Design and Material Selection of Automotive Wiring Harnesses

Automotive wiring harnesses are the network backbone of automotive circuits, with the improvement of safety, comfort, economy and emission requirements of cars，Car wiring harnesses are becoming more complex. But the space provided by car body for the wiring harness is getting smaller and smaller. Therefore, how to improve comprehensive performance design of automotive wiring harnesses has become the most important focus. Moreover automotive wire harness manufacturers are no longer solely engaged design and manufacturing of wire harnesses. It has become an inevitable trend to collaborate with automotive host manufacturers for early stage development. Based on our years of experience in wire harness design and manufacturing, let’s discuss the general design process and principles of wiring harnesses.

1. Automotive overall circuit design

1-1 Power distribution design

Whether the design of the power supply system is reasonable or not directly related to the normal operation of automotive electrical components and the safety of entire vehicle. Therefore, the starting point of automobile wiring harness design in the world is mainly promise safety. The entire vehicle electrical system is basically composed below these three parts.

Battery direct power supply system. The load connected to this part of the power supply is generally a safety or important component of the car, The main purpose is to control as little as possible when providing electrical energy to these devices, Ensure that these components can work normally for a short period of time even if the car cannot start. For example: the working power supply of engine ECU and engine sensors、the working power supply of the fuel pump、Power supply for ABS controller、Diagnoser interface power supply etc.

Power supply system by ignition switch. These electrical components are mainly used when the engine is running, The power source taken from the generator avoids the possibility of competing for power when charging the battery. For example：Instrument power supply、Brake power supply、Airbag power supply etc.

Unloading its’ load power supply when engine starts. Electrical components generally carry a large load, and there is no need to work when the car starts. Generally, cigarette lighter power supply、air conditioning power supply、entertainment power supply、wiper power supply... ...

1-2 Circuits protection design

Circuits protection is to protect the wires, Considering the protection of circuit electrical components. The main protective devices include: fuse、circuit breaker and fusible link.

1-2.1 Fuses selecting：

The engine ECU, ABS, etc. have a significant impact on the overall performance and safety of the vehicle, And electrical components that are susceptible to interference from other electrical equipment must be equipped with separate fuses.

Electrical components such as engine sensors、various alarm signal lights and external lighting、horns also have a significant impact on the overall performance and safety of the vehicle. But this type of electrical load is not sensitive to mutual interference. So this type of electrical load can be combined with each other and share the same fuse according to the situation.

The electrical loads of ordinary electrical components set up to increase comfort can be combined with each other and share the same fuse according to situation.

Fuses are divided into fast fusion and slow fusion. The main component of the fast melting fuse is thin tin wire. The chip fuse has the advantages of simple structure, good reliability, good vibration resistance and easy detection, so it is widely used. Slow melting fuses are actually tin alloy sheets. Fuses of this structure are usually connected in series to inductive load circuits, such as motor circuits.

The load of the resistor and load of the inductor should avoid using the same fuse as much as possible.

The fuse capacity is generally calculated and determined based on the maximum continuous working current of electrical components, It can be calculated according to empirical formulas: rated capacity of fuse=maximum working current of the circuit ÷ 80% (or 70%).

1-2.2 Circuit breaker：

The biggest characteristic of circuit breakers is their recoverability, but their cost is high leads to rarely used. Circuit breakers are generally thermal sensitive mechanical devices that utilize the different thermal deformations of two metals to make the contacts open and close or self connect. A new type of circuit breaker uses PTC solid material as an overcurrent protection component, which is a positive temperature coefficient resistor that opens or closes according to the current or temperature. The biggest advantage of this type of protection element is that it can automatically connect after troubleshooting, without manual adjustment and replacement.

1-2.3 Fusible wire

The characteristic of fusible wire is that when wire passes through a large overload current, the fusible wire can fuse for a certain period of time (generally ≤ 5s), thereby cutting off power supply and preventing the occurrence of malignant accidents. Fusible wire is also composed of a conductor and an insulation layer, The insulating layer is usually chloro-sulfonated polyethylene material. Because the insulating layer is thicker, it looks thicker than the wire of the same specification.

The fusible wire is generally connected to the circuit directly led out of the battery. The commonly used nominal cross-sections of fusible wires include 0.3mm2, 0.5mm2, 0.75mm2, 1.0mm2, 1.5mm2, and even larger cross-section fusible wires such as 8mm2. The length of wire segments for fusible wires can be divided into three types: (50 ± 5) mm, (100 ± 10) mm, and (150 ± 15) mm.

Fusible wires should have clear marks, and their marks should still exist for easy replacement when they are blown. The characteristics of fusible wires are shown in Table 1.

                   Table 1 Fusing Characteristics of Fusible wire

1-3 Selection and design of relays

Relays are divided into two types: current type and voltage type. The selection of relays is generally determined based on the power of the electrical appliance and the carrying capacity of the switch. Common relay equipment generally includes wiper, horn, defroster, headlamp, fog lamp, fan, blower, turn signal (flasher), etc. There are three types of relays: 6V, 12V, and 24V. Commonly used relays having a rated voltage of 12V.

Technical requirements for selecting relays: ① Good reliability; ② Stable performance; ③ Light weight, small in size, long service life, with minimal impact on surrounding components; ④ Simple structure, good processability, and low cost.

1-4 Design principles for grounding distribution

The engine ECU, ABS, and other components have a significant impact on the overall performance and safety of the vehicle, and are susceptible to interference from other electrical devices. Therefore, the grounding points of these components must be set separately.

For airbag system, it is best to use a dual grounding because its grounding point should not only be set separately, but also to ensure its safety and reliability. The purpose is that one of the grounding points fails, and the system can be grounded through another grounding point to ensure safe operation of the system.

The radio system also needs to be grounded separately to avoid interference.

The grounding of weak signal sensors should be independent, and the grounding point should be located closer to the sensor to ensure the true transmission of the signal.

Other electrical components can be combined with each other to share a grounding point according to the specific arrangement. The principle is to ground nearby to avoid unnecessary voltage drops caused by excessively long grounding wires.

Due to the large cross-sectional area of the battery negative wire, engine grounding wire, etc., it is necessary to control the wire length and direction to reduce voltage drop; To increase safety, the engine and vehicle body generally need to be separately connected to the negative electrode of the battery for grounding;

Grounding method: One is to ground through a hole type joint, which must be insulated with a heat shrink tube at the end of the joint; The second is to directly ground through the internal short circuited sheath.

2.    3D layout and direction design of wire harness

2-1 This process mainly simulates the routing and diameter of wire harnesses in different areas, considers the sealing and protection of wire harness vias, and simulates the fixing hole positions and fixing methods of wire harnesses, as shown in Figure 1. The main software used for 3D wiring includes PRO-E, UG, and CATIA and so on.

3. Selection and design of connectors

The connector is core component of wiring harness, and the performance of the connector directly determines the overall performance of the wiring harness, and plays a decisive role in the electrical stability and safety of the entire vehicle

3-1 Design principles for selecting connectors

The selection of connectors should ensure good contact with electrical components, minimize contact resistance, improve reliability, and prioritize use of dual spring compression structure connectors.

Reasonably select connectors based on the cross-sectional area of wire and size of the passing current.

Due to high temperature and humidity inside the engine compartment, as well as the presence of many corrosive gases and liquids, it is necessary to choose a waterproof sleeve.

If the same type of sleeve is used in the same wiring harness, its color must be different.

Based on the overall coordination of the car's appearance, black or dark protective sleeves should be preferred in engine compartment.

To reduce the types and quantity of sleeve used for wiring harness docking, hybrid components are preferred to facilitate assembly and fixation.

Gold-plated terminal should be priority used for airbags、ABS and ECUs because all them require high performance are preferred to ensure safety and reliability.

The internal part of the battery connector (battery clamp) is a cone with a taper of 1:9; The material of the battery clamp is tinned copper, galvanized copper, or lead antimony alloy.

Connectors of different specifications can carry the following currents: series 1, about 10A; series 2.2 or 3, around 20A; series 4.8 , around 30A; series 6.3 , around 45A; series 7.8 or 9.5, around 60A.

3-2 Performance analysis of raw materials (materials) for connectors

3-2.1 Shell material (plastic parts)

The commonly used materials include PA6, PA66, ABS, PBT, PP, etc. Their specific performance differences as shown in Table 2. When designing plugs, different materials can be selected according to different needs, and flame retardant or reinforcing materials can be added to the plastic according to actual situations to achieve the purpose of reinforcement or flame retardancy, such as adding glass fiber reinforcement.

               Table 2 Performance Differences of Connector Sleeve Materials

3-2.2 Terminal material (copper parts)

The copper used for connectors is mainly brass and bronze (the hardness of brass is slightly lower than that of bronze), with brass accounting for a larger proportion. In addition, different coatings can be selected according to different requirements.

Selection and design of wires

4-1 Selection of wire type

The selection of wire type for wiring harness design focuses on the environment and function in which the wiring harness is located. The design of wire harness mainly considers the environment and function of wire harness. For example, the ambient temperature around the engine is high and corrosive gases and liquids are abundant. Therefore, be sure to use high temperature resistance, oil resistance, vibration resistance, friction resistance wire. The wires on the luggage compartment cover should maintain their elasticity at low temperatures, so cold elastic wires should be selected to ensure their normal operation; The wires on the automatic transmission must be resistant to high temperature and hydraulic oil, and their temperature stability is good; Weak signal sensors should use shielded wires, such as knock sensors, crankshaft position sensors, ABS wheel speed sensors, etc; The requirement for bending resistance of the door inner line is high.

The commonly used wires for automotive wiring harnesses usually use stranded copper wires with PVC insulation material as the insulation material. The wires used for wiring harnesses should have characteristics such as temperature resistance, oil resistance, wear resistance, waterproofing, corrosion resistance, oxidation resistance, flame retardancy, etc.

The commonly used wire types for automotive wiring harnesses include several types, including Japanese standard (AVSS, etc.), national standard (QVR), German standard (FLRY), American standard, etc. The characteristic of AVSS (AVS) wire is thin skin insulation and good flexibility; The characteristics of QVR are thick insulation skin, relatively soft, and good ductility; German standard wires have thinner insulation and better flexibility; The insulation skin of American standard wires is generally made of thermoplastic or thermosetting elastomers, as well as processed through irradiation technology. The appropriate type of wire can be selected according to the needs of users and different working environments.

4-2 Calculate and select the cross-sectional area of wire

Calculate the current of the flowing wire based on the power of the electrical components; Electrical equipment that works for a long time can choose conductors with an actual current carrying capacity of 60%; Electrical equipment that operates for a short period of time can use wires with an actual current carrying capacity of 60% -100%.

Adjust the cross-sectional area of wire appropriately according to different working environments and temperature levels.

Adjust the cross-sectional area of wire appropriately based on the direction of the wire and the number of connectors (i.e. the magnitude of voltage drop).

Some experts have summarized some empirical formulas for calculating the cross-sectional area of wires:

I=P/UsA=I ρ L/Ud

In the formula: I - current; P - Power; Us - system voltage; A - cross-sectional area of the wire; Ud - maximum allowable voltage drop loss; ρ—— Copper resistivity; L - Wire length.

Or according to the following empirical formula:

I=A × 10+8/2

Table 3 Empirical Values of the Relationship between Permissible Current and Wire Cross Section Area

4. Design of the entire vehicle wiring harness seal (rubber part)

When passing through holes in car wiring harnesses, rubber parts are generally used for transition to provide wear resistance, waterproofing, sealing, and other functions. It is mainly distributed in the following areas: the interface between the engine and the cab, the interface between the front cabin and the cab (2 places in total), the interface between the four doors (or with a back door) and the carriage, and the inlet of the fuel tank.

The commonly used materials are natural rubber, chloroprene rubber, silicone rubber, EPDM, etc.

The characteristics of natural rubber: it has good elasticity and mechanical strength, excellent bending flexibility, high tear strength, and good cold resistance. Disadvantages: Not very good aging resistance, not resistant to oil and ozone, flammable.

The characteristics of chloroprene rubber include good resistance to ozone, heat aging, oil, and other properties, as well as flame retardancy and self extinguishing properties; But the low temperature resistance is not good.

The characteristics of silicone rubber include good heat resistance, cold resistance, and weather resistance; The disadvantage is that it is not oil resistant. The characteristics of EPDM include good weather resistance, ozone resistance, heat resistance, corrosion resistance, acid and alkali resistance, as well as high strength and high elongation; Disadvantages: Poor adhesion and elasticity compared to natural rubber, resulting in poor oil resistance.

By comparison, EPDM material generally was used for automotive wiring harnesses because the comprehensive performance of EPDM is better than others.

Design of whole vehicle wiring harness binding and fixation

6-1 Wire harness wrapping design

The wiring harness wrapping design plays a role in wear resistance, flame retardancy, corrosion prevention, interference prevention, noise reduction, and beautification of the appearance. The following wrapping design scheme is generally formulated based on the working environment and space size.

The working environment of the engine wiring harness is harsh, so it is all wrapped with corrugated pipes with high flame retardancy, waterproof, and high mechanical strength.

The working environment of the front cabin  is also relatively poor, and most branches are also wrapped with flame retardant corrugated pipes, while some branches are wrapped with PVC pipes.

The working space of the instrument is relatively small, and the environment is relatively good. It can be fully wrapped with tape or flower wrapped.

The working space for the door and ceiling lines is relatively small, and they can be fully wrapped with tape. Some branches can be wrapped with industrial plastic cloth; The thinner roof line can be directly adhered to the vehicle body with sponge tape.

The chassis wire is wrapped with corrugated pipes to prevent wire harness wear due to its frequent contact with the vehicle body.

6-2 Performance analysis of raw materials used for bandaging

6-2.1 Corrugated pipe

Corrugated pipes generally account for about 60% or even more in wire wrapping. The main characteristic is good wear resistance, with good resistance to high temperatures, flame retardancy, and heat resistance in high temperature areas. The temperature resistance of corrugated pipes is between -40-150 ℃. Its materials are generally divided into two types: PP and PA. PA material is superior to PP material in terms of flame retardancy and wear resistance; But PP material has stronger bending fatigue resistance than PA material.

6-2.2 PVC pipe

The function of PVC pipes is similar with corrugated pipes. PVC pipes have good flexibility and resistance to bending deformation, and they are generally closed. Therefore, PVC pipes are mainly used at the branch of wire harness bends to ensure a smooth transition of wires. The heat resistance temperature of PVC pipes is not high, usually below 80 ℃.

6-2-3 Braided tube

Braided pipe has strong elasticity and good scalability. It can be used normally between -40 ° C and 150 ° C when idle, and has a flame retardant function of 94-V0. It is highly flame-retardant, fireproof, and radiation resistant; Made of PET, nylon monofilament, and polyester fiber, it is wear-resistant and soft.

6-2.4 Adhesive tape

Tape plays a role in bundling, wear resistance, insulation, flame retardancy, noise reduction, and marking in the wire bundle, and generally accounts for about 30% of the bundling materials. The tape used for wiring harnesses is generally divided into three types: PVC tape, air flannel tape, and cloth based tape. PVC tape has good wear resistance and flame retardancy; The temperature resistance is around 80 ℃, with poor noise reduction and relatively low price. The material of flannel tape and cloth based tape is PET. Velvet tape has the best binding and noise reduction properties, with a temperature resistance of around 105 ℃; The cloth based tape has the best wear resistance, with a maximum temperature resistance of around 150 ℃. The common disadvantage of flannel tape and cloth based tape is poor flame retardancy and high price.

6-3 Wire harness fixing design

The central electrical box is generally fixed with steel plates, bolts, etc., or directly installed on the vehicle body with the fixed structure designed by the electrical box itself.

Each wire harness is generally fixed in car inner hole with plastic ties, hooks, etc. The inner holes are mostly circular or elliptical, with diameters ranging from 5mm, 6mm, and 7mm.

The sleeve that connect between each wire harness are generally collectively fixed with sheathing brackets and installed on vehicle.