Starter Motor

In this section of the course we looked at how the starter motor works and what its meant to do in a car. The only function the starter motor performs in the car is turn over the engine to get it started. That's it. The starter motor we looked at was the Pre-engaged Starter motor.

How does it work
How this works is that the pinion gear is spinning very slowly and engages with the ring gear on the flywheel before it starts to turn over the engine. Hence the name pre-engaged starter motor. This is good because the pinion gear suffers less wear as it is engaging with the ring gear. Behind the pinion gear is a one way clutch. This clutch ensures that the engine in turned over by the starter motor and not the engine turning the starter motor. This unit does this by locking up whenever the pinion gear is spins the other way. The pinion gear and the one way clutch are mounted on the armature shaft. The armature is a group of conductors that are mounted around a iron core in a cylindrical manner and are supported on a central pivot. Attached to this is the commutator. A commutator is a rotating switch that supplied the armature conductors with their own positive and negative. (Brushes supply the commutator with the positive and the negative). This creates a magnetic field around the conductors. Around the commutator attached to the inner body of the starter motor is the field coils. The field coils consist of soft iron pole shoes with heavy conductors wound around them. Current flows through this winding to produce a magnetic field. the pole shoes then intensifies this magnetic field making it stronger. This magnetic field then reacts with the magnetic field produced around the conductors causing the conductors to move. This then creates a motor. But the current through the battery first flows through the solenoid. 

Here's a diagram to help explain how the solenoid works.

  
When the starter switch is off, voltage is supplied to the battery side of the starter switch and the B terminal of the solenoid. When the starter switch in on the current flows through the hold in wind and to the earth creating a magnetic field. At the same time current flows through the pull in winding, the M terminal, then via  starter motor through the field coils, through the brushes, through the armature, through the negative brushes to earth. This produces a strong magnetic field around the pull in winding and turns the starter motor slowly. (this is the time when the pinion gear slowly attaches to the ring gear on the flywheel). The plunger moves up connecting the B terminal to the M terminal. At this point the pull in winding is turned off because of the equal pressure (voltage) at each end of the winding. The heavy current can now flow directly from the battery to B via M to the starter motor. (The starter motor then turns over the engine).

This is how the starter motor works.

Starter Motor Circuit Testing (on car)  
Before we test the circuit the first thing we have to do is to check the OCV of the battery. The OCV of the battery has to be above 12.4V (50%charge). After the OCV is recorded the EFI fuse (electronic fuel injection) is taken out. This is because all the testing is done when the starter motor is cranking the engine. By taking the fuse out we ensure that the engine doesn't start and we can keep cranking the engine and perform the tests. (when the engine turns on the starter motor stops working). The second thing you do is record the cranking voltage of the battery. The is the reading of how much voltage the battery is actually supplying while cranking. The figure should not be less than 9.5V. If is's lower than 9.5V it means that the starter motor is not receiving the voltage it requires. Therefore by receiving less voltage the starter motor will under perform (not work properly). You then need to check the voltage drops on the starter circuit. Three readings are taken.
1st from the battery positive post and solenoid starter stud (while cranking)
2nd accross the solenoid main input and output terminal stud (while cranking)
3rd between the battery negative and starter motor body (while cranking)
The maximum voltage drop allowed is 0.5V. Any higher and the starter motor wont receive the proper amount of voltage it requires to operate. 

We also check the current being drawn by the solenoid. The spec for this is between 120 to 150Amps. It is not a bad thing if the starter motor draws less current, all it means is that it is easier to get the starter motor working. There's nothing wrong with a lower reading. But if the solenoid is drawing too much current it means that it is harder to get the starter motor to work and this means that there is an internal fault in the motor. This internal fault is causing resistance within the motor. This resistance will weaken the magnetic field inside the starter motor and therefore resulting in more current (amps) to be drawn.    

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