1、 Introduction

The automobile wiring harness is composed of terminals, sheaths, wires, connectors, adhesive tapes, bellows, PVC pipes, heat-shrinkable tubes, fuses, fuse boxes and other accessories, which play the role of the whole vehicle's neural network, transmit signals, and implement electric energy. Due to the different functions of wire harness in the whole vehicle, it is divided into engine wire harness, front wire harness, instrument wire harness, chassis wire harness, door wire harness, top wire harness, etc.

2、 Withdrawal of needle

Harness is the neural network system of automobile, which is responsible for transmitting voltage, signal and a large amount of data during the whole vehicle operation. Especially in the context of the Internet and big data, it is not only required that the harness carrier play the role of on-off, but also put forward higher requirements for the data transmission rate and response ability. At the same time, due to the limited physical layout space of the harness, it brings greater challenges to its after-sales maintenance.

Terminal pin withdrawal is a common failure mode of harness. Pin withdrawal means that the terminal does not reach the expected position, thus disabling the connector function. The automobile wiring harness mainly depends on manual work, and the control difficulty can be imagined. In order to better prevent and control the problem of terminal withdrawal, the control is mainly carried out from the following aspects: design and selection, process protection, terminal crimping, assembly, electrical measurement, and assembly.

3、 Design selection

The quality is designed and manufactured, not tested. The prevention of terminal withdrawal should start with design and model selection. Here are five considerations.

(1) Insertion force: the difficulty of terminal assembly. The smaller the resistance in the terminal pre-assembly connector, the easier the pre-assembly is. Therefore, the first evaluation index when selecting is the insertion force. The smaller the insertion force, the easier the assembly is, and the lower the risk of terminal withdrawal.

(2) Retention force: the linear pull force (i.e. retention force) of the terminal from the sheath, the greater the retention force, the less likely the connector will pop up when inserting. In this design and selection, the connector and terminal with large retention force can be selected.

(3) Shake amount: when male and female connectors are inserted, the shaking amount of the terminals in the sheath will significantly affect the terminal being ejected. In order to reduce the risk of pin withdrawal during insertion, the terminals and connectors from the same manufacturer should be selected as far as possible during design and selection (purpose: to ensure the minimum shaking amount when terminals and connectors are matched).

(4) In place sound: the sound when the terminal assembly is in place. At present, the industry relies on manual work for the pre-assembly of terminals, and there is a risk of terminal withdrawal. How to make employees better identify the pre-installed terminals in place? Here is an assessment indicator, that is, the sound in place of the terminal assembly is higher than the ambient sound (the ambient sound level should be 30dB-50dB): 7dB before wet, 5dB after wet, or agreed by both parties.

(5) Error-proof structure of terminal hole: when the terminal is inserted in the wrong direction, the terminal cannot be inserted into the terminal hole or the insulation support and seal are exposed outside the terminal hole. In the process of solving the problem, we found that some terminals may be inserted into the connector in the wrong direction, which is difficult to identify when pulled back. Therefore, when designing and selecting the type, it is necessary to consider the difficulty of inserting the terminal in the wrong direction to ensure that the terminal cannot be in place when assembling in the wrong direction.

4、 Process protection

There are two factors that affect the pin withdrawal of the terminal: one is the deformation of the spring piece, and the other is the skewness of the terminal. Both are affected by external forces during the process, resulting in terminal deformation. In order to protect terminal shrapnel from deformation caused by external objects, the terminal head should be protected with a protective cup after crimping. The protective cup can only be removed after assembly. After the harness is assembled, the male end sheath shall be sealed with sealing tape or protective device to prevent the terminal from being skewed due to the action of foreign objects during transportation.

5、 Terminal crimping

Terminal crimping is a key link in the production process of automobile wire harness. Its main process is to connect the electrical system with terminals, and combine terminals with circuits through machine tool technology. Terminal crimping is also the influence factor of terminal pin withdrawal during harness production. "banana" terminal is a common problem during terminal crimping, resulting in excessive bending of the terminal. When the terminal is bent and assembled, the insertion and extraction force of the connector will increase slightly, and the male terminal will not be inserted into the effective area of the female terminal, resulting in terminal pin withdrawal. Adjusting the limit pin on the crimping device can solve this problem.

6、 Assemblage

Harness assembly mainly depends on manual operation. In order to better reduce the risk of terminal assembly not in place, the industry generally follows the principle of "one plug, two listen, and three pull". The first is to insert the terminal, the second is to listen to the sound of the terminal in place, and the third is to pull the terminal to see if it will come out after the terminal is inserted. An employee can easily cause work fatigue by inserting the terminal thousands of times a day. In order to better let the employee form muscle memory, we have made adjustments: first, "pull" before the shift, "pull" after the rest. Here, "pull" means to set up a tension machine at the work position, Before the start of the shift and after the rest, employees need to pull the dynamometer by hand to ensure the insertion strength of employees and form muscle memory; Second, "one look, two plug, three listen, and four pull". The purpose of adding "one look" is to identify terminal deformation and banana terminal, and ensure that the insertion direction is correct when inserting.

7、 Electrical survey

Harness electrical measurement is an important link in harness manufacturing. In order to ensure terminal skew and effectively identify and intercept terminal withdrawal, the electrical measurement equipment needs to meet the following conditions: first, the male terminal clamp must wear anti-skew grille to ensure that the skewed terminal cannot be connected; The second is that the electrical measuring probe adopts threaded step needle, which can effectively prevent the terminal from being ejected during the electrical measuring process; Third, calculate the movement of the terminal in the sheath according to the dimension chain, so as to manufacture the probe and establish the routine maintenance plan of the probe.

8、 Assembly

Case: Workshop A reported that it was difficult to install the plug of the left front door control panel of a vehicle in batches (terminal withdrawal). After field investigation and analysis, it was confirmed that the vehicle was moved from Workshop B to Workshop A for assembly, and the employee did not insert it correctly during assembly, resulting in the harness bus terminal popping out, and the fault disappeared after correct assembly. This case fully proves that employees need to ensure that the male and female end sheaths are aligned and inserted during the assembly process of the wire harness to reduce the risk of the terminal being ejected due to the malposition of the male and female end sheaths during the assembly process.

9、 Conclusion

In this paper, the factors that affect the pin withdrawal of automobile harness terminals are analyzed in depth, and detailed prevention and control research is carried out from the aspects of design and selection, harness manufacturing, process protection, assembly process, etc., which not only provides guidance for the design and selection of harness, but also provides specific suggestions for the control of harness manufacturing process, and provides specific methods for the analysis of failure modes.