TalonP

I have multiple issues right now with my wife's mercedes. Prior to some work, she had a red battery light indication come on. I went into the diagnostic menu (holding OK and the CALL button at the same time prior to starting the car) and saw a UB voltage around 12.2 and an IB voltage in the negatives. I therefore assumed the alternator had failed as the car is a 2012 and has 115,000 miles on it. I decided to go with an aftermarket alternator as Mercedes wants $800! for an alternator.

I put in the new alternator, reconnected the battery and prior to starting it for the first time, ran the diagnostic menu again. After starting, the voltage went up into the 13s and when accelerating would go to 14.6v and hold. So everything appeared good and working. After letting the car warm up for 15-20 minutes, I took it for a test drive etc. After about 5 minutes of the test drive at some point it just dumped back down to 12.3 and stayed there (actually dropped to 12.2 after a bit). Driving it again today, I now get the red battery light indication again.

Something else has got to be causing this right? The likelihood of this new alternator being bad seems VERY slim. I hear some people have issues with the voltage regulator but as far as I can tell there is no built in regulator on the alternator itself as it appears to be a single case unit (not something where you can pull off a component of it like some YouTube videos show). Perhaps I'm not correct though. Its the 150 amp alternator and the original mercedes Part# is A 000 906 30 00. I bought an aftermarket one though from Amazon (it's re-manufactured).

Glock_19_9mm

You didn't mention this, did you have the battery checked? If there is a shorted cell, that could pull down the alternator voltage.

Do you have a Multi Meter?

ihaveaquestion

I am having the red battery light on too.

I think it's because of my failed oil filler cap dripped engine oil on top of the alternator.

Battery voltage at 11.9v with no change when the car is running, suggesting the alternator is not charging. Should I fix it? Or should I just sell to CARMAX?

Glock_19_9mm

Yep.. sounds like you alternator is DOA. I would put the DMM right on the output wire from the alternator just to make sure. You can also pull it and have it tested, but based on your 11.9V battery voltage, you are not going to make it to the store :-) (much less carmax)

A Bosch is less than $225 from rock auto. Typically they are an easy swap, but I am not familiar with your MB.

As to whether to sell or not, that is a personal choice... If it were me and the rest of the vehicle is sound (and I liked it) I would swap the alternator and keep it running... it is cheaper than a car payment.

ihaveaquestion

I am afraid CARMAX is going to frown upon seeing the red battery light and give me a low quote.

I will see if cleaning the plug contact will fix the issue.