The starter motor field coil assembly becomes an electromagnet when energized forcing the armature assembly to rotate. It can be considered the lungs of the starter motor. If the field coil assembly does not receive electrical energy, then the starter will not operate similar to when lungs do not receive oxygen, then the human body will not function.
The field coil assembly is located inside the starter housing or yoke. It consists of two to four wire wound circles or coils encompassing two to four iron cores. The cores intensify the magnetic field created by the coils when energized and are secured to the inside of the yoke with flat head machine screws from the outside of the yoke.
Field coils can be electrically wound in series with the armature assembly, meaning one right after the other, for strength or torque; in parallel with the armature assembly, known as shunt wound, providing individually separate electrical paths for the field coils and armature assembly which provides speed. Compound wound field coil assemblies are a combination of both types of circuits, series and shunt, which maintain torque and limit the rotating speed of the armature assembly. Depending on design, up to three coils of the compound wound field coil assembly may be in series with the armature assembly, while up to two coils of the compound wound field coil assembly may be parallel to the armature assembly. The shunt coils may also be wound in the opposite direction of the series coils
How does the field coil assembly become energized? From where does the energy come? When the ignition key is moved to the start position, the starter control circuit becomes activated supplying energy to the starter solenoid windings causing the solenoid plunger to close the starter load circuit contacts in the solenoid. When the contacts are closed energy from the battery flows through the positive battery cable, to the battery terminal on the starter solenoid, through the contacts to the starter motor terminal. Depending on the type of starter, i.e., series, shunt, or compound, energy is routed from the starter motor terminal through the field coil and armature assemblies, and then finally to ground on the starter motor housing or yoke
The field coil assembly can be tested under two conditions, in the starter or out of the starter. In the starter is recommended because then the field coil assembly can be checked for a short to the yoke. If the field coil assembly is tested out of the starter, then it will have to be checked for short to ground after it is reinstalled in the starter. The field coil assembly can also be checked for continuity, internal short, excessive resistance, or open circuit while still in the starter.
If this procedure appears to be beyond the capabilities of the vehicle owner or driver, then testing and servicing the field coil assembly should be performed by a professional or ASE Master Certified mechanic. The vehicle would have to be taken to a repair shop that employs these types of mechanics such as A & M Alternator Services located at 2419 E. Jackson St. in Phoenix, Auto Electric Specialists located at 5216 W. Lamar Rd. in Glendale, Village Auto Electric Service located at 19 N. Miller St. in Mesa, All Start Electric located at 13501 E. Chandler Blvd. in Chandler, Jordan’s Automotive Specialists located at 8718 E. McDowell Rd. #3 in Scottsdale, Rob’s Quality Automotive located at 11801 N. Cave Creek Rd. in Phoenix, Scottsdale Pro Tech located at 8245 E. Butheruand Dr. #111 in Scottsdale, and Art’s Family Auto Repair located at 915 W. Hatcher Rd. in Phoenix.
Next issue will cover testing and servicing the field coil assembly.