Hello everyone,

this has happened to me for the second time in 2 years. This time I was on my way back from Munich this Thursday. Day driving was fine, A/C wasn't even on as it wasn't very hot. As the evening came I turned on my lights. So far so good. It started to drizzle a bit, I turned on the wiper. The first thing I noticed was the arm was rather slow. Hmm. At that very instant I didn't realise something was wrong, I only thought - uhm, how interesting! Then the radio turned itself off, soon followed by ABS and airbag control lights turning on. Then RPM gauge followed. That was when I knew my accumulator wasn't being charged. Just as the last time. Even before that, in the afternoon, I called my MB shop for an appointment because at idle the engine was shaking terribly. Something has happened to the electronic idle control. Now I know it was the low voltage in the system. (It's OK now with a charged accumulator). I was running from accumulator for half a day when it gave up on me in the evening when I turned on the lights. It is a 100 Ah Varta.

I took the alternator out (Valeo btw, MB didn't even bother to stock the car with a decent Bosch!). Sure thing, one of the brushes was used up - too short, no contact. The replacement was less than $2.

What surprised me was that for the second time no warning light came up. How is this possible? The light works when tested before starting the engine. Is there some special electronic rig to activate that light when there is no charging of the accumulator or is the light connected directly to the voltage regulator on the alternator and the voltage regulator itself decides whether to signal a no charge situation?

The regulator works fine, I checked with a voltmeter. The regulator connects to the alternator through these 2 brushes only. In this case the regulator would be dead and could not signal a no charge situation. Provided it is the regulator that tells the control light whether to light up or not.

So, how does the system know it is not charged and on what condition and what part lights the warning light?

Thank you very much gentlemen.

I know it sounds crazy, but the alternator light has no circuit when the brushes wear out. Unlike other brand cars I've had where if the light came on, the problem was most likely worn brushes.

As far as I'm concerned this is a definite shortcoming.

Have a great day,

This situation is the reason I like a voltmeter instead of the light.
Same scenerio has happened to me 8 -10 times over the years.