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C230 Mercedes - Battery/Altenator warning

Hello guys… I’m puzzled on how to proceed on this one. Basically, car works fine… but at least once a day it gives me a “BATTERY ALTERNATOR- VISIT WORKSHOP” message… only pattern I see is that its usually cold out, but it could just be a coincidence.

I took the car to a local shop that claims to work only on Mercedes Benz and they tested both the alternator and the battery, and said they couldn’t see anything wrong… they did see computer codes with spikes in voltage once in a while. So they recommended I change the alternator and battery as a precaution!!! What a crazy way to test something! I’m puzzle here… I can’t find solid information on how to proceed with this. Some sites say its the voltage regulator, others say replace the alternator… Any help would be appreciated.

Thank you,


I have the same issue. Anyone have any info?

Especially helpful would be links to step by step pics for DIY alternator or regulator replacement,


Andy in Tampa

Year and miles for this Mercedes Benz?

Voltage regulators are built into the alternators on modern vehicles. It’s been a long time since I saw a car with a separate voltage regulator. This is probably why they’re suggesting replacement of the alternator, but I don’t see any reason to replace the battery if it tested OK.

I’d want more proof of an alternator problem before spending money to replace it. Did they test the alternator under the same conditions that cause the message to appear?

I would suspect a bad connection is the real cause but the alternator may have an intermittent problem internally. The simple and no cost items should be done first. I would first clean the connections to the alternator. If you work on the main output lead of the alternator be sure to remove the ground lead to the battery first. The main alternator output lead is ‘hot’ (directly connected) to the battery. If the lead makes connection to ground without the battery being disconnected big sparks will fly.

The voltage spikes are something to check into. Someone with a multimeter would put the test probe on the alternator output and observe the dc voltage as different things (accessories) were turned on and off. Then, s/he would change the test meter to check ac voltage with different things turned on and off. Thought would be applied to the results.

The wires going from the alternator to the battery (and from the battery to the alternator) are “hot” —which means they have voltage and carry current.
They are always hot, whether the ignition is ON ,or OFF.

That is true, and it is incorrect to call the ground lead of the battery hot. However, the advise is good. If the OP disconnects the ground cable to the battery, then he can check and tighten the alternator wiring without worrying about his wrench causing any shorts.

I was refering to the alternator lead as being “hot”, not the battery ground lead. The way I stated it could have been better though. I will edit the statement.

When it is the coldest, and the warning light is most likely to come on, take your car to an auto parts store (free), or a shop, and get the battery and alternator checked in the cold. A car which is checked in a warm garage won’t show what is happening to it when it’s in the cold. The battery voltage may be going low because of the cold. When the battery is warmer, its voltage will go up.