Self Reflection

During the 4 week electrical course I think I have come to have a sound knowledge of the electrical systems used in the automotive trade. I feel that I have been more interested in the practical part of the course as it helps me understand the theory side a lot better. My workbooks from the past 4 weeks have shown that I understand the concepts and theories behind Charging Alternator systems, Charging Battery systems, Starting systems (starter motors) and Electrical Components (relays, diodes, resistors, capacitors).
With knowledge taken from my practical classes and Dad (who is an Auto Electrician) I have be able to develop in-depth blogs that explore and discuss the backgrounds of the Electrical systems.

From my timetable you can see that a lot of my time was taken up by my job, however I didn't let myself fall behind too much with my self directed study which I made time for when I could. though it may not show on the timetable but during my breaks at work I usually spent time on reading over notes and workbooks. The task where we got to make our own logic probes was by far the highlight of the course, as it let us use the knowledge we had learnt to apply in a practical construction of a working circuit.

Admittedly my theory test results could of been better but thats just something that you can't help past the fact. I found that my problem solving and practical ability were much better than I originally expected them to be. Though my Dad is an Auto Electrician I've never really taken an interest until now.

Overall I have enjoyed myself during this course and have taken a lot of practical knowledge and understanding away from it.  

Capacitors

the capacitor is an electrical device which has the capacity to hold and store an electrical charge. it is made of metallised plates layered with an insulating substance, rolled up tightly, and fitted inside a metal cylinder. A terminal at one end of the capacitor is connected to the moving contact, and the metal cylinder, acting as the other terminal of the capacitor, is connected to earth. The capacitor holds an electrical charge and releases its stored energy when connected in a circuit. The capacitor is connected across contacts of the contact breaker, and provides an alternative path for the current and accepts and stores and electrical charge from the primary coil until conditions are suitable for releasing it back into the circuit. This stored energy would otherwise cause destructive arcing across the contacts. The current thus ceases to flow, and the capacitor immediately discharges itself through the primary coil in the opposite direction to the flow of induced current. This reverses the polarity of the coil and increases the rate at which the field collapses. this in turn increases the voltage induced in the secondary winding. A capacitor takes time to charge, they charge faster when first connected to a power source and start to slow down as their charge builds up. The size of a capacitors capacitance is measured in Farads (F).

To calculate how fast a capacitor should charge you use the formula
                                           Resistance X Capacitance X 5 = Time required to charge
                                                       (R x C x 5= T)

In one of our practical lessons we had to build a circuit with
                                  1 x resistor
                                  1 x capacitor
                                  1 x bridging wire
                                  1 x voltage source (battery or power supply)

After building the circuit we had to time the capacitor to see how long high it would charge over the course of 3minutes (180secs). Recording the charge every 10seconds on a table (which can be found in my 'capacitor circuits workbook') We then had to graph the results. The results showed that during the first 90seconds the charging was going quite fast, shooting up from a starting voltage of 3.20v to 10.8v, the second half (90seconds) the charging slowed down because the electrical store was getting taken up. Kind of like how when you blow up a balloon and if you blow it up too much it gets harder to blow up, harder to store air. This made the second 90seconds only rise 0.8v as opposed to the 7v in the first 90 seconds. This graph can also be found in my capacitor circuits workbook.  

Relays

A relay is n electromagnetically-operated remote-control switch in which a small current operates a switch which controls a much larger current. examples of relays in a vehicle are the horn relay, starter relay, headlight relay and so on. the relay is usually mounted in a location that forms a short path between the battery and the operating unit. A relay consists of a soft iron core wound with many turns of thin insulated wire. the coil is insulated from the core and one end of the coil is connected to the battery and the other is earthed through, for example, the horn button. There is a soft iron frame around the outside and the core has a springloaded armature across the top. One contact is carried on the movable armature which is connected to the frame and then underneath the battery terminal. the other contact is mounted on a fixed support which is connected to the "horn" terminal. when the horn button is pressed, current is fed to the fine winding around the central soft-iron core to the earthed base of the relay and so back to the earthed terminal of the battery. the armature is attracted towards the core and closes the contacts. therefore when the contacts close, the horn is connected directly to the battery. An advantage of the relay system is that the horn button wire has to carry only sufficient current to energize the relay winding (usually around 0.5-1amp), and therefore the horn button contacts remain in good condition almost indefinitely. another advantage is that the voltage drop in the main horn circuit is reduced because of the shorter length of cable required.
Relays usually have a wiring diagram printed on their cover to show how the relay can be used. Most relays usually have pin numbers on them to designate what they are usually connected to

86- positive side of control circuit

85- negative side of control circuit

30- battery supply for switched circuit

87a- normally closed switch circuit

87- other switch circuit

During one of our practical lessons we had to wire up a circuit and test our relay, we wired up a parallel 3-bulb circuit, the use of the relay was to see if the switching circuit of the relay would turn on all three light bulbs. After wiring up the circuit we had to measure available voltage at the following terminal points when the circuit is off and then when the circuit is on. 

CIRCUIT OFF

86- 12.5V

85- 12.5V

30- 12.5V

87a- 12.5V

87- 0.0V

CIRCUIT ON

86- 12.4V

85- 0.01V

30- 12.4V

87a- 0.0V

87- 12.4V

There are three terminals where the most voltage change is seen, these are terminals 85, 87 and 87a. This is because when the circuit is off it is a closed circuit so voltage flows through it, because no components are using an current, so when you turn the circuit on there is current being used by the components so there is voltage changes in the relay terminals.

Testing Diodes

Diode testing.

To test a diode you need to use a multimeter set to Ohms range. A working diode should only allow current to flow in one direction; Anode to Cathode. The most reliable way to test a diode is with the ‘Diode test’ position on your multimeter. In this position a higher voltage is used and the meter shows the voltage drop required to push through the boundary layer.

When testing diodes in the ohms range, a measurement of more than 2K needs to be set, because 2K is not enough to measure the diode, you will only get a reading of infinity. Using the diode test range again only work in Anode-> Cathode direction, because Cathode -> Anode is blocked.

we built a circuit with a 1K resistor and use a 12v battery supply and measured the voltage drops across the resistor and the Diode, the voltage drop across the resistor was around 12.7v, the VD across the diode was 0.65v, the amp flow through the diode was 0.01amps. we then took a voltage reading from the battery which gave us13.39v, lastly we added the voltage drop across the resistor and the diode- VDr + VDd= 13.35V. The rules of electricity apply here because the available voltage from the supply was equal to the voltage drop across the resistor and the diode, which are the components of the circuit, and in a series circuit like this was, the voltage is maintained through-out the circuit.

If you swap the diode for an LED you get a bigger voltage drop across the LED because the LED uses more voltage then the diode because it has to light up, whereas the diode doesn’t have to. Apart from that everything else in the circuit is the same.

Identifying Resistors

Identifying, testing and combining resistors

Resistors are a piece of apparatus used mainly to add resistance to a circuit. Resistors are identified by a code using the colour and position of the bands on the resistor itself. To calculate the total resistance a resistor can hold, you use the colour coded bands to figure it out. The first two or three bands are the numbers to write down. The next band is the multiplier (how many zeros to add to the number) a gold multiplier makes one decimal place smaller, silver makes two decimal places smaller. The last band is the tolerance values. So if I had a resistor with the colour coding of green, blue, brown and gold, by using my table I can figure out this resistors low and high tolerance values.

·         GREEN=5

·         BLUE= 6

·         BROWN= 1 ZERO

The gold band is the indicator of which end to read from. So my value to start off with for figuring out this resistor is 560, now to calculate the low tolerance value you must minus 5% of that 560 from 560, so 5% of 560 is 28, so I minus 28 from 560, which gives me the low tolerance value of 532. To find out the high tolerance level, I add that 28 (5%) back onto the original 560 which gives me a high tolerance value of 588. To test that the resistor is working connect it to a multimeter set on Ohms range and if the value falls between that 532 and 588, you have a working resistor.

Charging Systems- Alternator Testing

Alternator output on car testing

Before working on any vehicle to do an alternator test you must do a check for safety and general preliminary checks before starting to test the charging circuit. These checks include

·         Is the park brake applied?

·         Is the vehicle transmission in park or neutral?

·         Does the charge warning light operate?

·         Are the ignition and all accessories turned off?

·         Have you carried out a visual inspection of the connections?

·         Have you checked the alternator mounting?

·         Check the condition of fuses and the fusible links

After inspecting these parameters you can start to carry out the circuit tests on the alternator. Firstly you must determine the condition of the battery by doing an OVC check on it. To check the OVC you place your voltmeter Positive and Negative to the batteries positive and negative terminals. The results you should be looking for are between 12.4v and 12.6v this is equal to 50% and 100% charge. Next, you have to determine the condition of the voltage regulator, the specifications for this is anything between 13.8v and 14.5v. to check the regulating voltage you attach he voltmeter to the battery again, turn the car on and rev to 3000RPM, what you are looking for in this check is a change in the voltmeters reading, as it should go up, if the regulating voltage is too high is would affect the charging system by blowing bulbs in the dashboard. Next you have to determine the ‘No load current’ the specs for this test is anything between 10amps-18amps for a fuel injected engine. To check the no load current you have to turn the car on and rev the engine to approximately 1500-2000RPM, clamp the ammeter to the B wire terminal coming off the alternator.

The next test is determining the voltage and output load; to perform a load test on the alternator you have to have the car running, connect the leads of the load tester to the positive and negative terminals, clamp the ammeter on the lead running from the battery to the alternator. Also the voltmeter must be connected to the battery as well. Turn on all accessories such as headlights, fans etc. What you are expecting to happen is the readings on the meters to increase from the no load test, as there is now load be introduced to the circuit. Lastly you need to perform a charging system voltage drop test. The voltage drops you need to carry out are

·         Positive side volt drop, the spec for this is <0.2v

·         Negative side volt drop, the spec for this is again <0.2v

To carry out the positive side volt drop, attach the positive of the voltmeter to the positive of the battery and the negative of the voltmeter to the alternator output (B+) while the engine is running. For the negative side attach voltmeter negative lead to the battery negative and the positive lead to the alternator body. The reading you would expect from a good system is less than 0.2v. a high voltage drop could affect the system by not letting it charge properly.

The alternator testing is now complete. 

Charging Systems- Alternators

Alternators

An alternator is needed on a vehicle because they are equipped with power consuming accessories such as radios, heaters, horns, lights etc. Because of these components there needs to be a generating system that can produce a considerable output of electrical energy. The construction of a alternator is made up of the following component parts

·         A stationary winding assembly, the stator

·         A rotating electromagnet, the rotor

·         A slip ring and brush assembly

·         A rectifier assembly

·         Two end frames

·         A cooling fan

The stator is composed of a cylindrical, laminated iron core with three sets of windings inserted in slots on the inside. These windings are arranged in such a way that a separate alternating waveform is included in each winding as the rotating magnetic field cuts it. The rotor is an electromagnet consisting of a coil wound on an iron core, which is pressed on a shaft. When current is passed through the winding, magnetic poles are established at the ends of the iron core and shaft. An iron end-piece is fastened on each side of the coil assembly so that projections on the end-pieces interlace. These projections take on the same polarity as the ends of the shaft on which they are mounted, forming pairs of north and south poles around the periphery of the rotor. The rotor rotates within the stator with a very small air gap between the two parts so that as strong a magnetic field as possible cuts the stator windings and the maximum possible current is induced in the windings. The ends of the rotor coil are connected to insulated slip rings mounted on the shaft. A current supplied from the battery passes through the brushes and slip rings to energise the rotor windings and produce the magnetic field.

Although the alternating current could be used for lighting or heating purposes, direct current is needed to charge the battery, this is why the rectifier is required. The operating temperature of the rectifier must be kept within reasonable limits. To prevent overheating, the rectifier assembly is mounted in a piece of heat-conducting metal known as the “heat sink”. A fan is also mounted on the front end of the shaft which provides a flow of air for cooling the assembly. Alternators are constructed with a three-phase stator winding and six diodes are required to provide full-wave rectification of the current produced.

Alternators are designed to prevent production of a current great enough to cause damage to the alternator parts. The charging current will not rise above the maximum safe output of the alternator irrespective of speed or electrical load, so that no external current regulator is needed. 

Charging Systems- Battery Testing

Battery testing

Before testing any battery you must go through a few simple steps/ tasks to make sure the battery is okay for testing. Firstly you must inspect the battery specifications, these are-

·         The make of the battery

·         The battery number

·         Battery capacity in cold cranking amps (CCA)

·         The type of battery (conventional, maintenance free, gel cell, orbital, etc..)

·         Whether you can get to the electrolyte.

Next you need to carry out visual checks, these include-

·         Are the terminals clean and tight?

·         Does the battery show signs of swelling?

·         Does any further action need to be taken?

Now that you have carried out these checks you can start to test the battery. The first test is checking the electrolyte levels in each cell. To do this you must first obtain a pair of safety glasses, and gloves as the electrolyte in the cells can cause severe damage, if it comes in contact with skin or eyes.  To check each cell you need to be able to get to the electrolyte, meaning, are you able to remove the cell-cover caps. At this stage we are only doing a visual check of the electrolyte (the hydrometer test will come later) so going through the cells one-by-one check the electrolyte levels and record whether they are High, OK or Low.

Once you have done this you can move onto doing a Battery open circuit voltage (OVC) test. To do an OVC test you need your voltmeter, set on 20V, attach your Positive probe to the positive terminal of the battery and Negative probe to the negative terminal of the battery and wait for the reading, hopefully it should fall between 12.4v and 12.6v which converts into between 50% and 100% charge. Anything less and the battery is too undercharged to continue. Though to continue you can start the car to charge the battery, keeping your probes on the battery watch as the voltage comes up to at least 12.6v, if the voltage goes above 12.6v this is called surface charge, which you can get rid of by turning the headlights on for a minute or so or until the voltage hits the 12.6v mark.  

The third test you do is testing the battery electrolyte specific gravity this is where you use your hydrometer. When doing the hydrometer test, you need to remove the cell caps like you did with the visual electrolyte test. Again, you can only do one cell at a time, because you can only check one cell at a time with the hydrometer. So, removing the first cap carefully, by unscrewing it with your fingers (place the cap somewhere nearby as not to lose it) squeeze the top bobbly bit of your hydrometer which expresses all the air out of it and place into the cell, then let go of the top and the hydrometer should suck up all the liquid in the cell. On the side of the hydrometer there is a scale which gives you the specific gravity of the electrolyte. At the same time as reading the specific gravity you must also check the colour of the fluid, it should be clear; if it is murky it means that the plates are disintegrating. The level you are looking for in your electrolyte is around the 1280-1270 mark, this means that the battery has a high electrolyte level. After recording all 6 cells results you need to figure out the specific gravity variation of the battery, to do this you just minus the highest result from the lowest result for example if cell #1 had the lowest reading of 1240 and cell #5 had a reading of 1270 your specific gravity variation of the battery would be 30. The allowable specific gravity variation specification of a battery should be between 25 and 50.

The last test to do is a High rate discharge test or a Load test. Before starting this test you have to determine what load current you are going to apply, which is figured out by halving the batteries CCAs so on a battery with a total CCA of 310amps you would apply a load of 155amps. A voltage has to be held throughout the test, this voltage must be above 9.5v and not over 12.6v, the load must be held for 15seconds if it is not held for 15sec then the test fails. also before using the load tester make sure the load control is off, to do this you just have to turn the centre knob till the needle hits zero. When connecting the load tester to the battery you connect the positive lead to the positive terminal of the battery first, then connect the negative lead to the negative terminal of the battery, apply the specified load (in this example 155amps) by turning the load control knob, wait for the specified time (15secs) and take voltage held and load current readings. When the load tester beeps after the 15secs, immediately turn off the load tester by turning the knob till the needle hits zero, then disconnect the load testers leads in the reverse order.  

Charging Systems- Battery

A battery is a name given to a group of two or more electric cells connected together in series. A 12volt vehicle battery has 6 cells, each of 2volts. A battery does not store electricity in the commonly understood sense of the term. The charging current causes chemical changes inside the cells, and most of the electrical energy supplied is converted chemical energy. When the cells are delivering a current, that is, discharging, the chemical energy is converted

Chemical action in a battery is the interaction of different substances to form new substances. For example, when carbon burns, oxygen has become intimately mingled with the carbon, and a substance is produced which is different from either carbon or oxygen it becomes carbon dioxide.

Electrochemistry is when positively charged atoms can be made to accumulate on a metal plate and negatively charged atoms or groups of atoms on another metal plate, the two plates being immersed in a liquid. The liquid used must be a conductor of electricity. If the two plates are connected by a wire, an electric current flows through the wire. This is the basis of the electric cell.

There are two main essentials for an electric cell:

1.       Two plates must be of different substances and both must be conductors of electricity.

2.       The liquid in which the plates are immersed must be such that the chemical action occurs between it and one or both of the plates when the plates are connected by a conductor.

The liquid in the cells is called the electrolyte, and the plates are known as electrodes.

Most vehicles use lead-acid batteries, the cells in a lead-acid battery consist of positive and negative plates, separators and electrolyte. The positive and negative plates correspond to the positive and negative electrodes of a primary cell. These plates, which are thin, are assembled in sets so as to expose as much plate surface to the action of the electrolyte as possible. Both sets of plates are constructed from lattice-like lead-antimony grids filled with the active material which is forces into the plate grids in the form of a paste. The active material of a positive plate is lead peroxide (PbO2), a chemical combination of lead and oxygen. The active material of a negative plate is form of pure grey lead (Pb). Separators are an essential part of a multiplate cell. They are insulators, and their primary objective is to prevent metallic contact between the alternate positive and negative plates, while permitting free circulation of the electrolyte. They are chemically inactive, being made from various insulating materials. The plates are immersed in an electrolyte- which is a dilute solution of sulphuric acid in water.

How a battery works

Action of a battery on discharge,

When a load is connected to the terminals of the battery, chemical action takes place. In a lead-acid battery the sulphuric acid in the electrolyte combines with the active material of the positive and negative plates, forming lead sulphate in each. The amount of active material converted to lead sulphate is directly proportional to the amount of current flowing. The battery is discharged when there is not enough sulphuric acid left in the electrolyte.

Action during charging

Recharging is brought about by passing a current of electricity through the battery in the reverse direction to the flow on discharge. The charging current causes a reversal of the chemical action, the positive plates being reformed to lead peroxide and the negative plates back to pure grey lead. When all the active material has been reformed, the battery is fully charged.

Measuring the state of charge of a battery

Because sulphuric acid is 1.835 times as dense as water, the electrolyte of a charged battery will be denser than that of a discharged one. Hence, the density of the electrolyte in each cell of a battery provides measure of the batteries state of charge. The relative density of the electrolyte (specific gravity) is measured by an instrument known as a hydrometer. The working principle of the hydrometer is that an object which floats in water will float higher in a liquid of greater specific gravity, such as sulphuric acid. For battery testing the hydrometer is contained as a float in a special syringe which enables the electrolyte to be drawn into a glass tube and its level observed in relation to the scale marked on the narrow steam of the float. 

Starting Systems- Starter Motors Testing

Starter Motor- On car testing

To test a starter motor for faults while it is attached to the engine there are steps you need to take to insure that the testing you do is right and accurate. Firstly you must determine whether the car is equipped with any device that requires a power source for its memory, and if it is, you must make sure no memory is lost before disconnecting the battery, to do this you can do either one of two things, firstly you can attach a 9v battery to the cigarette lighter of the car or attach a memory minder device to the cigarette lighter. This will protect any memory within the car i.e radio stations or keycodes you may need to operate the radio.

The preliminary check before carrying out a starter circuit voltage drop test-

• ·         Has the ignition or fuel injection system been deactivated?
• ·         Has the battery been checked for serviceability?
• ·         Have you selected the right range on your meter?
• ·         Is the vehicles transmission in neutral?

The OVC (open circuit voltage) of the battery must be checked, this is to insure that you will be able to test the starter with sufficient battery power. The OVC of the battery should be between 12.4v-12.6v this converts into between 50% and 100% charge. To do the OVC you attach your meter Positive Probe to Positive battery terminal and Negative meter probe to the negative of the battery terminal and wait for your reading. If you get a reading of over 12.6v this is known as surface charge, this can be drained by turning the lights or fan on for a few seconds or until you get a meter reading between 12.4v and 12.6v

 Once you have established the OVC of the battery you must check the available voltage drop across the battery terminal while cranking. To do this you again attach positive to positive and negative to negative and crank the engine for a few seconds, the reading on your meter should go down  and be less than your OVC of the battery but must be above 9.5v which is the universal Cranking voltage specification (spec) if the cranking voltage fails the battery must be retested to determine its condition before continuing with the test.

The next part of the test is to check the starter circuit for voltage drop. First check the loss between the battery positive post and the solenoid starter input stud while cranking. The spec for this test is less than 0.20volts, anymore and it is a fail. Next you check the loss across the solenoid main input and output terminal studs while cranking, the specs for this is less than 0.10volts, anything more is a fail. Lastly you check the loss between the battery negative and the starter motor body while cranking, the spec for this test is 0.20volts, again anything more is a fail. Now you add the voltage drops together to find the maximum voltage drop. The maximum allowed is 0.50volts, anything higher is a fail and there is something wrong with the starter wiring or magnetic fields.

The last test you have to do is checking the starter motor current draw, the starter current draw specifications is anything between 125 and 175amps. To check the current draw you need to use an inductive ammeter (clamp meter) which is designed to take up to 400amps. To use this clamp meter you must set it on 400 amps DC and push the zero button to zero the meter, once the meter is zeroed clamp it around the positive battery lead anywhere between the positive terminal and the ‘B’ terminal of the solenoid. To get a reading it is easier if you have someone crank the engine for you as it is cranking push the hold button, as this will retain your reading so you do not lose it.  

Starting Systems- Starter Motors

Starter Motor

The most important electric motor fitted to motor vehicles is used for cranking the engine, it is called the starter motor. the main parts of a starter motor are, the Armature, the Field windings, brushes and the solenoid.

Starter motors have to carry large currents neccessary for a high power output, this is why the there are a large number of conductors in the windings of the armature and field magnets, these windings are usually made from thick rectangular copper strips or bars. the number of conductors in each armature coil is often only one, but it is sometimes two. with such a short path for the current, and a large cross-sectional area in the conductors, the starter armature can handle large currents without excessive voltage drop.

since the current in a series circuit is the same in all parts of the circuit, the field windings carry the same current as the armature windings and should therefore not add any resistance to the circuit. field windings are made of insulated copper strip similar to that used in the armature windings. The brushes of a starter motor and the connecting leads must also be able to carry the same current.

The starter circuit, beginning at the live battery terminal goes,

·         Main insulated cable from the battery to the starter switch

·         Switch terminals and contacts

·         Cable from switch to starter

·         Starter terminal

·         Field winding

·         Insulated brushes in contact with the commutator

·         Armature windings with connections to the commutator

·         Earthed brushes

·         Earthed connection of the brush leads

·         Metal casing of the starter mounted on the engine

·         Engine

·         Heavy connecting cable from the engine to the frame

·         Frame

·         Battery earth strap with connections to frame and the earthed battery terminal

·         The battery itself.

Since a high resistance or a break in any part of the starter circuit will affect the operation of the starter, it is obvious that every part of the starter circuit must be maintained in a sound condition. This includes the battery and the terminal connections, as the large starting current must pass through the cells and the electrolyte of the battery.

There are several different arrangements of internal field windings and poles in a starter motor. One of the simplest is with a series winding, where the current is the same in every part of the circuit, both internally and externally. Another common winding arrangement is often referred to as a “series-parallel” starter, although the current travels through the field windings in two parallel paths, the field windings are in series with the armature windings and so that the starter is really series wound and has the characteristics of a series motor. In a series motor the field windings are in series with the armature windings so that when the armature current decreases at high armature speeds the field current, and the strength of the magnetic field also decreases. Thus the torque (turning effort) decreases at high armature speeds in the series motor but not in the shunt motor.

 The shunt wound circuit, where the field windings are connected in parallel (shunt) with the armature windings, which is direct to the battery. Regardless of the load on the armature, or its speed of rotation, the field current remains the same, since normal battery voltage is applied to the constant resistance of the field coils. Thus the strength of the magnetic field remains constant.

The compound-wound starter has both series and shunt-field windings; this is because engines with high compression ratios and large valve overlaps require a starting motor which can develop more torque at higher speeds than the series motor is capable of producing. In this type of starter, three or four field poles are series wound and the fourth one has a shunt winding consisting of many turns of fine wire. The purpose of the shunt winding is to maintain the magnetic field at a more constant strength than is possible in a series-wound motor. To understand how this is achieved the characteristics of both shunt and series-wound motors must be considered. When the armature of a motor rotates at speed, a generator effect occurs and the armature windings generate a voltage which opposes the voltage of the battery. The opposing voltage rises as the armature speed increases but never equals the voltage of the battery. The effect of this is to reduce the current drawn from the battery.

Most motors have 4 brushes: two insulated and two earthed. This reduces the amount of current carries by each brush to half the total starting current, and the arrangement is suited to the split-field connections. 

The last part of the starter system is the solenoid which receives a large electrical current from the battery and a small electrical current from the ignition switch.  The small electrical current from the ignition switch is sent to the starter solenoid when It has been turned on, and current then flows from the live starter terminal through the coil creating a magnetic field around it. At the same time current flows through the PULL IN winding to the M terminal via the 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.

The plunger is now magnetised and is drawn up into the centre of the hollow coil, this movement connects the B terminal to the M terminal. At this point the PULL IN winding is turned off because of the equal 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.

LED light Probe

A self directed lesson based on constructing a simple electronic circuit design as found in the automotive industry.

Materials included 

-Brass Rod 150mm long

-Red & Green LED 

-Black and Red wire 2m long

-2 Resistors 1K Ω

-Red and Black alligator clips

100mm plastic tube 7mm Id

Shrink Tubing

-Black 2.4mm diameter, about 300mm long

-Red 6.4mm diameter, about 150mm long

-Black 12.7mm diameter, about 125mm long

Given these simple materials, a workbook and tools to construct our LED Logic probes.
following the wiring diagram we were provided with I started with soldering the Green LED positive leg to the first 1k resister, this is because the LED lights only have a voltage of 0.7v and with the battery having a voltage of around 12.6v that without the resistor the LED would of blown because the circuit would have a very high level of current or the maximum amount that the voltage source could of provided. After soldering the Green LED to the 1K resistor I had to solder the piece to the red wire, this is how the current travels through the circuit.To test that my connections are right I test the circuit by touching the red positive wire to the positive of the battery source and the negative side of the LED leg to the earth terminal, this completes the circuit for now and my Green LED lights up. 
After finishing the Positive side of my Probe I start on the Negative side and Red LED light. I repeat the process as I did with the Green LED, with my Red LED connected to my resistor I connect that piece to my black wire and test the connection the same way I did with the green.
When I am happy that my connections are strong and stable and working well, I take my brass rod (which is a conductor, meaning that current will travel through it) I sharpen one end which will be the LED probe test end, at the same I tin the middle of the rod removing the outer layer of coating so that it is easier to solder the LED legs on later.  
Once I have done this I take some of my black heat shrink and cover the exposed LED legs and resistor wires, this insulates the connection so that if that connection were to touch another exposed piece of wiring it would create a spark when attached to the power source. Each wired piece has to be individually insulated, but must leave the unattached LED legs outside of the insulating heat shrink because they must be soldered to that brass rod.

Before I can solder the unattached LED legs to the brass rod i have to insulate both ends of the rod, with more black heat shrink, this is so that the brass rod doesn't come into direct contact with exposed wires causing a short circuit.   

before soldering the uninsulated legs of the LEDs to the brass rod I had to get some more heat shrink and pull it over the red and black wires and up onto the rod, positioning the two LEDs into the middle of the of the rod  where is has been tinned and readied for soldering. Heating the heat shrink completes the wrapping around the wires and the rod, this will support the weight of the wires and prevent the insulated LED legs from breaking off. However when positioning the LEDs you must make sure that they do not touch the brass rod, otherwise this messes with the current flow and the LEDs wont work when supposed too. To attach the uninsulated legs to the rod the legs must be twisted around the rod, using your fingers or bottle nose pliers, then solder to the exposed tinned section of the rod. to give the LEDs extra support so that they don't snap off use a Hot Glue gun to glue just under the LEDs. 

now the insulated piece must be covered in plastic tubing which gives the probe extra stability and strength, a piece must be cut out though to fit around the LED lights. this is almost the end of the construction of the LED light probe. the next few steps are just covering up the construction with a few more piece of heat shrink just to insulate the probe a bit more and make it look nice.

The end product should be a fully operational LED Positive/ Negative probe. Both lights should light up when connected to both the positive and negative terminals of a battery source, and only one LED should light up when connected to the battery source and touching either a negative or positive terminal with the brass rod.