I know there are a few threads on alternator and voltage regulator replacement but suggest just replacing it. I was over 200 miles from home and it became a two day adventure, don't let it happen to you.


If you have 70,000 or more miles and have not replaced your voltage regulator, just do it. Less miles if you race a lot. Eventually it will definitely wear out and will fail. Then you may have to deal with a day or two being ruined and over paying for the repair. For ~$20 and an hour or two to replace the voltage regulator you can possibly save yourself a lot of time and money later.


I have a C55, not sure if all Mercedes have replaceable voltage regulators but I suggest finding out. I googled/binged AMG C55 voltage regulator and got a bunch of very cheap options. If I still have the car in another 50k miles I'm swapping it again.


I swapped mine out in car by disconnecting the battery, pulling the alternator which required pulling the belt (17mm socket and nail/pin/small screw driver), unbolting the alternator (E12? socket), spinning it so the back is up, removing a few screws and 15mm nuts, installing the new one and putting everything back. Supposedly you can do it faster by removing the belly pan and replacing it from under the car but I had lots of tools but no jacks or stands when mine went out. Completely replacing the alternator requires more work than either option as it deals with cooling lines and fan shrouds as well. There are threads that detail these other options, just search the forum.


IN CASE IT GOES OUT, check the voltage and use it as a "range".


Key in, make sure display shows Mileage screen.
Key in, 1 click to position 1
Click the button to the left of the gauge cluster 3 times
you will see UB and a voltage #. 13.6-14.2 and holding steady is good. Less and falling is bad.
As you drive the volts will slowly slip down. I took it to as low as 10.4 volts and made it ~35 miles. We also charged it multiple times with another vehicle and watched the volts get up to 12.8 again before going again.


Most auto parts stores can and will check the system for free.


If you are not far from home and think you can make it there or to a shop make sure to turn everything off and try not to touch anything, even the brakes and blinkers, unless needed. If its night all of the lights will help to drain the battery quickly. Just find a safe place to park and call a tow truck.

 

So it was the voltage regulator...glad you figured it out.

I'm at 97k and about to go on this driving trip to WV. Have no idea if the previous owner ever replaced it or not. Luckily I've got one in the spare tire well along with my socket set so I hope I'm alright