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Battery Icon and "Visit Workshop"

7K views 4 replies 3 participants last post by  benz-SL500-newbie 
#1 ·
Hey everyone, I've been googling this and getting answers from bad BNS to bad batteries to alternators. So I thought I would post my situation and start a fresh discussion.

Warning comes on after about 10-15 seconds after car is started. Front starter battery is a mercedes battery less than a year old. Consumer AGM is a DieHard and has been having some problems as of late with "consumers off-line" after sitting for a week or so, so I figured I'd replace that before going any further. No effect.

I hookup up my SDS. Lots of control modules complaining about low voltage and other minor complaints. Clear all fault codes, and started vehicle again and took for short drive. Still getting dash message but NO FAULT codes being thrown by the BNS module or anything else.

Voltages are suspiciously low. With engine running at idle, only 12.3 at cigarette lighter socket. SDS says 12.1v on the title bar, and when I go into the BNS module and look at actual values, it says

#180 Terminal 30 11.6
#181 Terminal 30a 12.7

I saw a brief red error message flash abouts the braking system that went away...I presume a low voltage glitch.

Thoughts?

Is the alternator bad? Any fuses I should double check?

Darren
 
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#3 ·
Darren - You're only about 30 miles from me. :)
The errors sound like they are all related to low voltage. A fully charged battery will allow you to drive 10 miles, but when the voltage drops you get the errors you see, including the "NO BRAKES!" error which is an electric pump.

Sounds like a bad alternator (which was my problem), or regulator, or field voltage to the alternator (maybe a fuse?), or a bad 100 amp fuse under the passenger floor mat.
The voltage at the rear battery should jump to 13.5-14 volts after startup if the alternator is working.

I'm not sure about testing the alternator in the car, as they may not be able to, but it can be pulled in about an hour.
Take the belt off, disconnect the batteries (rear, then front), remove the diaper panel underneath, disconnect power lead from alternator, pull the alternator field power plug, unscrew a couple bolts, pull alternator out.

Any parts store can then test it for function.
I got an Autozone lifetime warranty alternator, which failed about 3000 miles later. So I got another free one, but I'm not sure how long it will last.
Keep us posted on the results.
 
#4 ·
Consider the voltage regulator. I had an issue with low charging voltage to the rear battery, resulting in alarms to stop car and brake failure! Fairly easy to pull regulator (from under car), and see that brushes were ~1cm shorter than new. I surmised lower alternator output was being directed to starter battery at the expense of charging the rear battery.
 
#5 ·
status update. It was the alternator. $400 parts and $200 labor. The mercedes indy said he replaced over a dozen alternators this week alone...a bizarre coincidence or something's up here in Colorado Springs.

Thanks for the advice kbob999. I figured labor would be minimal and I wanted to try out the indy I found. They fixed it SAME DAY!
 
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