7.2 Starting Circuit

The internal combustion engine is not capable of self-starting. Automotive engines (both spark-ignition and diesel) are cranked by a small but powerful electric motor. This motor is called a cranking motor, starting motor, or starter.

The battery sends current to the starter when the operator turns the ignition switch to start. This causes a pinion gear in the starter to mesh with the teeth of the ring gear, thereby rotating the engine crankshaft for starting.

The typical starting circuit consists of the battery, the starter motor and drive mechanism, the ignition switch, the starter relay or solenoid, a neutral safety switch (automatic transmissions), and the wiring to connect these components.

Starter Motor

The starting motor converts electrical energy from the battery into mechanical or rotating energy to crank the engine (Figure 7-18). The main difference between an electric starting motor and an electric generator is that in a generator, rotation of the armature in a magnetic field produces voltage. In a motor, current is sent through the armature and the field; the attraction and repulsion between the magnetic poles of the field and armature coil alternately push and pull the armature around. This rotation (mechanical energy), when properly connected to the flywheel of an engine, causes the engine crankshaft to turn.

Figure 7-18 — Starter.

Starting Motor Construction

The construction of all starting motors is very similar. There are, however, slight design variations. The main parts of a starting motor are as follows:

The armature shaft supports the armature assembly as it spins inside the starter housing. The armature core is made of iron and holds the armature windings in place. The iron increases the magnetic field strength of the windings.

The commutator serves as a sliding electrical connection between the motor windings and the brushes and is mounted on one end of the armature shaft. The commutator has many segments that are insulated from each other. As the windings rotate away from the pole shoe (piece), the commutator segments change the electrical connection between the brushes and the windings. This action reverses the magnetic field around the windings. The constant changing electrical connection at the windings keeps the motor spinning.

The brushes ride on top of the commutator. They slide on the commutator to carry battery current to the spinning windings. The springs force the brushes to maintain contact with the commutator as it spins, thereby no power interruptions occurs. The armature shaft bushing supports the commutator end of the armature shaft.

The pinion gear is a small gear on the armature shaft that engages the ring gear on the flywheel. Most starter pinion gears are made as part of a pinion drive mechanism. The pinion drive mechanism slides over one end of the starter armature shaft. The pinion drive mechanism found on starting motors that you will encounter is of three designs: the bendix drive, the overrunning clutch, and the dyer drive.

Figure 7-19 — Field winding configurations.

The two windings, parallel (the wiring of the two field coils in parallel) increases their strength because they receive full voltage. Note that two additional pole shoes are used. Though they have no windings, their presence will further strengthen the magnetic field.

The four windings, series-parallel (the wiring of four field coils in a series-parallel combination) creates a stronger magnetic field than the two field coil configuration.

The four windings, series (the wiring of four field coils in series) provides a large amount of low-speed torque, which is desirable for automotive starting motors. However, series wound motors can build up excessive speed if allowed to run free, to the point where they will destroy themselves.

The six windings, series-parallel (three pairs of series-wound field coils) provides the magnetic field for a heavy-duty starter motor. This configuration uses six brushes.

The three windings, two series, one shunt (the use of one field coil that is shunted to ground with a series-wound motor) controls motor speed. Because the shunt coil is not affected by speed, it will draw a steady heavy current, effectively limiting speed.

There are two types of starting motors that you will encounter on equipment: the direct drive starter and the double reduction starter. All starters require the use of gear reduction to provide the mechanical advantage required to turn the engine flywheel and crankshaft.

Direct drive starters make use of a pinion gear on the armature shaft of the starting motor. This gear meshes with teeth on the ring gear. There are between 10 to 16 teeth on the ring gear for every one tooth on the pinion gear. Therefore, the starting motor revolves 10 to 16 times for every revolution of the ring gear. In operation, the starting motor armature revolves at a rate of 2,000 to 3,000 revolutions per minute, thus turning the engine crankshaft at speeds up to 200 rpm.

The double reduction starter makes use of gear reduction within the starter and the reduction between the drive pinion and the ring gear. The gear reduction drive head is used on heavy-duty equipment.

Figure 7-20 shows a typical gear reduction starter. The gear on the armature shaft does not mesh directly with the teeth on the ring gear, but with an intermediate gear which drives the driving pinion. This action provides additional breakaway, or starting torque, and greater cranking power. The armature of a starting motor with a gear reduction drive head may rotate as many as 40 revolutions for every revolution of the engine flywheel.

Figure 7-20 — Gear reduction starter.


A starter motor’s operation is dependent upon the type of drive it contains. Below are the three drive systems, along with an explanation of the operation of each.

The Bendix drive relies on the principle of inertia to cause the pinion gear to mesh with the ring gear (Figure 7-21). When the starting motor is not operating, the pinion gear is out of mesh and entirely away from the ring gear. When the ignition switch is engaged, the total battery voltage is applied to the starting motor, and the armature immediately starts to rotate at high speed.

Figure 7-21 — Bendix drive starter.

The pinion, being weighted on one side and having internal screw threads, does not rotate immediately with the shaft but because of inertia, runs forward on the revolving threaded sleeve until it engages with the ring gear. If the teeth of the pinion and ring gear do not engage, the drive spring allows the pinion to revolve and forces the pinion to mesh with the ring gear. When the pinion gear is engaged fully with the ring gear, the pinion is then driven by the starter through the compressed drive spring and cranks the engine. The drive spring acts as a cushion while the engine is being cranked against compression. It also breaks the severity of the shock on the teeth when the gears engage and when the engine kicks back due to ignition. When the engine starts and runs on its own power, the ring gear drives the pinion at a higher speed than does the starter. This action causes the pinion to turn in the opposite direction on the threaded sleeve and automatically disengages from the ring gear. This prevents the engine from driving the starter.

The overrunning clutch provides positive meshing and demeshing of the starter motor pinion gear and the ring gear (Figure 7-22). The starting motor armature shaft drives the shell and sleeve assembly of the clutch. The rotor assembly is connected to the pinion gear, which meshes with the engine ring gear. Spring-loaded steel rollers are located in tapered notches between the shell and the rotor. The springs and plungers hold the rollers in position in the tapered notches. When the armature shaft turns, the rollers are jammed between the notched surfaces, forcing the inner and outer members of the assembly to rotate as a unit and crank the engine.

Figure 7-22 — Overrunning clutch starter.

After the engine is started, the ring gear rotates faster than the pinion gear, thus tending to work the rollers back against the plungers, and thereby causing an overrunning action. This action prevents excessive speed of the starting motor. When the starting motor is released, the collar and spring assembly pulls the pinion out of mesh with the ring gear.

The Dyer drive provides complete and positive meshing of the drive pinion and ring gear before the starting motor is energized (Figure 7-23). It combines principles of both the Bendix and overrunning clutch drives and is commonly used on heavy-duty engines.

Figure 7-23 — Dyer drive starter.

A starter solenoid is used to make the electrical connection between the battery and the starting motor. The starter solenoid is an electromagnetic switch; it is similar to other relays but is capable of handling higher current levels. A starter solenoid, depending on the design of the starting motor, has the following functions:

The starter solenoid may be located away from or on the starting motor. When mounted away from the starter, the solenoid only makes and breaks electrical connection. When mounted on the starter, it also slides the pinion gear into the flywheel.

In operation, the solenoid is actuated when the ignition switch is turned or when the starter button is depressed. The action causes current to flow through the solenoid (causing a magnetic attraction of the plunger) to ground. The movement of the plunger causes the shift lever to engage the pinion with the ring gear. After the pinion is engaged, further travel of the plunger causes the contacts inside the solenoid to close and directly connects the battery to the starter.

If cranking continues after the control circuit is broken, it is most likely to be caused by either shorted solenoid windings or by binding of the plunger in the solenoid. Low voltage from the battery is often the cause of the starter making a clicking sound. When this occurs, check all starting circuit connections for cleanliness and tightness.

Test your Knowledge

5. What type of starter uses gear reduction within the starter and gear reduction between the drive pinion and the ring gear?

A. Single reduction
B. Double reduction
C. Speed
D. High current

6. What term refers to the center housing of a starter that holds the field coils and pole shoes?

A. Field frame
B. Pinion drive assembly
C. Commutator
D. Commutator end frame