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Discussion Starter #1
Just had my alternator replaced by my personal mechanic on my 2007 c230 sport due to loud whining noise the bearing would make. First day everything seemed fine then today at work car struggled to start on. Finally got it to start and checked voltage numbers and they were going up and down all over the place. If i drive the car itll stay around 14v but when i pull over at a light for example the numbers go up and down as low as 12v, rpm starts to go up and down and car shakes a little. Ill post a video of what i mean to maybe help out a bit. Mechanic wont be able to see my car till next week so just trying to find out if this is normal or not

 

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Just had my alternator replaced by my personal mechanic on my 2007 c230 sport due to loud whining noise the bearing would make. First day everything seemed fine then today at work car struggled to start on. Finally got it to start and checked voltage numbers and they were going up and down all over the place. If i drive the car itll stay around 14v but when i pull over at a light for example the numbers go up and down as low as 12v, rpm starts to go up and down and car shakes a little. Ill post a video of what i mean to maybe help out a bit. Mechanic wont be able to see my car till next week so just trying to find out if this is normal or not
Unless your battery was run down, it shouldn't effect it starting.. Also notice that your RPMs seemed a little low (500 rpm).. At some point, the alt. won't put out as your rpms drop off.. Try holding down the gas peddle to get the car to run a little above 600 rpm,, See if voltage stabilizes..
 

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Discussion Starter #5
So it does stabelize if i run the car. Itll stay between 14-14.5v. But the moment i come to a stoplight like i did in the video the numbers go up and down. This morning the car had no starting issues, but the voltage numbers somewhat concerns me.
 

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At stop, car in park... tip into throttle to get the rpms a bit above 600 rpms.. what's the voltage doing?
 

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Discussion Starter #7
Hey sorry for late reply. So i tried what you said. When i park and say i hit 1000 rpm the voltage goes back up to 14.5ish. So the issue seems to be more when the car is stopped. At one point on my way from work it went all the way down to 12v at a stoplight and the car shook a lil before it went back up to around 13.7ish volts. On the freeway it stayed 14.5v.
 

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Discussion Starter #8
I have two videos of me again showing whats happening. First video is of me driving and stopping at stop signs. The second one of me is with the car parked and throttling it around 1000rpm.

 

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Just right at the end of your second video.. I thought I saw your voltage drop down into the 12s.. at 800rpm range.. Not good.. Something not right with alt..
 

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Discussion Starter #10
Just right at the end of your second video.. I thought I saw your voltage drop down into the 12s.. at 800rpm range.. Not good.. Something not right with alt..
So its most likely alternator related then not battery. I kind of guessed it was because even with the old alternator and how it would whine the voltage always stayed at 14.2v when driving. This one is just up and down. Not sure what brand this mechanic got but obviously its not working properly. Which brand is OEM to my car, any ideas?
 

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Discussion Starter #14 (Edited)
Check your serpentine belt and tensioner. Make sure the belt is not slipping. Classic symptom is low voltage at idle.

If you need a new alternator, do NOT buy a rebuilt one! A new one is not that much more here (doesn't appear to have a core charge either + free shipping!) :

Will do, the belt and tensioner were actually also replaced along with the alternator so it could be that. As for the alternator i have contacted my mechanic and currently waiting on a reply. If it comes down to it will return this one and purchase a new one.
 

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Discussion Starter #16 (Edited)
Just a quick update guys: So i messaged my mechanic and he ordered another alternator under the warranty this one was under. Fingers crossed that this next one will work better. Will ask him to check the serpentine belt and tensioner while he replaces the alternator. Just to be on the safe side i went to autozone to get my battery checked and it came back okay, so it just pretty much boils down to if its the alternator or not. Voltages have somewhat gotten worse even dipping into the 12.1 volts. Yet when the car drives it always floats between 13.8v-14.5v. Issue only seems to appear when at a stop or the car stops. Car doesnt start properly in the morning and i get the battery light, yet as soon as i give the gas pedal a little push and the rpms go up the car starts.
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That's actually normal. At idle the alternator stops so it doesnt overheat
That is not correct. The alternator on the W203 is directly driven by the serpentine belt, and has no clutch. The regulator is designed to provide a steady 14v output but it can fluctuate as power needs change from things like lighting, the engine electronics and coils, and accessories. But it should never drop lower than about 13.2v. In this case with the car idling and voltage at 12.1v, something is wrong. It's probably the alternator, but it could also be the battery. A bad cell in the battery can act like a big resistor. A quick test to see if it's the battery is to disconnect it when the engine is running. If voltage goes back to normal (14v) then you need a new battery. Of course that's no guarantee that the alternator is good.
 

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Discussion Starter #19
Thank you, very detailed answer!! We are in the process of waiting on the replacement alternator so hopefully the second one works properly. When i replaced the alternator i also had the serpentine belt and tensioner replaced so im not to sure if that could be related but i will have the mechanic double check those also. But will give an update once the alternator has been replaced again.
That is not correct. The alternator on the W203 is directly driven by the serpentine belt, and has no clutch. The regulator is designed to provide a steady 14v output but it can fluctuate as power needs change from things like lighting, the engine electronics and coils, and accessories. But it should never drop lower than about 13.2v. In this case with the car idling and voltage at 12.1v, something is wrong. It's probably the alternator, but it could also be the battery. A bad cell in the battery can act like a big resistor. A quick test to see if it's the battery is to disconnect it when the engine is running. If voltage goes back to normal (14v) then you need a new battery. Of course that's no guarantee that the alternator is good.
 

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Discussion Starter #20 (Edited)
Update: alright guys so since i last posted i had my mechanic replace the alternator on the car a week ago. Ive been driving it and the voltage stays at 14.2v to 14.5v 99% of the time regardless if car is at idle or driving. Had one occasion where it dropped to 13.7v for a few seconds and then went right back up to the 14v but nothing lower than that. I did find out that the previous alternator was a rebuilt bosch and this current one is also a rebuilt bosch. So im guessing i just had a bad alternator with the first one. Had the spark plugs changed while i was at it and the cars been driving smoothly as ever 😎.
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