A car starter is a powerful electric motor that cranks your engine to start your car in a hostile environment. The starting system consists of the motor itself, and the attached solenoid. The solenoid is what takes battery power and delivers it to the starter motor. It also pushes the starter gear forward, allowing it to mesh with the gear teeth of the engine’s flywheel.
A high current is needed to start your car, so your battery must have enough “juice” to enable the car starting system to work properly. Most of the time, when a car will not start and the battery is fully charged, the starter motor is to blame. Starters do wear out. They are the most important part of the starting system, or circuit, consisting of the following:
Flywheel Ring Gear
This is a toothed ring that is fitted to the outside of the engine’s flywheel. Matching teeth on the starter motor mesh with this gear to spin the crankshaft.
Starter Solenoid (Relay)
All cars are wired so that the battery’s main cable connects to the starter motor windings (the thick cable is needed for large current flow, right?). This wire must be switched on and off, of course, and it would be costly and inefficient to route it through the ignition switch (not to mention the size of the switch’s components required to carry such current!). Consequently, a relay is necessary…
Relays are devices that utilize a central iron core fitted closely to the inside of a coil of wire. When the wire is energized the iron core will be drawn down the length of the coil, the direction dependent upon the direction of current flow. If the relay’s iron core is fitted with large, high current-carrying contacts it can be used as a high-current switch. Relays are used throughout cars (for horns, electric fans, air conditioning clutches, etc.) and the most important one is the starter solenoid.
The starter solenoid has very large contacts to carry the battery’s full current. Its wire coil is actuated by a smaller current from the ignition switch, at which time the iron core slams down to make contact and turn on the starter motor. Most non-Ford starter motors employ a solenoid built into the motor itself. This type of solenoid not only provides the motor’s electrical power but also mechanically engages the starter’s drive gear onto the flywheel. It is commonly known as the BENDIX type of solenoid. Such solenoids operate in three stages, the disengaged, partially engaged and engaged. In the disengaged position the drive gear is released, and no current is flowing. In the partially engaged stage, current from the starter switch flows through both the pull-in and the hold-in coils. Both coils draw the plunger inward, causing it to pull the shift lever and engage the pinion gear. When the plunger is pulled into the coil all the way, the pinion fully engages the ring gear. Just as the ring gear is fully engaged, engine cranking begins. When the engine starts the hold-in coil will cut out and the plunger will move out, retracting the pinion and opening the starter switch.
A Car Starter Motor
This is a powerful electric motor that engages the car’s flywheel to spin the crankshaft. As in all electric motors, the starter is composed of windings of wire that form loops, ending at the commutator segments (remember these from the generator?). The armature coils are mounted on the motor’s central shaft (supported with bearings) and the field coils are formed into four or more “shoes”, placed inside the steel frame of the starter. Brushes are used to create electrical contact to the commutator segments and when current is fed into two of the four brushes, it flows through all the loops of the armature and shoe windings and out the other two brushes. This creates a magnetic field around each loop. As the armature turns, the loop will move to a position where the current flow reverses. This constant reversal of current flow allows the armature and field coils to repel each other and spin the motor. The greater the current flowing in the coils, the greater the magnetic forces, and the greater the power of the motor.
The copper loops and field windings are heavy enough to carry a large amount of current with minimum resistance. Since they draw heavy amounts of current, they must not be operated on a continuous basis for longer than 30 seconds. After cranking for 30 seconds it is wise to wait a couple of minutes to let the starter motor dissipate some of its heat. Starters heat quickly, so prolonged use can cause serious damage. A typical symptom of overheating starter motors is extremely slow, labored engine-cranking.
Various wiring designs are used in starter motors and one of the most popular is the four pole, three winding setup. Two of the windings are in series with themselves and the armature. One winding does not pass through the armature but goes directly to the ground. This Shunt Winding aids with additional starting torque. However, as the starter speed increases, the shunt still draws a heavy current and tends to keep starter speed within acceptable limits.
Starter motors fail mostly due to overheating. They are placed in a hostile, hot environment and cannot be expected to last indefinitely. Another mode of failure is a shorted or open winding. This exhibits itself as a “dead spot” on the commutator. If a brush lands on a dead spot the motor will not turn at all.
A third failure-mode is a faulty pinion engagement. Sometimes the pinion assembly gets stiff or stuck due to lack of lubrication or wear. Starter motor rebuilding or replacement is required for all these problems.
Before doing so, however, check to make sure the electrical connections on the starter (and the battery) are clean and tight. Most failures to start are due to lose or corroded battery cable connections or low-current solenoid connections, not to faulty starters.
Finally
A we have discussed previously, sometimes a car starter will simply go bad. This can be due to worn brushes, bad bearings, and various other problems, many of which will require the services of a mechanic to properly diagnose. Understanding how a car starter works will help you to diagnose the problem when it occurs, because at some point it will.
