I have a 1996 S420 (W140) that had the red battery indicator on the dash light up for about 5 minutes. I thought that that indicator meant that the alternator was bad...but checking the voltage it's at 14.1 - 14.3 with many of the cars lighting and accessories turned on. The battery is indicating just above 12 volts when the car is shut down.
My question is, why would the indicator light come on for only a few minutes? is it a battery going bad? or is it an alternator going bad?
Any ideas...

I'm still looking for some guidance...

You may have an intermittant short.

Do you remember using any electrical device at the time?

Try to isolate parts of the loom by pulling non-essential fuses and drive around for a bit. If it doesn't happen, start putting them back in again one-by-one. If it happens, check that the red light corresponds with a low voltage reading across the battery.

You may find that it was a one-off, caused by say, going through a puddle and soaking the belt.

If it doesn't happen again, I wouldn't loose sleep over it, life is full of unexplained events............

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Paul Gibbons
'93 320CE
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike

Thanks for your reply...it was a strange the way it came on then dissappeared so quickly. You can monitor the voltage through the climate control pushbutton and even this morning, everything looked great.
It just makes me a bit aprehensive because a bad alternator means that there is a high probability that I'll be broken down on the highway...and that would be a royal pain in the butt.
Thanks again

There are two possibilites which explain your intermittant change indication.

Firstly, that light will come on prolonged when battery needs substantial charging but isn't getting it. (you knew that but why?). But if your battery were quite (95%) discharged and your rpm is low and/or the alternator is busy furnishing current to headlights or other systems and was unable to provide substantial charging current to the battery for several minutes. Recall that a you may have a 70 amp alternator but you can have car circuits demaning 70 or 80 amps . Part of this possibility, your battery may have had a lead denderite shorting two plates which later cleared under charging current. Also, my F-car, which I do not drive daily, the charge light come typically for a few minutes on first use, very normal.

Second possibility - there are 3 diodes in the alternator which convert ac to dc. Very often, one of these will fail and although the alternator retains some ability to charge, the current is substantially reduced and you find out later that you need a new alternator. That later can be a month. If the diode fails shorted, it can discharge the battery in hours. Also, you may have a condition whereby one of those diodes is intermittant.

The situation calls for replacement of both alternator and battery. Or if you aren't working for the sultan, continued observation to see which one needs attention. Intermittants only get worse, usually waiting for the most inopertune moment, such as being later for an early morning flight.

Incidently, electrical things work because they are filled with smoke. I know this because when I see smoke coming out of them they quit! Sorry, this book needs an index.

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1993 MB190e 104k

Thank you for your reply...I haven't seen a red light since...but I never saw it before. I think that you are probably correct. The Alternator is acting up and probably needs some attention before I end up at the side of the road. I plan to get to it in the next few weeks. Hopefully that's not too late.

ckroeger,

Throw a spare battery in the boot for peace-of-mind, and forget about the problem until it shows itself again.

At least you won't be stranded..............

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Paul Gibbons
'93 320CE
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike

Very good idea...thank you.