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An internal combustion (IC) engine converts the chemical energy of fuel to mechanical energy when it is running on the power developed in its own cylinders. An internal combustion engine, however, cannot start by itself and needs to be cranked before the four operations of suction, compression, power and exhaust take place. These days, the process of cranking is achieved with the help of an electric starter motor which is also called the self-starter.


Objectives After going through this lesson, you will be able to: Remove the self starter from the engine Disassemble the self starter Inspect commutator, armature, field coil, brush, brush holder, springs, magnetic switch and other mechanical components of the self starter.

i. ii. iii.



The starter motor, shown in Fig. 3.1, draws a heavy current, of the order of 350 ampere, from the battery in order to produce a torque that is sufficient to crank the engine. This heavy current is conveyed to the motor by a heavy duty switch called the magnetic switch or the solenoid switch. In this lesson we will study the overhauling of the self-starter in detail and testing of the magnetic switch.

3.2 • •

Fig. 3.1: Starter motor.

Removal Unlock the bonnet, lift it up and then hook it. Loosen the clamp at the negative terminal of the battery and disconnect the cable from the battery. Unscrew the nut at the heavy duty terminal ‘B’ at the magnetic switch, with the help of a No.12 ring spanner. Fig. 3.2: ‘S’ Terminal.

Disconnect the cable coming from the positive terminal of the battery.

Screw in the nut on the terminal.

The figure shows the ‘S’ terminal of the switch that connects the magnetic switch with the ignition-cum-starter switch which is also called the main switch. •

Pull out the lead coming from the main switch and connected at the low duty terminal which is shorted with the heavy duty ‘M’ terminal of the magnetic switch. Pull out the lead from the ‘S’ terminal. As per color code, these leads are orange and black or yellow respectively.

• • • • •


Loosen the clamp and disconnect the cable from the positive terminal of the battery. Remove the battery from the car. Remove the battery tray. Unscrew the two starter motor mount bolts with socket and ratchet handle or ring spanner and remove the starter motor from the clutch housing. Screw back the mounting bolts.


Fig. 3.3 (a) shows the starter motor assembly and the magnetic switch. On the left side of the motor is the starter pinion which meshes with the fly wheel ring gear in order to crank the engine. In Fig. 3.3 (b) two heavy duty terminals can be seen. These convey heavy current to the motor from the battery for producing sufficient torque to rotate the engine from rest.

Fig. 3.3: Starter motor (a) Complete assembly. (b) Heavy duty terminals.

• • •

Unscrew the nut at terminal ‘M’ of the switch and remove the lead wire going to the motor. Unscrew the two bolts with the help of No. 10 open end spanner to disconnect the magnetic switch from the drive end housing. See Fig. 3.4. Pull out the magnetic switch housing.

Fig. 3.4: Magnetic switch and plunger.

• • • •

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Remove the plunger by disengaging its collar from the shift lever. Unscrew the 2 screws with a screw driver and remove the commutator end cap. Remove the armature plate and the armature spring. Unscrew the two through bolts, holding the commutator end housing and the drive end housing, with the help of a No.10 spanner and tap the commutator end housing with a plastic hammer to remove it from the yoke. Remove the protective insulation strip. Lift the brush spring with a needle and pull out the positive brush from the brush holder, and then remove the brush holder plate from the armature shaft.

Figure 3.5 shows the brush holders and springs with the positive brush removed from the holder. The brushes are made of copper alloy and current from the battery flows in the armature conductors through these brushes and the commutator. •

Remove the yoke from the armature.

Fig. 3.5: Brush holders and springs.

Fig. 3.6: Pole shoes and windings.

The four pole shoes and field windings can be seen in Fig. 3.6. When current flows through the field windings, the pole shoes get magnetized. Magnetic field

generated around the armature conductors, due to the flow of current in the armature conductors, interacts with the main field across the pole shoes and rotates the armature. The armature with the commutator at its end is shown in Fig. 3.7 (a). The other end is the drive end enclosed in the drive end housing. •

Remove the drive end housing from the armature.

Towards the left of the armature are the drive pinion, the overrunning clutch and the shift lever as shown in Fig. 3.7 (b). In front of the pinion is an armature stop ring. To dismantle the armature assembly, the armature stop ring has to be removed first. To do this: •

• •

Push the stop ring back towards the pinion and holding the armature in a vice, between 2 wooden blocks, remove the armature ring which holds the armature stop ring in place. Remove the armature stop ring. Remove the pinion and the over running clutch along with the shift lever and the spring.

Fig. 3.7: Armature and other components.

The spring behind the pinion serves to cushion the axial force of the shift lever. If the teeth of the pinion and the flywheel ring gear butt against each other and do not mesh then the spring gets compressed and as soon as the pinion rotates slightly further, the spring pressure pushes the pinion forward and the teeth of the pinion and the ring gear mesh easily. Fig. 3.8 shows the armature on a steel shaft. Towards the left side on the shaft, the splines on which the pinion moves axially can be seen. On the right hand side is the commutator.

Fig. 3.8: Armature on a steel shaft.

In the following sections, we will study the inspection of the various components of the self starter.

Self-check Questions 1. 2. 3. 4. 5.

What is the magnitude of current drawn by the self starter? What purpose does the magnetic switch serve? The magnetic switch is also called the _____________ What is the function of ‘S’ terminal in the magnetic switch? Starter pinion of the self starter meshes with the ________________ in order to crank the engine

3.4 • • •

Inspection of Commutator Check that the commutator is not dirty or burnt. Clean the commutator with a sand paper of No. 300 – 400 grit size. Check the commutator for uneven wear. For this, the armature is placed on Vblocks on a surface plate, the zero of the dial gauge is set with fine adjustment screw and the armature rotated slowly. If the deflection of the dial gauge pointer exceeds the limit, it should be repaired or replaced. If the deflection is excessive, it is checked that the armature shaft is not bent. A bent shaft must be replaced. Check the commutator for wear. If the outside diameter of the commutator reaches the limit of 27 mm, the commutator should be replaced.

Fig. 3.9: Depth of mica undercut.

Inspect the commutator for depth of mica under-cut. If the service limit has been reached, it should be corrected by increasing the depth or replace the armature. The standard depth is 0.5 to 0.8 mm and the limit of the under-cut is 0.2 mm.

The difference between the proper depth of the under-cut and when it has decreased beyond limit can be seen in Fig. 3.9.


Review of Analog Multimeter

An analog multimeter is used to do the ground test and the open circuit test on the armature. No deflection of the needle on the multimeter scale is interpreted as an open circuit whereas a full deflection of the needle on the scale is interpreted as a short circuit. In between the two extremes, continuity in the circuit is symbolized and the resistance in the circuit determines the position of the needle. The multimeter should first be checked for proper functioning by touching the probe leads together. Since there is no external resistance in the circuit, the multimeter should indicate a short circuit, which appears as a full scale deflection of the needle on the scale.

Self-check Questions 6. Commutator is cleaned with a sand paper of ___________ grit size 7. Depth of mica under-cut in commutator should not be less than ___________ 8. No deflection of the needle on the multimeter scale is interpreted as an ______ _________ whereas a full deflection of the needle on the scale is interpreted as a ________________ 9. When the probe leads of the multi-meter are touched together it shows a _________


Inspection of Armature

3.6.1 Ground Test •

Touch the multimeter probe leads to the commutator segments and to the armature core. If no deflection of the needle is seen, the armature is not shorted and is acceptable. If there is continuity between the commutator and the core, that is, the armature has been grounded and it should be replaced.

3.6.2 Open Circuit Test In the open circuit test, the armature winding is inspected for continuity. •


Touch the leads of the multimeter on commutator segments and check continuity. If an open circuit is detected at any point, that is, there is no deflection, the armature must be replaced.

Inspection of Field Coil

To inspect the field coil, the open circuit test is performed first. For this test, one must refer to the manufacturer’s manual as the requirement for continuity test varies with the design of motor.

In a particular type of field coil, as is shown in the figure, there should be continuity between the positive brush and the field coil lead wire but there should be no continuity between the brush and the yoke. If it is not found to be so, the field windings are open– or short– circuited respectively, and the field coil must be replaced. There should also be no continuity between the lead wire and the yoke.

Fig. 3.10: Field coil.

3.8 •


Inspection of the Brush Check the brush for wear. The length of the brush should not be 12 mm or less, otherwise, it should be replaced. The standard length is 18 mm.

Inspection of Brush Holders and Springs

The figure shows a brush holder plate on which 2 brush holders and 2 springs are clearly visible. The upper brush holder is insulated from the plate whereas the other brush holder, which is below it, is grounded with plate. These brush holders carry the positive and the negative brush respectively. The torsion springs help to push the brush inward over the commutator. •

• •

Check the movement of the brush Fig. 3.11: Brush holders and spring. in the brush holder. If it does not move freely, the brush holder should be checked for distortion and the sliding surfaces of the brush for contamination. Corrective action as necessary should be taken. Check the brush springs for wear, damage or other abnormality. Replace them if necessary. Check for continuity across the two brush holders. Touch the multimeter leads on the brush holders. There should be no deflection as one brush is insulated from the plate and the other brush is grounded. If continuity is detected, the positive brush holder is also grounded due to defective insulation and the

insulation should be replaced. We may also ascertain the continuity between the two brush holders and the plate one by one.

3.10 Inspection of Mechanical Components • • • • • •

Inspect the shift lever for wear. Check that the overrunning clutch locks up when turned in the direction of drive and rotates freely in the reverse direction. Inspect the pinion for wear, damage or any other abnormal condition. If the teeth are worn, the pinion should be replaced. Inspect the spline teeth for wear or damage and check that the sleeve moves freely on the splines of the armature shaft. Inspect the armature shaft bushes in the end housings for wear. The shaft should not be loose in the bushes at either end. Check the sleeve assembly for free movement. Check for wear.

3.11 Inspection of Magnetic Switch • • •

Inspect the plunger surface of the magnetic switch for wear or damage. Install the plunger return spring over the plunger and push it in the switch housing. The plunger should return freely and quickly to its original position. Check the pull-in coil for continuity. Touch the leads of the multimeter on the ‘S’ and ‘M’ terminals of the magnetic switch. A deflection shows that there is continuity between the terminals, that is, the pull-in coil is acceptable. Check the hold-in coil circuit, which is connected between the ‘S’ terminal and the switch case, that is, ground. Touch the leads of the multimeter on the ‘S’ terminal and the case. A deflection of the needle confirms continuity.

During inspection, if any part is found to be unacceptable due to abnormal wear or non-functioning, it should be replaced.

Self-check Questions 10. There should be/should not be continuity between the commutator segments and the armature core. Choose the correct alternative. 11. The minimum length of the brush is ____________ 12. How would you recognize the positive and the negative brush? 13. Continuity between the 2 brushes should be _________ 14. Pull-in coil is acceptable if ______________________ _____________ 15. hold-in coil is acceptable if ____________________________________

3.12 Summing Up An internal combustion engine can be cranked with the help of an electric starter motor which is also called the self-starter. The starter motor draws a heavy current, of the order of 350 ampere, from the battery in order to produce a torque that is sufficient to crank the engine. This heavy current is conveyed to the motor by

a heavy duty switch called the magnetic switch or the solenoid switch. In this lesson we have studied the removal and disassembly procedure of the self starter. We also studied how to test the armature, field and other components of the starter motor including its mechanical components and the magnetic switch.

3.13 Possible Answers to Self-check Questions 1. 350 amp 2. Conveys heavy current to the motor 3. solenoid switch 4. connects the magnetic switch with the main switch 5. flywheel ring gear 6. No. 300 – 400 7. 0.2 mm 8. open circuit, short circuit 9. short circuit 10. should not be 11. 12 mm 12. Positive brush holder is insulated from the plate whereas the negative brush holder is grounded with plate 13. absent 14. there is continuity between S and M terminals of the switch. 15. there is continuity between S terminal and the case

3.14 Terminal Questions 1. Describe the tests of the armature. 2. How would you test the field coils?

3.15 References 1. Maruti 800 Service Manual

3.16 Glossary Armature

A laminated, soft iron core wrapped by a wire that converts electrical energy to mechanical energy as in a motor or relay. When rotated in a magnetic field, it changes mechanical energy into electrical energy as in a generator.


A copper or carbon conductor used to transfer electrical current from or to a revolving electrical part such as that used in an electrical motor or generator.


A series of copper bars at one end of starting motor armature, electrically insulated from the armature

shaft and insulated from one another by mica. The brushes rub against the bars which form a rotating connector between the armature windings and brushes. Commutator segment

The name for the copper segments of the armature of a starter or dc generator.


A material or device that conducts heat or electricity.


Flow of electrically charged particles.


Device used to measure continuity, resistance, current and voltage.


The smaller of two gears. The pinion on the starter drives the ring gear of the flywheel.

Solenoid switch

An electromagnetic switch that uses a movable core.


A point of connection for closing an electrical circuit.