Note: Stretch did a write up on rebuilding an alternator a while back.

Here is how it works:
The shell of the alternator is wound with three circuits of copper wire spaced 120 degrees apart. This is called the stator.

If you were to take a bar magnet and rotate it inside the three coils of wire, then an electrical current would be produced in each of the three coils. Each coil produces a ac voltage 120 degrees offset from the others. As one reaches its positive peak, another is heading negative towards its negative peak and the third has passed its negative peak and becoming more positive.

You may ask why the three coils, that is because it is the most efficient.
And you may ask why are we making ac when we need dc for the battery, the answer is there are diodes that turn the three ac waveforms into one dc output. Using the three coils results in a much smoother output, (closer approximation to pure dc) than trying to convert one ac coil to dc.

Well now we have a voltage but no way to control it. So instead of a bar magnet we use a coil of wire on the rotating part, called the rotor. By varying the current through the rotor coil we can adjust the strength of the magnetic field and thus the output voltage at the stator.

This is where the regulator comes in. There are 3 connections on the regulator. The first two are the brushes, which touch the slip rings on the rotor coil. Slip rings allow the current for the rotor coil to transfer onto the rotating coil from the stationary regulator. The third connection on the regulator is battery voltage. It goes from the battery through the alternator light bulb to the regulator. This is the small wire in the 3 wire alternator harness, the two fat wires go to the battery and carry the output from the armature. (There are two thinner output wires instead of one fat wire since that absorbs the flexing better between the engine and the body.)

The regulator takes the voltage from the battery and does two things. First it measures the voltage level on the thin wire to know where to set the output voltage so it will correctly charge the battery (and not fry the electronics on board). Second it takes that battery voltage and uses it to create a controlled amount of current to excite the rotor coil. The amount of current sent from the regulator, through the brushes and slip rings, into the rotor coil, is directly proportional to the desired output voltage.

If the alternator is not charging then a voltage difference exists across the charge warning bulb so it glows. Once the alternator is charging, the voltage is equal on both sides of the bulb so the bulb does not glow.

Once the alternator gets up to speed there is an auxiliary stator winding and another set of diodes that is used to feed the regulator in addition to the battery feed through the charge warning bulb. That is why the alternator can produce power at high rpms but not at idle if the charge lamp is missing or burnt out.

I play with huge ones at work, the big boys work just like the one in your car except:
- we have a honking 60 L, 16-cylinder, quad turbo Diesel engine for a prime mover, the typical electrical output is 2.5 MegaWatts, your house uses less than 5 kiloWatts on average. The typical data center has no less than two, and sometimes up to 16 or more, of these generators.
- the engine typically burns 150 gal/hr at full load
- we don't use diodes, we want to stay with a 3 phase ac output (usually 480 V but sometimes 4160 or 13800 V)
- and I can assure you that if the regulator does not excite the rotor, we get no electricity...

My 124's alternator had bad bearings. Took it to a local motor shop and they were able to replace the bearings and the regulator since the brushes were almost gone, and it was about $50.

Final thought, if your brushes are wearing down, replace the regulator before the brush holders dig into the slip rings. If those get damaged severely, the alternator can become scrap metal...

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech