In the application of automobiles, the hidden dangers of wire harness faults are strong, but the advantages of fault hazards are significant, especially in cases of wire harness overheating and short circuits, which can easily lead to fires. Timely, fast, and accurate identification of potential faults in wiring harnesses, reliable repair of faulty wiring harnesses, or correct replacement of wiring harnesses, is an important task in automotive maintenance. It is an important measure to prevent car fire accidents and ensure the safe and reliable use of automobiles.
1. The function of automotive wiring harnesses
In order to facilitate the installation and neat layout of car wiring, protect the insulation of wires, and ensure the safety of car use, the entire car wiring (car high-voltage lines, UPS battery wiring harnesses) on the car are connected The use of cotton yarn or thin polyvinyl chloride tape wrapped and wrapped in bundles in zones (excluding starter cables) is called a wiring harness, which is generally divided into engine wiring harness, chassis wiring harness, and vehicle wiring harness.
2. Composition of wiring harness
The wiring harness is composed of wires with different specifications and performance requirements. The main specifications and performance requirements are as follows:
1. The cross-sectional area of the wire
According to the load current of the electrical equipment, the cross-sectional area of the wire is selected. The general principle is that for electrical equipment that works for a long time, a wire with an actual current carrying capacity of 60% can be selected, and for electrical equipment that works for a short time, a wire with an actual current carrying capacity between 60% and 100% can be selected; At the same time, voltage drop and wire heating in the circuit should also be considered to avoid affecting the electrical performance of electrical equipment and the allowable temperature of wires; To ensure a certain mechanical strength, the cross-sectional area of low-voltage conductors is generally not less than 1.0mm ²。
2. Color of wires
There are color and numbering features on car circuits. With the increase of automotive electrical equipment, the number of wires is also constantly increasing. In order to facilitate identification and maintenance of automotive electrical equipment, low-voltage wires in automotive circuits are usually composed of different colors and marked with letter codes of colors on the automotive electrical circuit diagram.
The color code (represented by one or two letters) of the wires is usually marked on the car circuit diagram. The colors of the wires on the car are generally different, and there are two commonly used selection principles: single color and dual color. For example: red (R), black (B), white (W), green (G), yellow (Y), black and white (BW), red yellow (RY). The former is the main color in the two tone line, and the latter is the auxiliary color.
3. Physical properties of wires
(1) Bending performance, door wiring harness between door and cross body（ https://www.shx-wire.com/door-wiring-harness-car-horn-wire-harness-audio-connection-harness-auto-door-window-lifter-wiring-harness-sheng-hexin-product/ ）It should be composed of wires with good winding performance.
(2) High temperature resistance, wires used in high temperature areas are generally coated with vinyl chloride and polyethylene with good insulation and heat resistance.
(3) Shielding performance, in recent years, the use of electromagnetic shielding wires in weak signal circuits has also been increasing.
4. Binding of wiring harnesses
(1) The cable half stack wrapping method involves applying insulation paint and drying to increase the strength and insulation performance of the cable.
(2) The new type of wiring harness is wrapped in plastic and placed inside the plastic corrugated pipe of the side cut, which increases its strength and better protection performance, making it more convenient to find circuit faults.
3. Types of car wiring harness faults
1. Natural damage
The use of wire harnesses beyond their service life leads to wire aging, insulation layer rupture, significant decrease in mechanical strength, causing short circuits, open circuits, grounding, etc. between wires, resulting in wire harness burnout. The oxidation and deformation of wire harness terminals can cause poor contact, which can cause electrical equipment to malfunction.
2. Electrical faults causing damage to the wiring harness
When electrical equipment experiences overload, short circuit, grounding and other faults, it may cause damage to the wiring harness.
3. Human error
When assembling or repairing automotive components, metal objects can crush the wire harness, causing the insulation layer of the wire harness to rupture; Improper position of wire harness; The lead position of electrical equipment is connected incorrectly; The positive and negative leads of the battery are reversed; Improper connection and cutting of wires in electrical harnesses during circuit maintenance can cause abnormal operation of electrical equipment, and even burn out the wire harnesses.
4. Inspection methods for automotive wiring harnesses
1. Visual inspection method
When a certain part of the automotive electrical system malfunctions, abnormal phenomena such as smoke, sparks, abnormal noise, burnt smell, and high temperature may occur. By visually inspecting the car wiring harness and electrical appliances through the sensory organs of the human body, such as listening, touching, smelling, and looking, the location of the malfunction can be determined, greatly improving the maintenance speed. For example, when there is a malfunction in the car wiring, abnormal phenomena such as smoke, sparks, abnormal noise, burnt smell, and high temperature often occur. Through visual inspection, the location and nature of the fault can be quickly determined.
2. Instrument and meter inspection method
The method of diagnosing automotive circuit faults using comprehensive diagnostic equipment, multimeter, oscilloscope, current clamp and other instruments and meters. For electric control system vehicles, a fault diagnosis instrument is generally used to search for fault codes to diagnose and measure the range of faults; Use a multimeter, current clamp, or oscilloscope to check the voltage, resistance, current, or waveform of the relevant circuit in a targeted manner, and diagnose the fault point of the wiring harness.
3. Tool inspection method
The lamp test method is more suitable for checking wire short circuit faults. When using the temporary lamp test method, attention should be paid to the power of the test lamp not being too high. When testing whether the control output terminal of the electronic controller has output and whether there is sufficient output, special caution should be taken to prevent overloading and damage to the controller during use. It is best to use a diode test light.
4. Wire Jumping Inspection Method
The jumper method involves using a wire to short circuit a suspected faulty circuit, observing changes in the instrument pointer or the working condition of electrical equipment, in order to determine whether there is an open circuit or poor contact in the circuit. Jumping refers to the operation of connecting two points in a circuit with a single wire, and the potential difference between the two points in the crossed circuit is zero, not a short circuit.
5. Repair of wiring harnesses
For minor mechanical damage, insulation damage, short circuit, loose wiring, rust or poor contact of wire joints in obvious parts of the wiring harness, repair methods can be used; To repair a wiring harness malfunction, it is necessary to thoroughly eliminate the root cause of the malfunction and eliminate the possibility that it may occur again due to the fundamental cause of vibration and friction between the wire and metal parts.
6. Replacement of wiring harness
For faults such as aging, severe damage, internal wire short circuits, or internal wire short circuits and open circuits in the wiring harness, it is usually necessary to replace the wiring harness.
1. Check the quality of the wiring harness before replacing it.
To ensure the safety and reliability of the wiring harness, strict control must be carried out before use, and certification inspections should be carried out. Any defects found should not be used to prevent the harm caused by unqualified products. If conditions permit, it is best to use instruments for inspection.
The inspection includes: whether the wiring harness is damaged, whether the connector is deformed, whether the terminals are corroded, whether the connector itself, the wiring harness and the connector have poor contact, and whether the wiring harness is short circuited or not. The inspection of wiring harnesses is essential.
2. Only after troubleshooting all electrical equipment on the vehicle can the wiring harness be replaced.
3. Wire harness replacement steps.
(1) Prepare wire harness disassembly and assembly tools.
(2) Remove the faulty vehicle’s battery.
(3) Disconnect the connector of the electrical device connected to the wiring harness.
(4) Make good work records throughout the entire process.
(5) Release the wire harness fixing.
(6) Remove the old wiring harness and assemble the new wiring harness.
4. Verify the correctness of the new wiring harness connection.
The correct connection between the wire harness connector and electrical equipment is the first thing to confirm, and it is also necessary to ensure that the positive and negative terminals of the battery are connected correctly.
During the inspection, it is possible to display a ground wire that is not connected to the battery, and instead use a light bulb (12V, 20W) as a test light. Before this, all other electrical devices in the car should be turned off, and then a test light string should be used to connect the negative terminal of the battery to the chassis ground. Once there is a problem with the circuit, the test light will start to turn on.
After troubleshooting the circuit, remove the light bulb and connect it in series with a 30A fuse between the negative terminal of the battery and the ground terminal of the frame. At this time, do not start the engine. Connect the corresponding power equipment on the vehicle one by one, and conduct a comprehensive inspection of the relevant circuits one by one.
5. Power on work inspection.
If it is confirmed that there are no problems with the electrical equipment and related circuits, the fuse can be removed, the battery grounding wire can be connected, and power on inspection can be carried out.
6. Check the installation of the wiring harness.
It is best to check the installation of the wiring harness to ensure that it is installed correctly and securely.
Post time: May-29-2024