Low Voltage Wiring Harness Design & Solutions

NVH is the acronym for Noise, Vibration, and Harshness. It is a comprehensive indicator to measure the quality of automobile manufacturing. In this article, wiring harness engineers mainly analyze automotive NVH and wiring harness noise from a design perspective, combined with actual vehicle synchronous development experience.
The comfort of the occupants in the car and the strength and lifespan of automobile parts caused by vibration are all within the scope of NVH research.

The importance of automotive NVH
Generally speaking, the performance of NVH directly determines the comfort level of the vehicle during driving. The higher the NVH score, the higher the overall comfort level of the vehicle, and vice versa. At the same time, vehicle NVH issues have always been one of the most concerning issues for major automobile manufacturers and parts supply companies. According to statistics, about 1/3 of vehicle failure problems are related to the vehicle’s NVH problems, and nearly 20% of the research and development expenses of major automobile manufacturing companies are spent on solving vehicle NVH problems.2. Causes of abnormal noise caused by wiring harness

12V power supply pre-wiring harness assembly margin in luggage compartment area

2.1 The reserve margin is too long
2.1.1 Excessive exercise margin
The movement margin of the wire harness branches of the moving parts is relatively long. When the related electrical components are moving, the wire harness branches are easy to come into contact with the surrounding environment, causing abnormal noise. During actual use, the motion-related parts of the car, such as car seats (usually the front seats), steering column, dashboard glove box, etc., will move frequently. When arranging the wire harness on the moving parts, it is necessary to increase the movement margin, that is, when the component moves, the wire harness cannot restrict its movement. Of course, the longer the exercise margin, the better. When designing, the wire harness path of the moving parts at the extreme positions of each direction of movement should be made, and the size of the longest path should be used as the tentative size of this wire harness segment, and then simulate again whether the tentative size will interfere with surrounding accessories at each position. If there is interference, you should reselect the wiring harness fixing point or negotiate with surrounding components to avoid them. If there is no interference after verification, the size of this section is the reasonable wire harness design size, as shown in Figure 1.

Reserve assembly allowance for docking baseboard wiring harness

2.1.2 Assembly allowance is too long
During the automobile production and assembly process, if the operating space is insufficient, an assembly allowance needs to be reserved. However, if the assembly margin is too long, it will easily cause the wire harness branch to interfere with the surrounding environment, causing abnormal noise. In the production workshop of the main engine factory, in order to improve production efficiency, some parts are usually separately assembled offline and then assembled on the final assembly line, such as the deputy instrument panel, front bumper and rear bumper, etc. At this time, we need to reserve a docking allowance in the early design to facilitate assembly by workers. For the assembly allowance of these Inline dockings, the wiring harness branch generally requires 120~140mm, as shown in the red mark in Figure 2.

In the assembly process of some electrical components, if the electrical components are fixed first, the connector at the wiring harness end will not be able to be assembled due to lack of operating space. This requires pulling out the connector at the wiring harness end to connect with the electrical component, then pushing it back, and finally fixing the electrical component to its original position. Then these wiring harness branches require us to reserve allowance for assembly. For the assembly margin of electrical components, the wiring harness branches generally need to be lengthened by 60~80mm, as shown in the red mark in Figure 3.

As long as the conditions for assembly are met, the shorter the assembly margin, the more controllable the wire harness direction will be, the lower the probability of contact with the surrounding environment, and the lower the probability of abnormal noise from the wire harness branches. In addition, velvet tape is preferred for the covering of the wire harness branches with assembly margin, which can reduce abnormal noise caused by interference between the wire harness and the surrounding environment.

The colored wiring harness is a simulated driver’s seat branch line

2.2 The distance between fixed points is too long
The distance between the two adjacent fixed points of the wire harness is long, which is prone to shaking and abnormal noise. The function of the buckle tie on the wire harness is to fix the wire harness branches and restrict their direction. The cost reduction of eliminating a buckle is minimal, but the side effects are very obvious: the actual wiring harness branch direction is very different from the status in the data. Therefore, the size of two adjacent fixed points should not exceed 200mm. If there is no interference point between the wire harness branch and the surrounding environment, and the wire harness branch segment is a straight segment, the distance between the two fixed points should not exceed 300mm. In addition, the size from the outlet point or fixing point of the wire harness branch to the end of the sheath should not exceed 150mm. When the distance between the two fixed points is too long and the gap with the surrounding environment is small, once the tolerances are corrected during the manufacturing of the wire harness, the wire harness branches will become loose or distorted during actual assembly, and it is easy to interfere with the surrounding environment. Abnormal noise is produced when the car is driving, which affects the riding experience.

2.3 Unreasonable selection of buckle ties
If the buckle tie is unreasonably selected and designed, it may shake or fall off, causing abnormal noise. Each snap tie has its own size and thickness to fit the mounting hole. Within its adaptation range, when the insertion force of the head is ≤60N and the extraction force is ≥120N, failure of the buckle tie can be avoided. If the buckle tie fails, not only will it produce noise when the car is driving, but the fixed wiring harness will also shake and come into contact with the surrounding environment, causing abnormal noise.
In the early design of the wire harness, we should give priority to the use of ordinary round waist ties, which can not only reduce the design cost, but also constrain the direction of the wire harness. Try to avoid using some special-shaped buckles and offset T-shaped buckles to prevent the buckles from shaking due to the vibration of the car itself, resulting in abnormal noise.

Reduce the NVH of the vehicle from the perspective of wiring harness design
3.1 Noise reduction in the wiring harness itself
When designing, take full consideration of the possible abnormal noise caused by the wiring harness itself. The abnormal noise produced by the wiring harness itself is the most direct to the occupants. Therefore, when designing, we must simulate a reasonable movement process and assembly process, select a reasonable wire harness fixing point, and select a suitable tie buckle. In addition, in areas where the wiring harness branches are close to the interior, it is recommended to wrap them all with velvet tape for noise reduction.3.2 Noise reduction in sheet metal
Try to avoid opening holes in the firewall sheet metal and lower body sheet metal. When a car is driving, part of the noise and vibration comes from the chassis. If more holes are opened in the firewall sheet metal and lower body sheet metal to fix the wiring harness, the car will cause greater noise and vibration when driving at high speed, increase NVH, and reduce the comfort of the car. . Therefore, stud cards should be used as much as possible to fix the wiring harness on the firewall sheet metal and lower body sheet metal. During the design, the sheet metal engineer was asked to weld studs with a diameter of 5mm or 6mm for wire harness fixation. If there are some unavoidable wire harness via holes, try to open smaller holes. The size of the opening is based on the largest sheath to be pierced. The common opening size is the diagonal size of the largest sheath to be perforated, plus 6mm as the diameter of the opening. That is, a 3mm gap is left on both sides to ensure that the largest sheath can pass through the sheet metal hole while reducing the sheet metal opening. Noise and vibration from the chassis can be minimized.

3.3 Noise reduction with sound insulation cotton and carpets
Try to avoid compressing the thickness of the insulation cotton and reducing the openings in the carpet. Sound insulation cotton and carpets that belong to soft interiors have the function of sound insulation and noise reduction. The thicker the thickness, the better the sound insulation effect.

3.3.1 Sound insulation cotton
The thickness of sound insulation cotton is generally between 10 and 20 mm, and it is mainly arranged in the passenger compartment cowl and the left and right rear wheel covers. It plays a vital role in the sound insulation of the passenger compartment. Because the sound insulation cotton is installed closely to the body sheet metal, and the wiring harness is fixed on the body sheet metal, as shown in Figure 4 (the brown part is the sound insulation cotton). Therefore, the thickness of the sound insulation cotton that causes contact with the wire harness cannot be too thick, otherwise the wire harness cannot be fixed on the sheet metal. In areas where sound insulation cotton exists, stud cards should be preferred for fixing the wire harness. In this way, the sound insulation cotton only needs to open small holes to avoid the studs. At the same time, the studs should also be given priority to longer specifications, such as studs with a height of 20mm. In this way, a thickness of 8~9mm can be left for the sound insulation cotton, thereby improving the sound insulation effect of the sound insulation cotton. If welding studs are not available in certain areas of the car body, you can use raised buckles with long round holes to achieve the same effect.

3.3.2 Carpet
The thickness of carpet is generally between 10~60mm, which can greatly reduce the noise and vibration from the chassis. The carpet is installed opposite the wiring harness and the soundproofing batt. The carpet is arranged on top of the wire harness, as shown in Figure 5 (the cyan part is the carpet). Therefore, the wiring harness under the carpet should try to avoid using elevated buckles and do not compress the thickness of the foam cotton in the carpet. If some wiring harness branches, such as seat wiring harness branches, need to pass through the carpet to connect to electrical devices, carpet openings are required. Generally, the size of the opening is 10mm larger than the diameter of the wire harness branch. At the same time, take the center of the hole as the origin and open a cross groove. The length of the groove is 20mm longer than the diagonal length of the maximum sheath that can be penetrated to facilitate the sheath to pass through and avoid opening larger via holes that will affect the sound insulation effect of the carpet.

04 Conclusion
A car is assembled from more than 20,000 parts, and car NVH is a comprehensive issue that involves almost every part of the car. The noise generated by the wiring harness, known as the “vessel of the car”, is often the most direct in the passenger compartment. Therefore, when designing the wiring harness, we should focus on the noise and abnormal noise that may be caused by the wiring harness arrangement to avoid vibration and abnormal noise when the car is driving due to unreasonable wiring harness layout. This in turn affects the driving experience of the car.