Effective Preventive Measures Against "Terminal Pin Withdrawing" Defect in Harness Failure Mode

Car Wire Harness consists of automotive Wiring Harness terminals, sheaths, wires, connectors, adhesive tapes, corrugated pipes, PVC pipes, heat shrinkable pipes, fuses, fuse boxes and other accessories, and plays the role of neural network of the whole vehicle, transmitting signals and executing electric energy. Due to the different functions of the wiring harness in the whole vehicle, it is divided into Engine Wiring harness, front wiring harness, Instrument Wiring Harness, chassis wiring harness, door wiring harness, top wiring harness, etc.

1. About the terminal needle withdrawal

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 background of Internet and big data, it is not only required that the wire harness carrier play an on-off role, but also put forward higher requirements for data transmission rate and response capability. At the same time, due to the limited physical layout space of the wire harness, it brings greater challenges to its after-sales repair.

Terminal withdrawal is a common failure mode of wire harness. The withdrawal of the pin means that the terminal does not reach the expected position, thus disabling the function of the Connector. Automotive wiring harness mainly depends on manual operation, and the difficulty of control can be imagined. In order to better prevent and control the problem of terminal needle withdrawal, the following aspects are taken into account: design and selection, process protection, terminal crimping, assembly, electrical measurement and assembly.

A. Preventive measures

a design and selection

It is designed and manufactured by quality, not tested. For the prevention of terminal withdrawal, the prevention should be started from the design and selection. Here are five considerations.

① Insertion force: the difficulty of terminal assembly. The smaller the resistance in the terminal pre-installed connector, the easier it is to pre-install in place. Therefore, the first evaluation index in selection is the insertion force. The smaller the insertion force, the easier it is to assemble and the lower the risk of terminal withdrawal.

② Holding force: the straight-line pulling force (i.e. holding force) of the terminal from the sheath. The larger the holding force, the less likely it is to be ejected when the connector is plugged. Here, the connector and terminal with high holding force can be selected according to the design and selection criteria.

③ Shake amount: When the male and female connectors are plugged, the shake amount of the terminal in the sheath will obviously affect the ejection of the terminal. In order to reduce the risk of needle withdrawal during plug-in, try to choose the terminal and connector as a unified manufacturer in the design and selection (purpose: to ensure the minimum shake amount when the terminal and connector are matched).

④ In-place sound: the sound when the terminal group is set up in place. At present, the preassembly of the terminal relies on manual operation in the industry, and there is a risk of terminal needle withdrawal. How to make employees better recognize that the terminals are pre-installed in place? Here, an assessment index is introduced, namely, the sound of the terminals standing in place is higher than the ambient sound (the ambient sound level should be 30dB-50dB): 7dB before and 5dB after being wet, or agreed by the supplier and the buyer.

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 problem solving, we found that some terminals can be inserted into the connector in the wrong direction, which is not easy to identify when pulling back. Therefore, it is necessary to consider the difficulty of inserting the terminals in the wrong direction when designing and selecting the type, so as to ensure that the terminals cannot be put in place when they are assembled in the wrong direction.

B. Process Protection

There are two influencing factors for terminal withdrawal: one is the deformation of shrapnel, and the other is the deviation of terminal. Both of them are affected by external forces in the process, which leads to the deformation of the terminal. In order to protect the terminal shrapnel from the deformation caused by foreign objects, the terminal head must be protected by a protective cup after the terminal is crimped, and the protective cup can only be removed after assembly. After the wiring harness is assembled, the male end sheath should be sealed with sealing tape or protective gear to prevent the terminal from being skewed by foreign objects during transportation.

C. Terminal Crimping

Terminal crimping is a key link in the production process of automotive wiring harness. Its main technological process is to connect the electrical system with the terminal, and combine the terminal with the circuit by machine tool technology. Terminal crimping is also an influencing factor of terminal needle withdrawal in the process of wire harness production. "Banana" terminal is a common problem in the process of terminal crimping, and the problem of crimping die leads to excessive bending of the terminal. When the terminal is bent and assembled by crimping, the plugging and unplugging force of the connector increases, while when it is heavy, the male terminal cannot be inserted into the effective area of the female terminal, and the terminal pins withdraw. Adjusting the limiting pins on the crimping equipment can solve such problems.

Case1: The assembly shop reported that the terminal pins of an AC socket of a certain vehicle had retreated in the process of plug-in, and the terminals had two states. By comparison, it was found that the faulty terminal was banana-shaped, and it was verified that there was no needle withdrawal phenomenon in plug-in. Then, after adjusting the limiting pin of crimping equipment, this problem was completely solved.

D. Organization

Harness assembly mainly depends on manual operation. In order to better reduce the risk of terminal group not standing in place, the industry generally follows the principle of "one plug, two listening and three pulling". One plug refers to inserting the terminal, the second listening refers to listening to the sound of the terminal being inserted into place, and the third pulling refers to pulling the terminal to see if it comes out after being inserted. An employee inserts the terminal thousands of times a day, which is prone to operational fatigue. In order to better enable employees to form muscle memory, we have made adjustments as follows Take a break "pull". Here, the pull refers to setting up a tension machine at the operating post. Before and after the start of class, employees need to pull the dynamometer by hand to ensure the insertion strength of employees to form muscle memory; The second is "one look, two plugs, three listens and four pulls". The "one look" is added to identify the terminal deformation and banana terminal, so as to ensure the correct insertion direction when inserting.

E. Electrical Measurement

The electrical measurement of wire harness is a very important level in the manufacture of wire harness. In order to ensure that the terminal is skewed and the terminal withdrawal needle can be effectively identified and intercepted, the electrical measurement equipment needs to meet the following conditions: First, the male terminal fixture must wear an anti-skew grid to ensure that the skewed terminal cannot be conducted; Second, the electrical measurement probe uses a threaded stepped needle, which can effectively prevent the terminal from being pushed out in the electrical measurement process. Third, the movement amount of the terminal in the sheath is calculated according to the dimension chain, so as to make the probe and establish the routine maintenance plan of the probe.

F. Assembly

Case: Workshop A reported that the plug of the left front door control panel of a certain vehicle was difficult to install (terminal withdrawal). After on-site investigation and analysis, it was confirmed that the vehicle was moved from Workshop B to Workshop A for assembly, and the female terminal of the wiring harness was pushed out because the employees did not insert it during assembly, and the fault disappeared after assembly. This case fully illustrates that employees need to ensure that the male and female sheaths are aligned and inserted during wiring harness assembly, so as to reduce the risk of terminal ejection caused by misalignment of male and female sheaths during assembly.

2. Concluding Remarks

In this paper, the factors that affect the withdrawal of automotive wiring harness terminals are deeply analyzed, and the prevention and control are studied in detail from the aspects of design and selection, harness manufacturing, process protection, assembly techniques, etc., which not only provides guidance for the design and selection of harness, but also provides specific opinions for the control of harness manufacturing process, and provides specific methods for the analysis of failure modes.