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Mike, Mercedes Technician
Category: Mercedes
Satisfied Customers: 39198
Experience:  Tech Expert
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HiI have a merc b160 petrol 2010.Battery light came on

Customer Question

Hi I have a merc b160 petrol 2010  blue efficiency. Battery light came on . so changed battery. Light still on. Now changed second hand Mercedes identical alternator. Battery still draining and light on dashboard still there. Its a start stop type alternator ECU regulated charging so the re is no separate regulator on the alternator. The alternator seems to be getting hot on idle wuth no extra electrical units running in the car. Help??

Submitted: 2 years ago.
Category: Mercedes
Expert:  Mike replied 2 years ago.
Hello and Welcome -
My name is ***** ***** Im happy to help -
|Can you tell me what fault codes have been retrieved - then we can progress from there -
Rgds, MIKE........................
Customer: replied 2 years ago.


Cannot say what fault codes have been reported as they are not able to read the codes

In your experience could 2 alternators be at fault and is the reason why the battery isn't charging?

Or what is the most likely explanation for the new battery going flat? As you are aware the battery sign appears on the dashboard.

Expert:  Mike replied 2 years ago.
Hi again Jay-
Thank you for your reply..
Its somewhat ominous that they have comms failure when trying to get fault codes out -
Basically this is a "smart charge" system - which means that the charging system runs yes from the alternator but through the Ecu ( main computer / ecu ), when reading your question initially, I felt your issue was either wiring / connections , in the circuit..or, ecu. And now you say they have issues with comms to the ecu - again - the wiring etc all needs to be checked from the alternator back to the ecu.. If no faults found, I would suggest send the Ecu to a test company - such as bba-reman or whichever you might choose.. ( Garages dont have the test equipment to run the tests needed for this )
I have yes had TWO faulty second hand parts in my time - but , well.. you could ask the supplier for another - try it -- but if that doesn't work, its def not that.... and then you need to progress as above to get this one sorted out for you Jay.
( I take it they checked for fuse issues on the diagnostic / obd circuit.. )
Let me know how you go..and / or let me know if you need anything further on this..
~~~~ Meantime - Please do click on the Exc / Ok rating for me at this time.. "Thank you" in advance for that .
.. Note that you can get back to me anytime at all after doing the rating - it doesn't close you out --
Best Rgds, MIKE.
Customer: replied 2 years ago.


Can y help and explain. They are now saying it is the CONTROL UNIT that has burnt out. The unit that controls the alternator regulation.

Can you give me please the exact part and what its called so I can look into it and get the cheapest price for this.

Expert:  Mike replied 2 years ago.
Hello again Jay -
There are several control units on that car - more than that in fact and some inter with each other -
Are they talking about the "main" ecu..?
Can you get the part number off the "unit" they are saying it is - then I can look further at that for you..
Rgds, MIKE...................