Research on Five-Anti of Commercial Vehicle Harness

This paper sorts out and analyzes the common failure modes of commercial vehicle Wiring Harness, sums up five failure modes: water ingress, wear, stress, virtual connection and burning, and then puts forward five prevention measures for wire harness, namely, waterproof, anti-wear, anti-stress, anti-virtual and anti-heat source.

Automotive wiring loom, as the nervous system connecting the electrical appliances of the whole vehicle, carries the task of transmitting current and signals. Once the cable harness fails, it will seriously affect the function of the whole vehicle and driving safety. Common failure modes of commercial vehicle harness system mainly include short circuit, open circuit, poor contact, etc. If the reasons are investigated, the failure modes can be further subdivided into water intake, wear, stress, virtual connection, scalding, etc. This paper mainly analyzes the five failure modes and puts forward the corresponding solutions.

1 Waterproof

QC/T 29106 "Technical Conditions for Automobile Wire Harness" defines the dry area and wet area of the whole vehicle. Dry area refers to the area where the electric wire bundles installed in the cab, passenger compartment, luggage compartment, etc. do not need special waterproof protection treatment, and wet area refers to the area where the electric wire bundles need special waterproof protection treatment except the dry area. Waterproofing is mainly aimed at the wiring harness in the transition area between wet and dry areas, including chassis wiring harness, bumper wiring harness, front wall wiring harness, etc., as follows.

1.1 Connector Waterproof

Connectors should be matched with each other, and waterproof connectors with IPX7 protection level and above should be selected.

The wire diameter specification should be selected within the allowable range of the Connector. The sealing plug must be matched with the connector and wire diameter, and the insulation with sealing plug is crimped. The wire insulation layer and the end of sealing plug should be visible in the connection B area. The sealing plug should be prevented from being punctured when the wire is sealed or crimped through the sleeve. A blind plug is added to the vacant hole of the connector, and the blind plug must match the connector.

1.2 Parallel Line Point Waterproof

Wire parallel points should be scattered instead of concentrated, and the parallel points should be welded by ultrasonic welding machine or the parallel terminals should be crimped, and the joints should be smooth to avoid burrs protruding. External contact transparent thickened double-wall heat shrinkable tube with glue (easy for inspection) is required to be protected. It is required to bake a gelatinous substance, so that the gelatinous substance can fill the gap and play a waterproof role.

The parallel connection points of shielded twisted-pair wires used in communication lines shall be pressed by parallel terminals, and the length of the rubber of shielded twisted-pair wires shall not exceed 150mm. The shielding layer shall be reliably grounded after the whole section of heat-shrinkable tube is heat-shrunk, and copper wires are not allowed to be exposed.

Both ends of the heat-shrinkable tube are wrapped at least 1cm. If unilateral wiring is adopted, at least 1cm shall be reserved at the wireless end of the connected heat-shrinkable tube. For heat shrinkage, the wire diameters at both ends should be reasonably arranged, and the cross-sectional area of the wire diameters at both ends and the outer diameter of one end of the multi-wire should be less than 3 times of the outer diameter of the less-wire side, so as to ensure the heat shrinkage quality of the heat shrinkable tube.

1.3 Anti-Siphon

The siphon phenomenon is mainly caused by the temperature rise in or around the vehicle's electrical appliances (such as urea pump, nitrogen and oxygen sensor, headlamp, etc.) when they work. After the vehicle stops working, the temperature changes inside the electrical appliances generate negative pressure relative to the outside world, and at this time, water vapor will enter the electrical appliances along the wiring harness through the connector in a small amount.

In view of the failure caused by siphon, the tail of the connector can be changed into one-piece injection molding process by low-pressure injection molding, and the anti-siphon cable (tinned copper wire core and coated with specific waterproof ointment) can be matched. The injection molded connector mainly prevents water vapor from entering the electrical appliance from the root of the connector, and the anti-siphon cable can solve the problems of copper wire corrosion and product short circuit caused by water vapor entering the connector through the cable.

The improved weight-closing connector can meet the test standards: 1m deep immersion test (IPX7) and multi-angle high-pressure and high-temperature water jet test (IPX9K).

1.4 High-Pressure Water Gun Washing Direct Injection Protection

Urban environmental protection has put forward higher requirements for the cleanliness of urban muck trucks. Repeated high-pressure water gun washing of vehicles entering and leaving the construction site has a great adverse impact on the watertightness of wiring harness connectors. Washing and direct injection protection is mainly applied to this kind of working condition solution, as follows.

1) Tail clamp protection: Plastic or rubber tail clamp protection is added to the tail of the harness connector in the wet area and the transition area between wet and dry areas. The tail clamp can prevent the high-pressure and high-speed water flow from directly impacting the connector root to damage the seal to a certain extent.

2) Protective cover protection: mainly aimed at the protection of heavy-duty connectors, that is, the protection of connectors that affect driving safety or precision electronic components, such as engine ECU, etc.,.

1.5 Waterproof Treatment of Reserved Connector

The reserved connector shall be waterproof in the form of plug-in, and the waterproof grade of plug-in connector shall meet IPX7 requirements and above.

2 Anti-wear

Harness wear is mainly caused by interference, which is related to harness layout, fixing method and protection selection. It can be divided into static interference wear and dynamic interference wear.

2.1 Static Interference Wear Solution

Static interference wear often occurs in the pipe beam, bumper, frame or sheet metal through hole of instrument panel.

2.1.1 Pipe Beam Harness of Instrument Panel

There are many switches and electrical appliances on the instrument tube beam harness, and the arrangement is relatively compact. Due to space constraints, plastic trunking or cable protection boxes are often used to effectively fix and protect the harness. Nylon (PA66) and glass fiber composite materials are generally used as materials in our company, Kable-X.

2.1.2 Bumper Harness

Bumpers have many sheet metal and tough edges, and most of them are fixed by edge buckle tie, wedge-shaped or fir-tree type snap tie and bolt tie, as shown in Figure 5.

2.1.3 Frame or Sheet Metal Through Hole

Rubber sleeve protection is added at the via hole. If the outer diameter of the branch of the via hole harness is not less than 25mm, it should be considered to add a guide bracket and use soft harness protection material to adjust the softness of the harness according to the actual situation, so as to avoid excessive extrusion of the harness on the rubber sleeve.

Soft protective materials can be used in combination with PVC sleeves and self-rolling textile sleeves.

2.2 Dynamic Interference Wear Solution

Unreasonable fixing of the harness between components with relative motion or insufficient reserved safety distance will often lead to dynamic interference and wear of the harness. Common faults include door harness, seat harness, chassis-cab harness, engine harness, transmission harness, rear battery frame harness, etc.

Corrugated pipes, which are often used as protective materials, will also cause a certain degree of wear on cables. When designing the wiring harness, it is necessary to consider reasonable fixation, and at the same time, it is necessary to reserve enough length at the joint, and prevent excessive redundancy accumulation of the wiring harness.

2.2.1 Simulation of Harness Flexibility

It can simulate the flexible movement path of the wire harness under external force and gravity, dynamically display the spatial deformation, bending and torsion state and stress and strain distribution of the wire harness, and effectively avoid the risk of wire harness wear and failure caused by dynamic interference.

2.2.2 Selection of Bellows

UFW super flat corrugated pipe should be used as much as possible. The width of the wave trough is 3~5 times that of other waves, which can effectively increase the contact area between the wire and the inner wall of the corrugated pipe, thus playing a better role in protecting the PVC skin of the wire.

3 Stress Prevention

The harness failure caused by stress mainly occurs at the bend of the harness and the harness between the relative moving parts, and its manifestations are mostly harness breakage, connector loosening and falling off, terminal withdrawal and so on. The solution is as follows.

1) Requirements of bend. For the wire harness with a diameter D of less than or equal to 35mm at the bend, the radius R of the fillet of the wire harness should be greater than D; When the wire diameter D is greater than 35mm, the radius R of the fillet in the wire harness should be greater than 1.2d.

2) Fixing and checking of wire harness between relative moving parts. The moving body connector (such as engine ECU) needs to be provided with a fixed point at a distance of 100 ~ 150 mm from the tail of the connector, so as to reduce the vibration of the terminals in the connector and the quality of the wiring harness, and a fixed point should be provided at the branch of the main wiring harness.

3) Softness and protection requirements of wire harness. The softness and hardness of the wire harness will affect its own stress, so the outer protection of the wire harness between the relative moving parts should be made of soft materials as much as possible to better cushion the stress caused by the relative movement.

The battery harness can be made of ultra-soft PVC insulated low-voltage cable, and the conductor material is bare copper soft conductor. Self-rolling corrugated casing can be used for external protection.

4 Anti-Virtual Connection

There are many reasons for virtual connection, which can be divided into connector quality, connector looseness, slight terminal withdrawal, terminal corrosion and oxidation, unreasonable fixing method, improper assembly or debugging, etc. The solution is as follows.

1) The quality of connector mainly includes matching degree and terminal crimping quality, and QC/T29106 "Technical Conditions of Automobile Wire Harness" puts forward clear requirements for terminal crimping.

2) The connector is loose and the terminal needle is withdrawn. Mainly affected by three aspects, one is related to the quality of the connector; Second, it is related to the stress of the connector; Third, it is related to the defects of the locking mode of the connector itself, such as the wheel speed sensor connector, which can be solved by adding connector connecting cards.

3) The terminal is corroded and oxidized. Corrosion and oxidation of terminals are mainly caused by water ingress into the sheath.

4) During assembly in the workshop, it should be avoided that the connector or terminal is damaged due to the drag of the harness; After the connector is assembled and pulled back, it will be marked after passing the inspection.

5) At present, the common connector terminals of commercial vehicles are mostly chip-connected. Improper use of debugging tools, such as needle picker and multimeter, on the one hand, will lead to the damage of the sheath or terminal, and on the other hand, the gap between the female terminals will become larger. Commissioning personnel should use special terminal withdrawal jig and taper probe multimeter.

5 Heat Source Prevention

Commercial vehicle heat sources mainly include engine, exhaust pipe, muffler, air pump pipe, etc. Common thermal protection measures include: aiming at the heat source part with a temperature of ≥200℃, setting an insulation board (aluminum-plated steel plate) between it and the wire harness, and ensuring that the distance between the wire harness and the insulation board is ≥ 50mm; Wrap heat insulation materials on the heat source parts, such as basalt or aerogel insulation layer on the outer wall of the exhaust pipe; Wrap thermal insulation materials, such as aluminum foil sleeves, aluminum foil tape, etc., outside the bundle, and ensure the spacing of more than 50mm.

6 Total Knot

The protection design of wire harness is an important and complicated project, which often needs to consider various factors. In this paper, the five common failure modes of commercial vehicles are analyzed in detail and the corresponding solutions are put forward, which has certain reference significance for improving the protective performance of wiring harness.