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.
The first thing to check is the battery light on the instrument cluster. This light bulb is part of the alternator field circuit; if it burns out the alternator will not charge. Simply turn the key on and see if it illuminates briefly then goes off. If the light does not illuminate, you'll need to replace it before continuing.
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 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, 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.
The alternator is located on the lower right side at the front of the motor. If your Mercedes still has the under engine tray you will need to remove it before you begin as well as safely jack up and support the vehicle. You are going to be removing the alternator through the bottom of the vehicle and will need to have room to work. You will also need to wear safety glasses, since you're working under your car. Please see our articles on safely jacking up and supporting your vehicle as well as removing the under trays.
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Time: 2 hours
Tab: $240 to $370
Tools: 15mm, 13mm, 8mm sockets, :www.pelicanparts.com/Mercedes..., screwdriver:www.pelicanparts.com/Mercedes...
Parts Required: New or rebuilt alternator:www.pelicanparts.com/cgi-bin/...
Hot Tip: ALWAYS disconnect the battery before beginning
Performance Gain: Car charges again
Complementary Modification: Change serpentine belt:www.pelicanparts.com/techarti...
Applicable Models:www.pelicanparts.com/catalog/...
Mercedes-Benz 260E (1987-89)
Mercedes-Benz 300CE (1988-93)
Mercedes-Benz 300E (1986-93)
Mercedes-Benz 300TE (1988-92)
Mercedes-Benz E320 (1994-95): www.pelicanparts.com/catalog/...