Lesson Notes By Weeks and Term v3 - Senior Secondary 1
AUTO-WIRING SYSTEM
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AUTO-WIRING SYSTEM
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Subject: Auto Mechanical Works
Class: Senior Secondary 1
Term: 3rd Term
Week: 5
Theme: Transmission & Braking System
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State reason for usingdifferent wire gauge and colour coding Connect cables usingjoining, terminals and connectors. Use soldering iron and crimpers.
while common colours exist, technicians must be aware that exact colour codes can differ significantly between manufacturers (e.g., Japanese, European, American vehicles) and even models. Consulting wiring diagrams specific to the vehicle is always the best practice.
C. Connecting Cables: Joining, Terminals, and Connectors
1. Cable Joining (Permanent Connections): Used to extend a wire, repair a broken wire, or make a permanent tap into a circuit. a)
Solder Joint: Description: A metallic bond created by melting a filler metal (solder, typically tin-lead or lead-free alloy) between two cleaned, twisted wire ends using a soldering iron.
Advantages: Excellent electrical conductivity, high mechanical strength (if done correctly), highly resistant to corrosion and vibration, long-lasting.
Disadvantages: Requires skill, heat can damage insulation or sensitive components if not careful.
Procedure for a Quality Solder Joint:
1. Preparation: Strip about 1-2 cm of insulation from both wire ends using a wire stripper. Ensure no strands are cut. Clean the wires if they are dirty or corroded.
2. Twisting: Twist the bare wire ends together tightly (e.g., Western Union splice or simple pigtail twist) to create a strong mechanical connection before soldering. This is crucial for mechanical integrity.
3. Heating: Heat the twisted wire joint with the soldering iron tip. Do not apply solder to the iron; heat the wire itself.
4. Applying Solder: Once the wires are hot enough (solder will melt and flow into the joint readily), touch the solder wire to the heated joint, not the iron tip. The solder should flow smoothly into the strands by capillary action, creating a shiny, smooth, cone-shaped joint. Use just enough solder to cover the joint completely.
5. Cooling & Inspection: Remove the iron and allow the joint to cool naturally without disturbing it. A good joint will be shiny and smooth. A dull, grainy joint (a "cold joint") indicates insufficient heat or movement during cooling and is weak electrically and mechanically; it must be redone.
6. Insulation: Insulate the joint immediately after cooling using heat-shrink tubing (preferred for automotive due to its durability and moisture resistance) or high-quality electrical tape. b)
Crimp Joint: Description: A mechanical connection made by deforming a metal terminal or splice barrel around a stripped wire end using a specialized crimping tool.
Advantages: Quick, requires less skill than soldering, no heat involved, often preferred for automotive applications where vibration is high (as a properly crimped joint is very robust).
Disadvantages: Requires the correct crimping tool and terminal for the wire size, a poor crimp can be unreliable.
Procedure for a Quality Crimp Joint:
1. Preparation: Strip insulation from the wire end, ensuring the bare wire length matches the barrel length of the terminal or splice. Avoid cutting any strands.
2. Insertion: Insert the bare wire completely into the barrel of the terminal or splice. For insulated terminals, ensure the insulation grip part also covers the wire's insulation.
3. Crimping: Select the correct crimping jaw on the crimping tool that matches the size of the terminal. Place the terminal in the jaw and squeeze the crimper handles firmly until the crimp is complete (most good crimpers have a ratchet mechanism that won't release until a full crimp is made).
4. Inspection & Pull Test: Visually inspect the crimp for proper deformation. Perform a gentle "pull test" on the wire to ensure it is securely held within the terminal. The wire should not pull out.
5. Insulation (if not pre-insulated terminal): For non-insulated terminals or splice barrels, insulate with heat-shrink tubing or electrical tape.
2. Terminals (Temporary or Semi-Permanent Connections): Definition: Metal components attached to the ends of wires, designed to facilitate connection to other wires or electrical components (e.g., switches, relays, motors, battery posts).
Purpose: Provide secure, low-resistance electrical connections, allow for easy disconnection and reconnection, and protect the wire end.
Types Common in Automotive: Ring Terminals: Looped end, designed to be secured under a screw or nut (e.g., grounding points, battery terminals).
Spade Terminals (Fork Terminals): U-shaped end, designed to slide under a screw or stud.
Bullet Connectors: Cylindrical, push-together connectors for quick, single-wire connections. being used. A mismatch will result in a poor connection.
Stripping: Correctly strip the wire to the appropriate length, ensuring no copper strands are cut.
Placement: Insert the stripped wire fully into the terminal barrel.
Crimping: Squeeze the handles firmly and completely.
Safety: Keep fingers clear of the crimping jaws to prevent pinching injuries.
Worked Example 1: Selecting Wire Gauge for a Headlamp Scenario: A mechanic in Aba needs to replace the wiring for a single 55-watt (W) headlamp on a commercial bus operating on a 12-volt (V) system. The existing wire is damaged. What wire gauge is appropriate?
Solution:
1. Calculate Current (Amperage): Power (P) = Voltage (V) × Current (I) So, Current (I) = Power (P) / Voltage (V) I = 55 W / 12 V = 4.58 Amps
2. Consider Safety Factor: It's good practice to add a safety margin (e.g., 20-30%) for continuous operation and potential voltage spikes. Let's aim for a wire that can comfortably handle 6-7 Amps.
3. Consult a Wire Gauge Chart: (Teacher would provide or display a simplified chart during lesson) A typical automotive wire gauge chart might show: 22 AWG: max 5-7 Amps 20 AWG: max 7-10 Amps 18 AWG: max 10-15 Amps
4. Selection: Based on the calculated 4.58 Amps (and adding a safety factor for 6-7A), a 20 AWG wire would likely be sufficient, but an 18 AWG wire would provide an even larger safety margin, lower voltage drop, and better mechanical strength for a critical component like a headlamp, especially over a longer run. For headlamps, 18 AWG is commonly used.
Worked Example 2: Making a Solder Joint Scenario: A learner is repairing a broken wire in a vehicle's audio system. The wire is 18 AW
G. Procedure (Step-by-step for the student):
1. Obtain two pieces of 18 AWG wire, a soldering iron (e.g., 40W), rosin-core solder, wire strippers, and heat-shrink tubing.
2. Cut about 2cm of heat-shrink tubing and slide it onto one of the wire ends before joining.
3. Carefully strip approximately 1.5 cm of insulation from the end of each wire, ensuring no strands are cut.
4. Twist the bare copper strands of the two wires together tightly.
5. Heat the soldering iron and clean its tip on a damp sponge.
6. Place the hot iron tip against the twisted wires to heat them up.
7. Once the wires are hot, touch the solder to the wires (not the iron tip). The solder should melt and flow smoothly into the twisted strands, coating them completely and appearing shiny.
8. Remove the solder and then the iron. Allow the joint to cool undisturbed.
9. Once cooled, slide the heat-shrink tubing over the soldered joint and apply heat (e.g., with a heat gun or carefully with a lighter from a distance) until it shrinks tightly, providing insulation and mechanical protection.
Commentary: This example emphasizes the importance of proper technique (heating the work, not the solder) and insulation for a durable and safe connection. This section provides a comprehensive explanation of the core concepts related to auto-wiring systems, including detailed steps and Nigerian-contextualized examples.
A. Wire Gauge and Its Importance Definition: Wire gauge refers to the physical thickness or diameter of an electrical wire. The most common standard for automotive wiring in Nigeria and globally is the American Wire Gauge (AWG) system. Gauge Number vs.
Diameter: It is crucial for students to understand that in the AWG system, a smaller gauge number indicates a thicker wire, while a larger gauge number indicates a thinner wire. For example, a 10 AWG wire is much thicker than a 22 AWG wire.
Reasons for Using Different Wire Gauges:
1. Current Carrying Capacity: Thicker wires (smaller gauge numbers) have less electrical resistance and can safely carry a higher amount of electrical current (amperage) without overheating. Thinner wires (larger gauge numbers) have higher resistance and can only carry smaller currents.
Example: The starter motor of a vehicle, which requires a very high current (e.g., 100-300 amps) to crank the engine, uses very thick cables (e.g., 0 AWG or 2 AWG). In contrast, a small dashboard indicator light (e.g., 0.5 amps) uses a very thin wire (e.g., 20 AWG or 22 AWG). Using a thin wire for a high-current application will cause it to overheat, melt its insulation, and potentially cause a fire.
2. Voltage Drop: Electrical resistance in wires causes a reduction in voltage along the wire's length, known as voltage drop. Thicker wires have less resistance and thus less voltage drop over a given distance. Excessive voltage drop can lead to components not receiving enough power to operate correctly (e.g., dim headlights, slow electric windows).
Example: For long runs of wire or high-current components like headlamps on a commercial bus (e.g., a "MOLUE"), thicker wires are used to minimize voltage drop and ensure the lamps receive adequate voltage for maximum brightness.
3. Mechanical Strength: Thicker wires are generally more robust and less prone to breaking due to vibration or mechanical stress, which is important in the harsh environment of a vehicle engine bay or chassis.
B. Colour Coding Definition: Colour coding in automotive wiring is the use of different colours for the insulation of electrical wires to identify their function, circuit, or destination within a vehicle's electrical system.
Purpose:
1. Circuit Identification: Allows technicians to quickly identify which wire belongs to which circuit (e.g., ignition, lighting, charging, sensors).
2. Troubleshooting: Facilitates fault diagnosis by making it easier to trace specific wires from a component back to its power source or control unit. If a headlamp is not working, a technician can quickly identify the power wire (e.g., red) and the ground wire (e.g., black) by their colours.
3. Safety: Helps prevent incorrect connections during repair or installation, which could damage components or create hazards.
4. Standardization: While specific colour codes can vary between manufacturers, there are common conventions that help in general understanding.
Common Colour Code Examples (General): Red (R): Main power, battery positive (+), unfused power.
Black (B) or Brown (BRN): Ground, battery negative (-).
White (W): Power with ignition on, accessory power.
Yellow (Y): Turn signals/indicators, sometimes power to accessories.
Blue (BLU): High beam headlights, sometimes interior lighting.
Green (G): Low beam headlights, sensor signals (e.g., oil pressure).
Violet (V) or Purple (P): Interior lighting, sometimes accessories. Often, wires have a base colour with a contrasting stripe (e.g., Red with White stripe, R/W), indicating a specific sub-circuit or function.
Nigerian Context: Many vehicles used in Nigeria are imported, and while common colours exist, technicians must be aware that exact colour codes can differ significantly between manufacturers (e.g., Japanese, European, American vehicles) and even models. Consulting wiring diagrams specific to the vehicle is always the best practice.
C. Connecting Cables: Joining, Terminals, and Connectors
1. Cable Joining (Permanent Connections): Used to extend a wire, repair a broken wire, or make a permanent tap into a circuit. a)
Solder Joint: Description: A metallic bond created by melting a filler metal (solder, typically tin-lead or lead-free alloy) between two cleaned, twisted Definition: Metal components attached to the ends of wires, designed to facilitate connection to other wires or electrical components (e.g., switches, relays, motors, battery posts).
Purpose: Provide secure, low-resistance electrical connections, allow for easy disconnection and reconnection, and protect the wire end.
Types Common in Automotive: Ring Terminals: Looped end, designed to be secured under a screw or nut (e.g., grounding points, battery terminals).
Spade Terminals (Fork Terminals): U-shaped end, designed to slide under a screw or stud.
Bullet Connectors: Cylindrical, push-together connectors for quick, single-wire connections. Push-on/Quick-disconnect Terminals (Blade/Tab Terminals): Flat male or female connectors that slide together, commonly used for connecting to relays, switches, and components.
Battery Terminals: Large, heavy-duty clamps for connecting battery cables to battery posts.
Application in Nigeria: Seen everywhere from battery connections in "Okada" (motorcycles) to dashboard connections in commercial buses.
3. Connectors (Multiple Wire Connections): Definition: Housings that contain multiple terminals, designed to connect multiple wires simultaneously, usually for specific components or harnesses.
Purpose: Organize wiring, protect multiple connections from moisture and physical damage, prevent incorrect connections (keyed housings), and allow for quick and easy disconnection/reconnection of entire sub-assemblies (e.g., headlamp assembly, engine sensors, control modules).
Types Common in Automotive: Multi-pin Plastic Connectors: The most common type, often with locking tabs to prevent accidental disconnection. They house individual male and female terminals.
Weatherpack/Deutsch Connectors: Sealed, robust connectors designed for harsh environments, providing excellent protection against dust and moisture, often seen in engine bays or chassis wiring of heavy-duty vehicles or high-end cars.
Application in Nigeria: Ubiquitous in modern vehicles. A common sight is a mechanic checking the multi-pin connector for a faulty injector or a sensor on a customer's car.
D. Tools for Wiring
1. Soldering Iron: Description: An electrical tool that heats up a metal tip to a high temperature (typically 300-400°C) to melt solder.
Types: Pencil-style: Common for general electronics and lighter automotive work. Vary in wattage (e.g., 25W for small wires, 40W-60W for heavier wires).
Soldering Gun: Pistol-grip style, heats up very quickly, good for occasional heavier work.
Soldering Station: Features temperature control, allowing precise heat settings, ideal for delicate or varied work.
Proper Use: Preparation: Ensure the tip is clean ("tinned" with a thin layer of solder for good heat transfer). Use a damp sponge or brass wool to clean the tip.
Heating: Allow sufficient time for the iron to reach operating temperature.
Contact: Apply the tip to the work (wires/component lead) to heat it.
Solder Application: Apply solder to the heated work, not the iron tip directly. The solder should flow quickly and smoothly.
Safety: Always wear eye protection. Work in a well-ventilated area to avoid inhaling solder fumes (especially lead-based solder). Be aware of the hot tip (can cause severe burns) and ensure the iron is placed in a safe stand when not in use. Disconnect from power when finished.
2. Crimpers (Crimping Tool): Description: A hand tool specifically designed to mechanically deform a metal terminal or splice barrel around a stripped wire end, creating a strong electrical and mechanical connection.
Types: Insulated/Non-insulated Crimpers: Have different jaws for crimping terminals with or without insulation.
Open Barrel/Closed Barrel Crimpers: Specialized tools for different terminal barrel designs.
Ratchet Crimpers: Provide a full, consistent crimp by not releasing until the crimping cycle is complete, improving reliability.
Proper Use: Matching: Always use the correct crimper type and jaw size for the specific wire gauge and terminal being used. A mismatch will result in a poor connection.
Stripping: Correctly strip the wire to the appropriate length, ensuring no copper strands are cut.
Placement: Insert the stripped wire fully into the terminal barrel.
Crimping: Squeeze the handles firmly and completely.
Safety: Keep fingers clear of the crimping jaws to prevent pinching injuries.
Worked Example 1: Selecting Wire Gauge for a Headlamp Scenario: A mechanic in Aba needs to replace the wiring for a single 55-watt (W) headlamp on a commercial bus operating on a 12-volt
Entrepreneurship and Self-Employment: The skills learned in auto-wiring are directly applicable to establishing small businesses. Graduates can become independent auto electricians, specializing in diagnosing and repairing electrical faults in "Okada" (motorcycles), "Keke Napep" (tricycles), and various types of cars and buses common across Nigeria. This skill offers a viable path to economic empowerment, especially for youth seeking vocational opportunities.
Vehicle Maintenance and Road Safety: Understanding auto-wiring enables vehicle owners and commercial drivers to perform basic electrical maintenance, such as replacing a faulty fuse or repairing a loose connection for a headlamp or horn. This saves money on professional repairs, reduces downtime for commercial vehicles (like "long-distance" buses or "trucks"), and crucially, enhances road safety by ensuring critical components like brake lights, indicators, and horns are always functional. This directly contributes to reducing road accidents, a significant concern in Nigeria. Modern Vehicle Customization and Adaptation: Many vehicle owners in Nigeria customize their vehicles with additional electrical accessories like improved sound systems, security alarms, GPS tracking devices (for fleet management or theft recovery), or extra lighting for rural driving. The knowledge of auto-wiring is essential for safely and effectively integrating these modifications without damaging the vehicle's existing electrical system or creating fire hazards. This application also extends to adapting vehicles for specific uses, such as converting a passenger vehicle for cargo transport, which often involves reconfiguring electrical components.