HELP, I think my alternator is gone
MBWorld Fanatic!
Joined: Jan 2005
Posts: 2,848
Likes: 15
From: Orange County
2005 C Wagon (No snickering please!)
There is no relay in the alternator or the charging circuit. The alternator has an integrated diode bridge and regulator. The regulator measures the battery terminal voltage and supplies current as necessary by passing a small current through the rotating coil in the alternator. The current creates a magnetic field that generated a higher current in the fixed coils of the alternator. The regulator is a solid state type, and active components are subject to failure. It is simple to replace the regulator assembly. Other failure modes are bearings screaming and the stator brushes wearing out. Very seldom does the fixed coil go open or short.
As the battery reaches full charge the alternator reduces current in the field coil and charging stops. At this point the internal resistance of the battery keeps the voltage near the nominal 12.6 volts. When running the car needs 25~35 amps. The alternator supplies this to keep the charge in the battery at maximum. The regulator monitors the battery and adjusts the current in the field coil to keep up with the consumers in the car. e.g. play the radio, open windows, move seat, run the wipers, etc.
The battery is the chief supplier of current while parked, starting, & driving.
