Design of automobile harness grounding wire

With the improvement of automotive electrical integration, the automotive harness becomes more and more complex, and the design and production process control become more and more difficult. One of the most important factors in automobile circuit is the design of iron point.

1. Design principle of tower distribution

1.1 introduction to grounding types.

Vehicle ground: as the name suggests, it is the site of the vehicle circuit. The negative pole of the battery is directly connected to the vehicle body, so that the whole vehicle body is connected with the negative pole of the battery. All grounding points are grounded through the body, so the grounding in the car circuit is also called grounding.

Power: it mainly refers to the grounding of high-power electrical equipment, such as engine cooling fan, wiper motor, window lifting motor, air conditioning blower, seat adjustment motor, sunroof motor, door lock motor, etc. The current of these appliances is generally large, which will interfere with other weak current or signal lines.

Signal location: generally refers to the grounding of electrical equipment with signal transmission. The transmission signal includes analog signal, digital signal, etc. the signal is generally sensitive and vulnerable to interference. Grounding of shielding layer: for entertainment system antenna and high-voltage working appliance, due to their great impact on the surrounding electromagnetic field, single core shielded wire must be used to ensure the accuracy of received signal and minimize the impact on the surrounding harness electromagnetic field. Moreover, the shielding layer of single core shielded wire is directly connected to the ground of the whole vehicle through the grounding point. If the power supply circuit of the engine ignition coil works, tens of thousands of volts high voltage will be generated, which will cause great interference to the surrounding signal lines, and even lead to the EMC failure of the whole vehicle. Single core shielded wire shall be used, and the shielding layer shall be connected with the vehicle body.

1.2 grounding principle.

Generally speaking, there are three principles for the distribution of grounding points.

(1) Separate strong and weak current grounding principle. If the motor product is a high current electrical appliance, it shall be separated from small current grounding such as signal line and control circuit.

(2) The principle of separate grounding of safety components, such as airbag module, ABS, ECM and other modules that have a great impact on the performance and safety of the whole vehicle, shall be grounded separately; For headlamp grounding, considering that one grounding fails and the other can continue to be used, the left and right headlamps must be grounded separately.

(3) In the principle of nearby grounding, considering economy, small voltage drop and minimum interference, the grounding point shall be as close as possible to the module end, so that the grounding wire is short, the wire cost is low, the voltage drop of harness circuit is small, and the possibility of interference is reduced, especially the grounding of weak current signal to ensure the real transmission of signal.

In addition to the above three basic principles, according to the module characteristics, there are the following points:

(1) Airbag module is a module with high safety level. In addition to separate grounding, it also needs backup grounding as a backup scheme to ensure the accurate and timely operation of airbag system;

(2) All grounding points shall be protected from painting / pollution, and the grounding shall not be lower than the wading height;

(3) Battery negative wire, engine iron wire, etc. Due to the large cross-section of the conductor, the length and direction of the conductor must be controlled to reduce the voltage drop;

(4) In order to improve the safety, the engine body generally needs to be connected with the negative electrode of the battery separately; At the same time, if the crimping terminal is greater than 10mm, tinning and spot welding are required.

2. Grounding form

(1) Body grounding form. At present, there are two main grounding forms for mainstream models. The first is the body welding nut, which is used to fasten the harness grounding terminal with the chip removal function bolt; The second is to weld the grounding bolt on the body and fasten the harness grounding terminal with nuts.

These two forms have their own advantages and disadvantages. Body welding nuts are mostly used in Japanese and Korean cars, and the process is complex. Both hands are required to cooperate during assembly, and the requirements for grounding bolts are high. The screw is required to have a certain self tapping function. When fastening the grounding terminal, the paint and oxide layer of the welding nut can be scraped off to achieve good grounding performance. This form of grounding generally protects the body welding nuts, that is, add process screws before body electrophoresis, and take out process bolts after electrophoresis to ensure that the grounding points are free of paint and good grounding performance. This form is more beautiful after assembly.

Body welding bolts are mostly used in European and American cars, and the assembly process is simple; The welding bolts shall be fastened with process nuts before electrophoresis, and then the nuts shall be loosened when assembling iron terminals. The advantage of this design is that it can well protect the grounding bolt, will not be covered by electrophoretic paint, and can ensure good grounding performance. However, because this form is welded bolts, the car body will highlight a part, which is not beautiful, so it should be easy to install in the selection of location, take into account the beauty, and set it where it is not easy to find.

(2) Form of harness grounding terminal. The selection of harness grounding terminal is mainly based on the following information: current carrying, grounding screw or nut size, layout position, whether anti rotation is required, etc. The following are several mainstream grounding terminals and their applications.

① Large terminals are generally selected for large current such as battery negative pole and engine grounding, which are suitable for harness with wire diameter greater than 10mm2. Tinned spot welding and heat shrinkable tube with glue are often used at the crimping part of the harness.

② The general grounding terminal is generally of combined type. In order to prevent the harness grounding terminal from following during installation, the grounding belt anti rotation mechanism is selected.

③ If there are multiple grounding circuits at the same grounding point, the grounding circuits can be spliced in the following three forms to reduce the number of grounding points.

Form 1: all grounding circuits at the grounding point are combined into one grounding circuit through connecting nails, and only one grounding terminal is crimped, which is mostly used for Japanese and Korean cars. Advantages: low cost; Disadvantages: the process is complex, the circuit has small current and large voltage drop, which may interfere with each other.

Form 2: all grounding circuits at the same grounding point are crimped with grounding strips separately, and grounded through the combination of grounding strips, which is mostly used for European vehicles. Advantages: simple process and reliable performance; Disadvantages: high cost. After the grounding circuit is distinguished according to the current at the same grounding point, the small current grounding circuit can be combined and crimped to the same grounding plate, and then combined for grounding, which is mostly used for American cars. Features: moderate process complexity and cost.

3. Vehicle grounding status and engine grounding design examples

(1) Grounding status. In the whole vehicle harness, the engine compartment has the most grounding points. Try not to select the grounding point on the instrument panel bracket, because the bracket is connected with the body through bolts, the contact resistance may be too large, affecting the grounding performance. The grounding wire accounts for about 15% ~ 20% of all wires of the whole vehicle harness. According to different functions, there are about 18 ~ 20 grounding points of the whole vehicle, which can basically meet the needs of the whole circuit design.

(2) An example of engine grounding design. Because the negative pole of the electrical parts on the engine (generator, starter and many other electrical parts) is directly connected and sent.

The engine block is rigidly connected, and the engine block is grounded through the grounding cable to form a grounding circuit. Once the engine is not in bad contact, the starter is poorly grounded and cannot be started when stopping; When driving, the generator is poorly grounded, works abnormally, and the whole vehicle loses power, resulting in the sudden flameout of the engine, and some contact points are ablated due to excessive contact resistance. The most effective engine grounding: directly connect the engine block to the negative pole of the battery. However, due to the layout, it is difficult to connect directly to the negative pole of the battery. Therefore, it is best to connect the engine block with the transmission housing to ensure good grounding performance.

Selection of engine grounding:

① The preferred option is to connect the engine suspension or cylinder block. Advantages: reliable grounding, short grounding wire and low cost;

② The second option is the cylinder block connecting the transmission and the engine. Advantages: reliable grounding, but long grounding wire and high cost. Because this part is connected to the cylinder block or transmission body through two mounting bolts, if other parts are assembled too much, there will be the hidden danger of poor grounding contact. If the mounting bolt torque is not in place, or the engine vibrates too much, or the engine tilts forward and backward, it will cause intermittent poor contact between the engine and the suspension, resulting in excessive voltage drop and some engine electrical parts do not work.

Automobile harness grounding design is an extremely important link in automobile circuit design. It reflects the design level of automobile harness and plays an important role in realizing the stability and reliability of automobile circuit. The grounding design of automobile harness should form grounding design strategy and solidified grounding design scheme. The solidified grounding scheme is a design scheme verified by vehicle model and a large number of tests. It can be used directly, improve the harness design quality and shorten the development and verification cycle.