Lesson 1/Learning Event 2
The lower contact connects to battery voltage as does the operating coil. A resistor is connected
from the battery to the field connection.
The lower contact normally is connected to the center contact because of spring tension. As the
magnetic coil is energized, the movement of the upper and lower contacts will disconnect the
center and lower contacts. As they move further, the upper contact will become connected to the
As the operation begins, the center contact is connected to the lower contact, sending full battery
voltage to the field winding. This will cause the alternator to produce full output.
As the alternator raises system voltage, the force exerted by the magnetic coil increases. This
causes the upper and lower contacts to move, which, in turn, breaks the connection between the
center and lower contacts. The field then receives reduced voltage from the resistor, causing a
corresponding reduction in alternator output. The resulting lower system voltage decreases
magnetic coil force, allowing the lower and center points to come together again. This is a
constantly repeating cycle (many times a second) that serves to limit electrical system voltage. The
magnetic coil force and spring tension are calibrated to maintain the desired voltage, which is
usually approximately 13.2 to 13.8 volts in commercial vehicles.
During periods of light electrical loads, particularly at high speeds, the system voltage may go too
high even with reduced field voltage from the resistor. When this happens, the magnetic coil will
pull the upper contact into connection with the center contact. This will shunt all field current to
ground, causing the alternator to stop producing current.