The alternator charges the battery and provides your car with a constant source of electricity while the engine is running. Over time, the alternator will begin to fail and cause trouble with the various electrical devices in the car. If you suspect alternator trouble, you need to check to see that it is operating correctly, and is indeed the cause of the problems with your charging system. Sometimes, bizarre electrical problems can be caused by a number of faults other than the alternator. It's important to troubleshoot the system prior to replacing your alternator.
Inspect the belt that drives the alternator. Is it tight and amply turning the alternator? If not, then check that the belt tensioner is working correctly. Modern belts seldom break, but they get brittle and glazed with age, and can slip on their pulleys. Replace the belt with a new one as required.
The next item to check is the voltage at the battery. Before performing any tests of your alternator, charge and test your battery. If the battery is faulty, your alternator tests will not be accurate. This should read a little more than 12 volts with the engine off. Next check the voltage of your charging system under load, the voltage should be within 13.2: 14.5 volts d/c. To load, have the engine running at idle, turn on the headlights and the HVAC blower motor. Never disconnect a battery cable while the engine is running to test the alternator, you may cause damage to the alternator or other electrical components from the surge in amperage. If your battery appears to be leaking, then your voltage regulator has probably failed.
The battery will usually only leak acid if it has been overcharged at a much higher voltage. If the voltage measured at the battery is more than 14.5 volts when the engine is running, then the regulator is probably bad. If your battery has boiled over and has acid overflowing out the top, make sure that you clean up any spilled acid immediately. Dousing the area with a water and baking soda solution should help considerably to neutralize the acid and prevent it from eating away at the metal.
An important item to check on your car is the engine ground strap. The engine is electrically isolated from the chassis by rubber motor mounts. If the engine ground strap is damaged or disconnected, you will have problems, including electrical system malfunctions and/or difficult starter cranking.
If you've checked all of these things and you still have charging problems, it's likely the alternator will need to be replaced.
You will need to safely raise and support the vehicle; please see our article on safely raising and supporting your vehicle for additional instructions.
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Tab: $400
Tools: E12 Torx:www.pelicanparts.com/Mercedes..., 13mm socket:www.pelicanparts.com/Mercedes..., screwdriver:www.pelicanparts.com/Mercedes..., floor jack, jack stands:www.pelicanparts.com/Mercedes..., wheel chocks:www.pelicanparts.com/cgi-bin/..., safety glasses:www.pelicanparts.com/Mercedes..., torque wrench:www.pelicanparts.com/Mercedes...
Parts Required: New or rebuilt alternator:www.pelicanparts.com/catalog/...
Hot Tip: ALWAYS disconnect the battery before beginning
Performance Gain: Car charges again
Complementary Modification: Change the drive belt: www.pelicanparts.com/techarti...
Applicable Models:www.pelicanparts.com/catalog/...
Mercedes-Benz C350 (2008-15)
Mercedes-Benz GLK350 (2010-15)