How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask MB Paul Your Own Question
MB Paul
MB Paul, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3567
Experience:  Qualified Mercedes Benz Diagnostic Technician - 20+ years experience - Workshop Foreman for Mercedes
Type Your Mercedes Question Here...
MB Paul is online now
A new question is answered every 9 seconds

1997 Mercedes S500 Sedan is charging at 12.5 volts at idle,

Customer Question

1997 Mercedes S500 Sedan is charging at 12.5 volts at idle, and at 1500 RPM, with the head lights. radio and AC on or off .
. The battery charge light comes on when the ignition is turned on, and always goes out when the engine is running. It has never come on when the engine is running. I noticed recently that there was a drag , hesitation and slower cranking speed in the starter. It finally quit cranking altogether. I installed a new battery and the starters spins fast and the engine starts immediately. The old battery was more than five years old. It was in the car when I purchased it five years ago ,but I do not know how long it had been in the car.
This low charge rate of 12.5 volts has been going on for months. At this point, I do not know if the battery went bad due to age or insufficient charging rate. The battery shows 12 volts with the engine off and 12.5 when the engine is running ,regardless of engine speed or electrical load. I owned a starter and alternator rebuilding business for 11 years, and rebuilt about 5000 a year. I also installed some, and tested the charging system on many cars. I have never seen a situation where charge light went out and the battery was not being charged between 13.5 to14,5 volts. My question : is the charging rate on my car sufficient to keep the battery up ,or do I need to replace the alternator?
Submitted: 8 months ago.
Category: Mercedes
Expert:  david craig replied 8 months ago.

Hi Im David. replace your alternator. anything less than 14.4 vdc at the battery while running is insufficient to charge a battery to a constanst 12.5vdc.

Expert:  david craig replied 8 months ago.

do you still need assistance?

Expert:  david craig replied 8 months ago.


Expert:  Lou P. master tech replied 8 months ago.

Hello are you still needing assistance?

Lou P.