a few times as well as between 14.5-14.7v fairly often.
It’s most common reading was between 14.1-14.3v and stayed there for most of the ride.
Thoughts on these voltages and if it’s a sign of the alternator failing mechanically as well?
Joined: Nov 2016
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From: Maryland
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
those voltages look pretty normal. You should look over these posts. If you disconnect the alternator LIN wire, you will see a steady voltage probably at 14V or 14.1V.
https://mbworld.org/forums/mercedes-...s-voltage.html
https://mbworld.org/forums/e-class-w...eboot-how.html
https://mbworld.org/forums/mercedes-...s-voltage.html
https://mbworld.org/forums/e-class-w...eboot-how.html
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From: MA Coast
W204 2010 C300 4matic Sport M272
When I thought I have an electrical issue I drove around with a wireless bluetooth battery monitor on the battery so i could see where the voltage was at. From memory the voltage was lowest when coming to a stop and rpms were lowest, maybe 13.9v. Then taking off and higher rpms it was around maybe 14.7 highest.
I was doing this because the red battery warning would pop up on the dash when starting the car after not driving it for 2+ days. As soon as i started driving the warning came on, I would rive to the end of the street and turn off the car and restart it. The warning wouldnt come back on after that. The issue seemed to fix itself, havent had the warning pop up in over a year. I was ready to replace the voltage regulator.
Point of my story is I didnt have to do anything.
though you say the pulley seems broken.
I was doing this because the red battery warning would pop up on the dash when starting the car after not driving it for 2+ days. As soon as i started driving the warning came on, I would rive to the end of the street and turn off the car and restart it. The warning wouldnt come back on after that. The issue seemed to fix itself, havent had the warning pop up in over a year. I was ready to replace the voltage regulator.
Point of my story is I didnt have to do anything.
though you say the pulley seems broken.
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Joined: Oct 2024
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From: Pennsylvania, US
2008 C300 4Matic
I’m going to attach a video of my alternator. From my understanding, if you block the alternator fan from spinning then the pulley should spin freely one way and lock up in the other. Mine is locked in both suggestions which would mean it is seized, but I want to make sure I’m correct in my test first.
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From: MA Coast
W204 2010 C300 4matic Sport M272
I dont think my pulley has a clutch in it, I think it is just mounted solid. Does your pulley have a plastic cap on the end?
Just watched your video. My alternator looks the same.
I dont know much about alternators so could be totally wrong.
Just watched your video. My alternator looks the same.
I dont know much about alternators so could be totally wrong.
Last edited by TimC300; Mar 26, 2025 at 06:02 PM.
Thread Starter
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Joined: Oct 2024
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From: Pennsylvania, US
2008 C300 4Matic
It doesn’t have the cap, but from what I was reading on a few forums it has an internal clutch built in behind the pulley, so it’s not able to be seen without taking it all apart. That’s why I wanted to come on here and ask though, as I’ve never heard of such a thing before now.
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MBWorld Fanatic!
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From: MA Coast
W204 2010 C300 4matic Sport M272
I cant find anything about an internal clutch. I think its either a solid one way pulley or a free wheeling clutch type pulley. Ours appears to have a solid one way pulley.
I want to say from all my reading I would expect the red battery warning light to come on if there were an issue with your alternator.
I want to say from all my reading I would expect the red battery warning light to come on if there were an issue with your alternator.
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Joined: Nov 2016
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From: Maryland
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Thread Starter
Member
Joined: Oct 2024
Posts: 225
Likes: 29
From: Pennsylvania, US
2008 C300 4Matic
