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Auto Psychic
Auto Psychic, Mercedes Technician
Category: Mercedes
Satisfied Customers: 1437
Experience:  semiretired mercedes benz master guild technician w 4+ decades of diagnosis and repair experience
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Mercedes-Benz E320 Tonight the battery warning light came

Customer Question

Tonight the battery warning light came on in my wife's 03 320. The message says "Visit Workshop". The alternator belt is fine. How do I tell if the problem is the alternator, the main battery, or the auxillary battery?
Submitted: 3 years ago.
Category: Mercedes
Expert:  Auto Psychic replied 3 years ago.

Auto Psychic :

hi, i'll be happy to help you in anyway i can.

Auto Psychic :

sorry to hear of your misfortune :(

Auto Psychic :

does it still start ok?

Auto Psychic :

gary

Customer:

Yes, it starts fine at the moment.

Auto Psychic :

thats good news.

Auto Psychic :

are you handy with tools and want to give this a go yourself, or do you just need advice?

Auto Psychic :

gary

Customer:

I would say that I am quite handy with tools and mechanical aspects of cars. Electrical items are another matter.

Auto Psychic :

do you have a voltmeter?

Customer:

Yes

Auto Psychic :

ok, lets start with the alternator.

Auto Psychic :

start the car up and put your voltmeter across the battery.

Auto Psychic :

anything less than 13 volts is a bad alternator.

Auto Psychic :

after that turn the headlites on with the engine off for 1 minute.

Customer:

OK. That will take a couple minutes, I need to clear a lot of crap out of my wife's trunk. Back in a few minutes after I do that start the car and check the voltage.

Auto Psychic :

if the voltage is less than 11ish replace the main battery.

Auto Psychic :

beyond thats its more likely the battery control module.

Auto Psychic :

dont worry about the aux battery for now.

Auto Psychic :

gary

Auto Psychic :

aint this remote diagnosis a hoot lol

Customer:

OK. Good news. 12.2 volts with engine off, 14.0 volts with engine running. I assume this means the alternator isworking.

Auto Psychic :

very good news. alternator ok, main battery probably ok.

Auto Psychic :

probably a bad battery control module.

Auto Psychic :

additional good news, believe it or not it may just be software.

Auto Psychic :

will require a trip to the dealer, i'm afraid.

Auto Psychic :

be prepared to pay 1 1/2 hours labor for diagnosis. if the repair seems reasonable, just do it.

Auto Psychic :

if the come back and say you need an $1800 something or other, check w me before you buy.

Auto Psychic :

good luck,

Auto Psychic :

gary

Customer:

I did not try the light test, as I left to check voltage before that came through. Should I still do it. Is the battery control module expensive, and does it need programming, or is it a DIY?

Customer:

Also, I am new to this caht stuff, how do I get back to you after I see what the dealer says? Finally, given that the alternator seems to be generating power, is it OK to drive it to the dealer (about 5 miles)?

Auto Psychic, Mercedes Technician
Category: Mercedes
Satisfied Customers: 1437
Experience: semiretired mercedes benz master guild technician w 4+ decades of diagnosis and repair experience
Auto Psychic and 3 other Mercedes Specialists are ready to help you
Customer: replied 3 years ago.

Gary -

 

See last couple follow up questions.

 

Scott

Expert:  Auto Psychic replied 3 years ago.
looks like my final answer didnt go thru.
get it jeopardy, final answer, or was that who wants to be a millionaire lol
driving to the dealer is no problem.
battery control modules are relatively inexpensive by mercedes standards, and are diy.
and we can chat as long as you want.
always around if you need me.
gary
Customer: replied 3 years ago.

Gary -

 

I think that covers it. I am thnking that I buy the BCM and install it to see if that fixes the problem Cheaper than 1.5 hours of diagnosis.

Expert:  Auto Psychic replied 3 years ago.
maybe, unless its just software.
but the new bcm will come with new software.
your call.
here if you need me.
gary
Customer: replied 3 years ago.
Thanks so much.
Expert:  Auto Psychic replied 3 years ago.
my pleasure,
gary

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