PART 1. WIRING PRINCIPLES

Simple Electrical Circuit

The typical electrical circuit as we know it comprises any component, such as a lamp for example, connected to a supply – which may be a battery – by two wires or cables; a feed wire and a return. Any electrical installation em-ploying this arrangement is known as an insulated return system, and for some time the electrical equipment on vehicles followed this principle.

Figure 1. Insulated Return System.

Earth System

An alternative arrangement suitable for many applications, uses an insulated cable as a feed wire to the component, the return being obtained via earth, which on a vehicle is of course the steel chassis.

Such an arrangement reduces both the amount of cable necessary and the complexity of the wiring circuits.

This single pole or earth return system soon became standard practice for vehicle work in general. But for many of the larger vehicles such as passenger-carrying and certain heavy commercials, which include petrol carrying vehicles, the insulated return system is still used.

Negative Earth System

Figure 2. Negative Earth System.

With the earth return system the orthodox arrangement was to earth the negative pole of the supply, and this became known as the single pole negative earth system, which was used for vehicle work for several years – on British vehicles up to about 1936.

Positive Earth System

Figure 3. Positive Earth System.

It was then found that certain specific advantages were obtained by earthing the positive pole of the battery instead of the negative. Thus we had the introduction of the positive earth system, which is almost universally used today, except for those specialised vehicles previously mentioned.

One of the main advantages gained by earthing the positive side of the supply is that the polarity of the spark plug central electrode is made negative, which results in improved spark plug performance and longer service life of the spark plugs and the H.T. cables.

At the battery itself the formation of electrolytic sulph-ation at the positive lug is reduced, and also the marked tendency to electrical leakage from the cells to earth, the result of the presence of acidulated moisture. The corrosion effects at switch contacts, cable connectors, soldered joints, etc. are also reduced, especially under conditions of excessive humidity.

Connecting Up And Terminal Markings

Figure 4. Positive Earth Return.

A point to be remembered is that this reversal of polarity makes no difference whatsoever to the working of the system or testing for faults, but it has to be borne in mind when fitting the batteries and connecting up cables. Terminals are all properly marked without reference to the polarity, as shown in our picture, i.e. the dynamo and field terminals D and F. (The D terminal being the larger of the two).

Methods Of Wiring

Figure 5. Cables Grouped and Braided.

Now we come to the cables and the method of wiring which is generally by means of a harness. We shall confine ourselves to British vehicles, although most others follow the same general pattern. Our picture shows a wiring loom in course of manufacture.

Multi-coloured cables are employed as a means of facilitating assembly both at the electrical manufacturer’s works, and also on the vehicle makers’ assembly lines.

Next, the cables are grouped together and braided into looms with individual conductors emerging where required, thus forming main, branch and trunk cables.

The development of this arrangement not only facilitates assembly but also provides considerable protection against chafing on metal edges and subsequent wiring faults in service.

Additionally, sub-assemblies such as the complete car body, or say a steering column may be fully assembled and wired before being positioned on the vehicle. To facilitate the fitting-up, various junction boxes and snap connectors are now widely used.

Cable Loom

On some vehicles one complete wiring loom may be used; on others as many as four separate looms – one main and two or three subsidiaries are needed and these are finally connected up to the main loom by means of snap connectors or junction boxes. Altogether there is a vast variety of wiring looms made up for different makes and models.

So, unless the vehicle is specially laid out, the stocking and general use of wiring looms is not practicable. For this reason the renewal of damaged wiring is more easily and economically handled in service by putting in single cables and employing either junction boxes or snap connectors to re-join to the undamaged parts of the existing loom.

Figure 6. A Completed Loom.

Junction Boxes And Cable Connections

For general purposes, a range of junction boxes and cable connectors is freely available.

The snap or spring connectors shown, greatly facilitate re-wiring and general service work. They are made in numerous combinations.

These spring connectors must be borne in mind when fault finding.

Figure 7. Lucas Junction Box and Wiring Connectors.

Types Of Cable In Use

Now before going into circuits and colour schemes, let us examine the types of cables which should be available before embarking on any vehicle wiring repair work.

They fall into three groups :

1. Starter cables, of which there are three sizes.

2. Car wiring cables, of which there are four popular sizes and as many colours as can be conveniently stocked by the average motor trader.

3. Ignition cables which are subdivided into two types:

a) High tension cable, i.e. for spark plugs, etc.

b) Low tension cable, for primary, or low voltage circuits.

Starter Cables

1. The most generally used pattern is a fairly light type of jute covered cable as shown. This comprises 37 strands of No . 20 SWG (Standard Wire Gauge) tinned copper wire.

It is suitable for most light vehicle work where the starter motor current does not exceed 400 amperes.

Figure 8. Examples of Starter Cables

2. Used in conjunction with this is a similar size earth braid which should always be fitted between engine and chassis as a bonding strip when rubber engine mountings are used.

This standard braid is also suitable for use as the earth lead from the positive terminal of the battery to the earth connection on the scuttle or chassis.

3. The two heavier starter cables are of similar con-struction to the light one, i.e. jute covered, but have 61 strands of No. 20 or 18 SWG, and are used for starter currents up to 700 amps.

Where this size cable is used for the starter supply, further lengths should be made up as a bonding strip and also as the battery earthing lead.

Circuit Wiring Cables

Next we come to the cables necessary for general wiring on the vehicle.

These are all special cables for the work, and comprise a number of copper strands in rubber and fabric sheaths, specially treated to be highly resistant to petrol, water and oil. Such cables are freely available on the market as auto-cable.

There are five sizes of cable altogether in general use on the 12-volt system, but for most purposes three are com-monly used. These are:

1. The battery feed circuit cable, comprising 44 strands of 0·012 copper, generally described as 44/012. This cable has a current carrying capacity of approximately 22 amperes.

2. Main generator or headlamp circuit cables, com-prising 28 strands of 0·012 copper (28/012) with a current carrying capacity of 14 amperes. For most purposes this is adequate.

3. Side and tail lamp wiring, accessory, ignition and generator field circuits comprising 14 strands of 0·012 copper (14/012) with a current carrying capacity of 7 amperes.

4. For panel lamp wiring and other incidentals a cable comprising 9 strands of 0·012 copper (9/012) is the most convenient size.

It should be realised that this is only a general guide to the cables used in the different circuits. New vehicles with Lucas wiring are carefully studied to determine the correct cable sizes. Where long lengths of cable are used, a larger gauge wire may be necessary to prevent excessive voltage drop.

Figure 9. Examples of Circuit Wiring Cables. All of these would be colour coded in the Lucas system.

For example, a vehicle with an exceptionally long cable run for the battery feed, six feet or more may be installed with an oversize cable, such as 65/012. If at any time this cable has to be renewed, a similar size replacement should be installed. This last ruling also applies to re-wiring on vehicles equipped with the 6-volt system, particularly if voltage drop trouble is encountered.

Equivalent Cables – Current Carrying Capacity

It may be useful to examine how the cable sizes which have been given, compare with similar cables identified by other standards of measurement. Some cable man-ufacturers describe their cables in terms of standard wire gauge and this illustration shows the equivalent sizes which will carry the same amount of current.

For example: Our 44/012 will compare with 44 strands of 30 SWG wire or 100 strands of No. 36 SWG, all having the same cross sectional area of copper which deter-mines the amount of current which can be carried with a specified minimum voltage loss, i.e. voltage drop.

Car Type Cables

9/·012 (30 SWG) EQUIV. 23/36

14/·012 (30 SWG) EQUIV. 40/36

28/·012 (30 SWG) EQUIV. 70/36

44/·012 (30 SWG) EQUIV. 100/36

Ignition Cables

1. The 7 mm high tension rubber cable is generally used for spark plugs and distributor or magneto leads.

There are numerous grades of this widely used cable at varying prices, but it is always good policy to buy high quality in order to obtain maximum durability in service. This avoids failures as a result of deterioration and cracking, which may cause elusive and annoying misfiring or, perhaps, complete breakdown.

The top three cables shown in the illustration are Lucas H.T. cables which all use Neoprene outer casings. This material has proved to be the best possible protection against heat, oil, petrol and water.

2. The 4 and 5 m/m low tension cables, which will carry up to about 7 amperes, are widely used for motor cycle work and are equally suitable for any exposed working conditions, where the cable is not subjected to oil or petrol.

We have mentioned altogether ten cables, commencing at the heavy type starter cable carrying up to 700 amp-eres, and finishing with the 4 and 5 mm mainly used on motor cycles. These ten different sized cables, together with earth braid, constitute the minimum range which should be stocked for general motor work.

The Wiring Circuits

Whilst a car wiring layout as a whole may appear com-plicated at first sight, the complete arrangement may be broken down into separate circuits as follows:

1. Battery and Starter Circuits.

2. The Ignition Circuit.

3. The Charging Circuit.

4. Lighting Circuits.

5. Accessories Circuit.

Circuit Identification – Battery, Ignition And Charging

To distinguish these circuits, a distinctive colour scheme is used. The value of such a colour scheme on the car assembly line is obvious, and, once it is understood, it is of equal value in service fault finding. So before we discuss all the wiring circuits we must know about the colours employed.

Eight basic colours are used as follows, with an approp-riate coloured tracer.

1. Plain, brown cable or brown with coloured tracer in it, is used as the current supply or feed wire to all services.

2. White for the ignition circuit, and all component feeds which are essential when the ignition is switched on.

3. Yellow or yellow with coloured tracer is used for the generator and field circuit.

Circuit Identification – Lighting And Accessories

4. Red for the side and tail lamp circuits, starting from the lighting switch.

5. Blue for the headlamp lighting circuit, also starting from the lighting switch.

6. Purple is used for the auxiliary circuits which are fed from the ammeter and protected by a fuse A2.

7. Green is used for the auxiliary circuits which are fed through the ignition switch and protected by a fuse, A4.

8. Black is used for the earth circuit. That is, if a com-ponent is not fixed directly to the chassis, a cable must be taken to a good earthing point on the chassis, and this cable will always be black.

The Typical Wiring Circuit

We can now deal with the wiring circuits as applied to modern (circa 1952) vehicles.

Generally, the electrical system of a motor vehicle can be considered as a series of simple circuits, each con-sisting of the component (Item 1), its switch, (Item 2), and three wires, comprising the feed, switch wire and return (Item 3), this return being provided by the frame of the vehicle, although, in the case of components insulated from the chassis, an earthing lead is also necessary.

Figure 10. A typical wiring circuit diagram

Some variations are to be found, such as fuses, two-way switching and so on, but the principle of feed wire, switch wire and return remains, and it is upon this that the Lucas wiring colour scheme is based.

Feed wires carry braiding of a main colour only. Switch wires have the main colour of feed with a coloured tracer woven spirally into the braiding. The return or earthing leads are black.

The Standard Wiring Diagram

Here we have a typical wiring diagram of which there is one published for most models and makes of vehicles. These diagrams are of considerable value in service, but a certain amount of difficulty may be experienced by the non-specialist in following them out.

It is however, quite simple if each component circuit is considered individually. which we shall proceed to do, commencing with the starter system.

Figure 11. A typical motor vehicle’s wiring diagram is shown on Page 8.

Key To Cable Colours

1. Blue 2. Blue with Red

3. Blue with Yellow 4. Blue with White

5. Blue with Green 6. Blue with Purple

7. Blue with Brown 8. Blue with Black

9. White 10. White with Red

11. White with Yellow 12. White with Blue

13. White with Green 14. White with Purple

15. White with Brown 16. White with Black

17. Green 18. Green with Red

19. Green with Yellow 20. Green with Blue

21. Green with White 22. Green with Purple

23. Green with Brown 24. Green with Black

25. Yellow 26. Yellow with Red

27. Yellow with Blue 28. Yellow with White

29. Yellow with Green 30. Yellow with Purple

31. Yellow with Brown 32. Yellow with Black

33. Brown 34. Brown with Red

35. Brown with Yellow 36. Brown with Blue

37. Brown with White 38. Brown with Green

39. Brown with Purple 40. Brown with Black

41. Red 42. Red with Yellow

43. Red with Blue 44. Red with White

45. Red with Green 46. Red with Purple

47. Red with Brown 48. Red with Black

49. Purple 50. Purple with Red

51. Purple with Yellow 52. Purple with Blue

53. Purple with White 54. Purple with Green

55. Purple with Brown 56. Purple with Black

57. Black 58. Black with Red

59. Black with Yellow 60. Black with Blue

61. Black with White 62. Black with Green

63. Black with Purple 64. Black with Brown

PART 2. INDIVIDUAL WIRING CIRCUITS

Common Battery Supply Circuit

Figure 12. The common battery supply circuit.

The current supply for all circuits on the vehicle com-mences either direct from the battery negative terminal, or from the battery side of the starter switch.

It comprises a heavy brown cable, first running from the battery to the control box ‘A’ terminal, and through the load windings of the regulator to the terminal ‘A1’. From here a brown with blue cable leads to the lighting switch where it loops off to one side of the ignition switch (terminal ‘A’).

If an ammeter is installed, it will be placed in the brown lead between the source of supply and the control box ‘A’ terminal, the cable becoming brown with white between the ammeter and the control box.

The Starter Circuit – With Manual Switch

The first and most elementary circuit is that of the starter motor system. This motor can be either manually or solenoid operated.

Figure 13. Starter circuit with manual switch.

Our picture shows the manually operated starter motor circuit. The current path is from the negative terminal on the battery to the starter switch (Item 1), across the switch contacts to the insulated terminal on the starter motor (Item 2), through the starter to the engine block, then via the braiding strip to the chassis (Item 3), returning to the positive terminal of the battery via the battery earth cable (Item 4).

The earth cables 3 and 4 are most important to the suc-cessful operation of the starter, particularly under cold starting conditions and must always be well maintained.

Where a solenoid starter switch is fitted, an additional relay circuit is introduced.

The Starter Circuit – Solenoid (Relay) Operated

In this arrangement of the main starter circuit we have to add a solenoid operating circuit, and this is under the control of the ignition switch. That is, the starter solenoid can only be operated with the ignition switched on.

Firstly, there is the common supply from the battery to the ignition switch for all the circuits.

Our solenoid supply is taken from the other side of the ignition switch (A3) to the solenoid operating push by a white cable, and from this push to the solenoid winding by a white with red cable. The winding is earthed to the casing of the solenoid itself, thus completing the earth side of the circuit back to the battery earth.

Figure 14. The starter circuit with solenoid operation.

The Ignition L.T. Circuit

A white cable, commencing at the A3 terminal of the ignition switch is taken to the A3 terminal at the control box, and provides a common supply for the ignition units (coil and distributor) together with all the ignition acces-sories – fused and unfused – which are under the control of the ignition switch.

Figure 15. Wiring diagram of ignition L.T. circuit.

Using the A3 terminal on the control box as a junction point, a white (28/012) cable connects directly to the SW terminal of the ignition coil. A white with black cable joins the CB terminal of the ignition coil to the L.T. terminal of the distributor.

The unfused accessories such as the petrol pump and automatic choke will also be supplied from the A3 control box terminal using the white cable as the feed wire.

The ignition warning light feed wire, also a white cable is taken direct from the A3 terminal of the ignition switch.

The Charging Circuit

The main components involved in the charging system are the generator, control box and battery.

Figure 16. Diagram of a typical charging circuit.

That part of the circuit connecting the battery with the control box has already been dealt with as the brown battery supply circuit.

The generator circuit itself consists of a yellow (28/012) cable from generator ‘D’ terminal to control box ‘D’, and a yellow with green (14/012) from generator ‘F’ terminal to control box ‘F’. Additionally, there is a yellow (I4/012) cable connecting from control box ‘D’ to one side of the ignition warning lamp.

The Ignition Warning Lamp

This indicator lamp performs two functions:

1. It indicates that the battery current is switched on to the ignition and ignition-fed accessories.

2. It indicates that the generator is charging when the engine is turning at charging speed.

Follow the previous circuit through. Commencing at the A3 terminal of the ignition switch, an extension of the white lead is carried to one side of the warning light. From the other side, a yellow lead is taken direct to the ‘D’ terminal at the control box.

When the ignition switch is closed, current feeds from terminal A3 at the switch, through the warning lamp to the ‘D’ terminal of the generator, the circuit being com-pleted through the generator winding to earth. The lamp therefore, lights up.

When the engine is started, and the generator voltage builds up to 12-volts, it opposes and equalises the battery voltage previously applied to the lamp, and no current will flow through it. The light goes out, and remains so, until the generator ceases to charge, and its voltage falls. Battery current will then pass through the lamp again and it will remain alight until the ignition switch is moved to the ‘off’ position.

Sidelamp Circuit

This picture features the sidelamp circuit.

Commencing at the lighting switch terminal S1 (Item 2), a separate red cable runs to each side lamp via a snap connector (Item 3).

Figure 17. Sidelamp wiring diagram.

The return cable is black and you will notice that, in this case, it is a full return through snap connectors (Items 4) to a special earthing terminal on the chassis (Item 5).

This was evidently required on this particular model to assure a good return path, and it would be very necessary to check this if any trouble were experienced with the sidelamps.

Rear Illumination

Figure 18. Wiring diagram for rear illumination.

In this example the rear illumination comprises tail lamp, one on each side, and also the number plate box lighting comprising two bulbs.

Commencing at the S1 terminal on the lighting switch (Item 1) from which the side lamp feeds are also con-nected, another red cable runs directly to the first tail lamp (Item 2). This red cable ‘loops’ out again to a snap connector located in the luggage boot (Item 3). From this snap connector, two more red cables feed the second tail lamp (Item 4) and the two bulbs wired in parallel in the number plate box (Item 5).

You will notice particularly that a black earth wire con-nects from both tail lamps and the number plate box, to the earth terminal on the control box, and thence to the chassis earth (Item 6).

The Head Light Circuit

We have already explained that all the main current passes over the load-turns on the voltage regulator to the lighting switch.

So we can commence at the S2 terminal on the lighting switch (Item 1).

Figure 19. Wiring diagram for headlamps circuit.

From this point, a blue cable runs via a snap connector (Item 2) direct to the foot dipper switch (Item 3).

The dipper switch is a two way switch. From one terminal two blue with white cables connect through snap con-nectors (Item 4) to the main filament in each headlamp bulb (Item 5).

From the other terminal on the two way switch, two blue with red cables run through snap connectors (Item 6) to the dip filament in each headlamp bulb.

Here again, separate black earth cables are fitted to ensure a good return path for the lamps. These earth cables connect direct from the bulb-holders in the lamp, via the control box earth terminal, to the common earth point on the chassis.

The Instrument Panel Lighting

The last item is the lighting on the instrument panel.

Figure 20. Wiring diagram for instrument panel lighting.

This is generally fed from the sidelamp terminal on the lighting switch.

On this model a red feed cable, commencing from the lighting switch (Item 1), supplies the panel switch (Item 2), and this loops as a red with white cable to a snap connector and the individual panel lights. In this case there is no separate earth cable.

From the panel switch (Item 2), there is a red with green cable (Item 3), which feeds the interior lamp (Item 4), and also an earth cable (Item 5), which is a cable direct from the lights to the chassis.

Notice that the interior light is wired directly to the panel light switch, but, generally, interior lights are fused, taking their supply from the A2 fuse.

The Complete Lighting Circuit

Figure 21. Diagram of the complete lighting circuit.

Here we have the wiring diagram of the complete lighting installation; we have seen just how simple this is by breaking it up into individual circuits.

The Long Range Driving Lamp

The most satisfactory arrangement for installing this lamp is such that when the head lights are dipped the SLR lamp should automatically go out, thus avoiding the necessity for two separate operations.

Figure 22. Wiring diagram for installing a long range driving lamp.

This result can be conveniently obtained by taking a red with blue (28/012) cable direct from the main beam terminal of the dipper switch.

An additional hand control switch may then be inserted in this feed line to enable the driver to have the SLR on with his head lamps on main beam. With this arrange-ment the lamp will immediately go out when the head lights are dipped.

Fog And Pass Lamps

When fitted as initial equipment, fog and pass lamps will usually take their current supply from the ‘S’ or ‘T’ terminals on the lighting switch so that they may be auto-matically switched off with the side lamps. If such lamps are fitted subsequently, this is still the most suitable method.

Figure 23. Wiring diagram for fog or pass lamp.

It is usual to wire these lamps by means of a red (28/012) cable from the lighting switch to the lamp switch, and follow with a red with yellow (28/012) to the lamp itself, making quite sure that the lamp has a good earth. If there is any doubt about this, a separate black earth wire should be installed. The relatively large size of cable is necessary if full brilliance from the lamp is to be assured.

Auxiliaries

There are three groups of auxiliaries and accessories, which take their current supply from either the ‘A3’, ‘A4’, or ‘A2’ terminals on the control box.

Those supplied from the ‘A3’ and ‘A4’ terminals are under the master control of the ignition switch. Since they are only required when the engine or vehicle is in motion, this practice precludes the possibility of their being accidentally left on, and so running the battery down.

The unfused components supplied from the A3 terminal will comprise the electric petrol pump, auto choke, petrol reserve solenoids etc.

The fused components supplied from the A4 terminal consist of trafficators, stop lamp, reverse lamp, wind-screen wiper, fuel tank unit, demister and heater motors when fitted.

The A2 terminal and its fuse takes current direct from the battery through the load windings of the regulator and is used for interior lights, door lights, low current horns.

The heavy-current horns, such as the wind-tone models, and also radio sets will preferably have separate fuses, the initial supply being taken from the ‘A’ terminal of the control box, that is, directly from the battery and not through the load windings of the regulator.

There remain a few additional items, which, for special reasons, may take current direct from the battery. Two popular ones will be the cigar lighter, which takes a very heavy current and the inspection lamp sockets, which may be required when everything else is off.

We shall now examine all these auxiliary circuits individ-ually commencing with the ‘A4’ fused auxiliaries under the control of the ignition switch.

Fused Auxiliary Circuits On A4

Now let us examine these A4 circuits on an actual vehicle. They are the most complicated of any, and we shall consider each one separately.

Figure 24. The ‘A4’ circuit and components.

As this picture shows, there are only five altogether on this particular model, i.e.:

1. Trafficators.

2. Stop Lamp.

3. Reverse Lamp.

4. Windscreen Wiper.

5. Fuel Tank Unit.

The Trafficator Circuit (Refer To Figure 25)

Trafficators are supplied from the A4 (Item 1), 35 amp fuse, local control being provided either by a self-cancelling two-way and off switch on the steering column, or alternatively by a similar type of switch on the panel. The most usual ‘run’ of this circuit is as follows:

A green cable (14/012) from A4 on the control box feeds to a twin spring connector (Item 2).

Two leads branch from this connector, one to the stop lamp switch, the other to a further spring connector (Item 3) located at the bottom of the steering column. From here one lead branches off to the reverse lamp switch, the other to the trafficator and horn switch at the top of the steering column (Item 4).

Two cables green with white, and green with red, lead from this switch and connect, each to one trafficator, through an additional spring connector (Item 5) also located at the bottom of the steering column.

Figure 25. Wiring diagram for the trafficator circuit.

In the majority of cases, a separate black earth lead will be run from each trafficator to an earthing point, as shown at (Item 6).

The Stop And Reverse Lamp Circuits

We can now examine the stop lamp and reverse lamp circuits together.

You remember the common green feed wire from A4 on the control box branched away, still continuing as a green cable from the first snap connector (Item 1) for the stop lamp. This further green cable leads direct to the stop lamp switch (Item 2), sometimes a small hydraulic switch, mounted on the master brake cylinder. From the switch, it proceeds, via a rubber-covered snap connector (Item 3) direct to the stop lamp (Item 4), and thence to earth (Item 5).

Figure 26. Wiring diagram for stop and reverse lamp circuits. Note use of snap connectors.

In the case of the reversing lamp, the feed comes off the second snap connector (Item 6) and proceeds still as a green cable to the reverse lamp switch (Item 7) usually mounted on the side, or end of the gearbox, and actu-ated when the reverse gear is selected.

From the switch, the switch wire proceeds as green with blue via another snap connector (Item 8) direct to the reverse lamp (Item 9) and thence to earth (Item 10) through the bulb.

The Screenwiper Circuit

A second green feed wire from the A4 on control box (Item 1) is used to supply the screen wiper and the petrol tank unit. This wire runs direct to one side of the screenwiper motor switch (Item 2) which also acts as a junction point for the feed wire to the fuel tank unit. From the other side of the switch, the wire becomes green with yellow to one side of the screenwiper motor (Item 4). From the other terminal on the motor (Item 5) the cable is a black return to the ‘E’ terminal on the control box direct to a good earth on the chassis (Item 6).

Figure 27. Diagram of circuit for screenwiper motor.

The Fuel Gauge And Petrol Tank Unit

As previously stated, the green feed wire for this com-ponent is junctioned off the screenwiper switch (Item 1). From there it connects – still as a green cable – to the fuel gauge unit (Item 2). From the gauge, the colour changes to green with black connecting through one or more snap connectors (Item 3) to the petrol tank rheostat (Item 4). The circuit is completed through this unit to earth.

In the event of any erratic reading on the gauge, the first thing to do would be to check the terminals for tightness, and open and remake the joints at the snap connectors, because either a loose or corroded connection will seriously affect the operation of the tank unit rheostat.

Circuit illustration is on Page 15.

Figure 28. Wiring diagram for fuel gauge and tank unit.

The Auxiliary Circuits:

A And A2 On Control Box

This picture shows a fairly typical layout for auxiliaries which are not under the master control of the ignition switch, and which may vary somewhat on different vehicles.

Figure 29. Components connected to A and A2 at the control box, and their wiring.

Top centre of the picture shows the inspection lamp sockets and a cigar lighter. It is usual to place these in the battery feed circuit before the ammeter. It would serve no useful purpose for the heavy current discharge from the cigar lighter to register on the ammeter; it would merely tend to alarm the driver. Similarly if the inspection lamp is dropped, and shorts as a result of bulb breakage, this would probably damage the ammeter. On the other hand a radio set for instance, which will have its own fuse, may be left on accidentally, or it may be in circuit for prolonged periods. It is therefore desirable that the discharge should be shown on the ammeter. Similarly, with heavy-current horns such as the Windtones, to con-nect them through the ammeter would merely register as a violent and alarming oscillation of the needle when the horn button is closed, so a separate fuse is usually sup-plied with them, and the supply taken direct off the battery line.

Now examine this picture in detail. Such components as interior lights, door lights, etc., which may be susceptible to wiring troubles, will be placed on the ‘A2’ fuse cable colour purple. Small-current horns such as the Altette horn may also be connected to this fuse. The radio set with its own fuse, however, can most conveniently be connected from the ‘A’ terminal of the control box. The supply for the cigar lighter and inspection plug sockets may be taken from the battery side of the starter switch as a matter of convenience. The supply for heavy duty horns may be taken from this point also, or, if no ammeter is installed, the Windtone horn supply may be taken from the ‘A’ terminal at the control box. At least one car maker who does not install an ammeter takes the Windtone horn supply from the ‘A2’ fuse, thus eliminating the additional horn fuse. Since there are no other com-ponents on this fuse, there is no possibility of putting the interior lights out of action because of a fault on the horn wiring. We have covered this item in some detail as a number of exceptions to the standard recommended layout may be found on different vehicles.

The RB.106 Control Unit And Fuse Base

Where the RB.106 control unit is installed, a separate fuse base is used.

Figure 30. Layout of the RB.106 control unit and fuse base.

There are two separate current supplies to this base:

1. From the ‘A’ terminal on the control box a brown (44/012) cable connects to the terminal ‘A1’ on the base. The incidental accessories are connected at ‘A2’, that is, through the fuse.

2. The supply to the ‘A3’ terminal is taken from the ignition switch ‘A3’ by means of a white (28/012) cable. The ignition accessories, fused and unfused will be connected at ‘A4’ and ‘A3’ respectively.

PART 3. REWIRING IN SERVICE

General

Vehicle re-wiring work properly organized is capable of producing a useful revenue, as well as providing an essential service facility which no motor engineer can really afford to neglect.

The work as a whole falls into four different categories:

1. Incidental wiring work such as may be involved in the fitting up of accessory lamps and other components.

2. Renewing single cables within the wiring harness or external to it.

3. The re-wiring of complete sections, following damage by collision, fire, etc.

4. Complete rewires involved in major vehicle over-hauls.

In this latter category it is feasible to fit a completely new wiring harness, but owing to the multiplicity of models it is not practicable to make them available as spares; the delay involved in obtaining a special harness would be completely prohibitive.

In any of the categories mentioned, in order to carry out a good quality job on an economical and profitable basis, properly organised stocks of cables, sleeving, jointers, clips, etc. must be available and the range and quantities of these components will vary according to the amount of work anticipated.

If rewiring work is to be made a speciality, a wide range of stock controlled components will be required, other-wise the work will quickly become uneconomical as the result of delays and time lost in making-up special bits and pieces.

Within our experience, service complaints following re-wiring work can be all too frequent, and in this section of the course we shall attempt to offer some useful guid-ance to those undertaking it.

In every case, the fundamentals of a good quality, economical job may be assured if:

1. The correct size and quality of cable is used.

2. Adequate stocks of the basic colours are maintained.

3. Suitable stocks of protective sleeving are available.

4. A full range of quick jointers, junction boxes and other incidentals are at hand.

Where the work is to be extensive, additional facilities in the way of guillotines, wire strippers, and other special tools for fitting up will go far in making the work more profitable.

Finally, although such work may have to be carried out by relatively inexperienced personnel, a good quick job can only be expected from an experienced electrician. At the same time, practice will soon result in increased speed and proficiency.

Cable Stocks Sizes And Colours

The following cables constitute the minimum range that is necessary. Quantities can be adjusted to suit the conditions and volume of work, availability, etc.

The essentials extracted from our Catalogue No. 502F are as follows:

Starter Cables:

1. 37/20 – for the 12-volt system.

2. 61/20 – for the 6-volt system. .

3. 61/18 – for heavy C.V. work.

4 Earthing braids.

Circuit Wiring Cables:

5. 44/012 max. 22 amperes brown.

6. 28/012 max. 14 amperes yellow, blue, white, purple and black.

7. 14/0l2 max. 7 amperes red, green, white and yellow.

When all the coloured cables are not obtainable, short lengths of coloured sleeving will aid identification.

Ignition Cables:

8. 7 mm H.T. Neoprene UM.827.

For motor cycle wiring and miscellaneous purposes:

5 mm Low Tension, rubber covered.

Multi-Core Cables:

An assortment of these is essential for steering column re-wiring and they are available with from two to seven cores according to requirements.

Protective Sleeving And Rubber Grommets

It is essential that all new wiring should be protected against chafing and exposure at vulnerable points.

For this purpose the most generally suitable material will be lengths of oil-proof plastic sleeving (PVC) which can be cut down to suit individual requirements.

The taping together of runs of cable is unsatisfactory in service, and is instantly recognisable as bad workman-ship. The following minimum range of six sizes will be required:

Sleeving:

5 mm dia. to carry one cable.

7 mm dia. to carry two cables.

11 mm dia. to carry three to four cables.

15 mm dia. to carry five to six cables.

18 mm dia. to carry seven to nine cables.

22 mm dia. to carry ten cables and over.

Rubber Grommets:

¼” bore to fit ½” hole.

½” bore to fit ¾” hole.

1” bore to fit 1¼” hole

Jointers, Snap Connectors And Junction Boxes

The comprehensive range of connectors and junction boxes illustrated in our Cable Catalogue No. 502F will cover all requirements.

The only alternative to these connectors is twisted and soldered joints, a slow business.

In no circumstances should dry joints be permitted.

These deteriorate badly in service and are visible evi-dence of bad workmanship.

Cable Clips

A good range of cable eyelets or terminals, and also spring and screw fixing clips are other essentials to economical rewiring work, and a comprehensive range is illustrated in our Catalogue No. 502F.

Typical samples are as follows:

1. Junction Boxes: These are available with 2, 4, 6 and 8 terminal positions.

2. Cable Clips: The essentials here are single and double cable clips.

3. Cable Harness Clips and Spring Clips for chassis fixing.

4. Cable Eyelets, or Terminals. ³/₁₆”, ¼”, ⁵/₁₆” and ⅜” eyelets will cover all requirements.

5. Starter Cable Terminals.

6. Battery Lugs and Earth Straps.

This is the simplest form of wiring work.

Wiring Up Accessory Lamps Or Renewing Single Cables

1. Always use the recommended size of cable and the correct basic colour, which makes for easy identi-fication subsequently, and also enhances the appearance of the job.

2. Take great care to ‘feed’ the component from the correct fuse or terminal point, as we have already detailed.

3. Protect the new wire with sleeving where necessary.

4. Clip up sufficiently close to prevent sagging in service which would give a consequent untidy appearance.

5. Avoid close proximity to any ‘Hot Spots’ and any moving parts such as brake cables, etc.

It happens that sometimes a single cable in the harness may require renewal, and the obvious way to do this is to run a new cable of the correct size, and major colour, outside the loom. This may be clipped on to the loom at intervals. Finally cut off the old cable ends where they enter the harness.

Partial Rewires

A typical service rewiring job will be where a section of wiring has become damaged due to a smash or a fire.

In this circumstance the best and quite adequate method is to cut away all the damaged wiring, if possible at a common point. Then, using either a multi-point terminal block, a junction box or a few snap connectors, run new cables of the correct size and basic colour to the various components. In many cases it will be possible to re-use the old cable run, including the rubber grommets, clips, etc.; otherwise, fit new pieces of insulating sleeving, rubber grommets and cable clips as required.

Complete Rewires

In view of the large amount of labour involved in the re-wiring of any modern vehicle, this job merits careful consideration and a strictly methodical approach.

Job instructions may only specify a re-wire, but it is obviously futile to re-wire a vehicle to find afterwards that when the vehicle is put back into service the generator doesn't charge, or the battery is flat, and so on.

So, the very first thing to do is to make a general check-over of all the units, including the lamps and ascertain that they are in a serviceable condition.

Proceed as follows:

Whilst in contact with the customer make a visual check of the following:

1. The battery.

2. The distributor, leads, and ignition coil.

3. The generator, belt, commutator, brushes, bearings.

4. The control box condition.

5. The starter commutator, brushes and bearings.

Listen to the engagement for undue noise.

6. The lighting – check the condition and operation where possible. Note cracked glasses, ill-fitting rims, etc.

7. The accessories: This general check-over will take about a quarter of an hour.

Remove any units which may require attention in the way of overhaul or minor repairs.

The main job of re-wiring the vehicle can then be tackled.

Methods Of Rewiring

Where only inexperienced labour is available, the best method is to disconnect the leads from their com-ponents, and then remove the complete harness intact if possible.

During the removal of the main harness, some decision will have to be made as to whether such additional cables as trafficator leads, interior lights, etc., which are in the overhaul, require renewing. Generally this will not be necessary. On the other hand the leads passing into the steering column assembly will almost invariably need replacing. Some care should be taken over this, as such rewiring necessitates the use of the correct multi-core cable.

Lay the harness carefully on a suitable size bench and position it with a few stout nails to act as locating pins.

A complete new set of cables can now be run, using the old loom as a pattern. Tie the cables together at junctions and apply sleeving where required.

Finally, re-assemble the complete new harness on to the vehicle and re-connect to the components.

The more experienced man, who fully understands his circuits, and is thoroughly familiar with the work, will prefer to chop the old harness out piecemeal and run his new wiring direct in situ.

Whichever method may be employed, a thorough and final check is necessary when the rewire is completed in order that a clean and effective job is assured.

This final check should comprise the following operations:

1. Replace the charged battery on the vehicle, making sure that earth connections are clean and tight – leave the main battery lead off.

2. Check that oil pipes, speedometer drive, etc., are properly connected.

3. Check that ALL switches are in the ‘OFF’ position, and that no odd leads are left disconnected.

4. Connect the main battery lug but do not tighten – this facilitates emergency removal.

5. Switch on ignition and start engine.

6. Check the charging.

7. Check all lights in turn.

8. Check all accessories in turn.

9. If everything is in order tighten the main battery lug.

Summarising

Whilst it is not possible to cover every variation in circuit arrangements that a particular vehicle manufacturer may adopt, the various circuits which have been provided in this book represent the standard layout employed for most vehicles. If these are properly understood, very little difficulty will be experienced when confronted with the wiring arrangement on occasional special models, or even on new model cars now being produced. Such arrangements will vary only in detail from the general pattern.

In the same way, satisfactory re-wiring operations can-not be covered in any great detail. Various methods may be employed which are equally satisfactory, always providing that adequate stocks of cables and compon-ents are at hand.