Comprehensive knowledge about automobile circuits

General tips for reading automobile circuit diagrams

1. Read the captions carefully several times

The legends describe the names and digital codes of all the electrical devices in the car. By reading the legends, you can get a preliminary understanding of which electrical devices are installed in the car. Then, you can find the electrical devices in the circuit diagram by their digital codes, and further find out the interconnections and control relationships.

2. Remember electrical graphic symbols

Automobile circuit diagrams use electrical graphic symbols to represent their composition and working principles. Therefore, it is necessary to remember the meaning of circuit graphic symbols in order to understand the circuit schematic diagram.

3. Remember the circuit symbols

In order to facilitate the drawing and reading of automotive electrical circuit diagrams, some electrical devices or their terminals are given different symbols and codes.

4. Keep in mind the characteristics of automotive circuits

The characteristics of automotive circuits are:

⑴ Single-wire system

⑵ Negative ground

⑶ Parallel connection of electrical equipment

5. Keep the loop principle in mind

Any complete circuit is composed of power supply, fuse, switch, control device, electrical equipment, wire, etc. The current must flow from the positive pole of the power supply, through the fuse, switch, control device, wire, etc. to the electrical equipment, and then through the wire (or ground) back to the negative pole of the power supply to form a loop. Therefore, there are three ways of thinking when reading circuit diagrams:

Idea 1: Follow the flow of circuit current, starting from the positive pole of the power supply, and trace back to the electrical equipment, switches, control devices, etc., and then return to the negative pole of the power supply.

Idea 2: Go against the direction of the circuit current, starting from the negative pole of the power supply (ground), passing through electrical equipment, switches, control devices, etc. and returning to the positive pole of the power supply.

Idea three: Start from the electrical equipment, and find its control switch, connection wires, control unit in turn, to reach the positive pole of the power supply and the ground (or the negative pole of the power supply).

In actual application, different ideas can be selected depending on the specific circuit, but one thing is worth noting: with the widespread application of electronic control technology in automobiles, most electrical equipment circuits have both main circuits and control circuits, and both circuits must be taken into account when reading the diagram.

6. Browse the entire map and divide each unit system

To understand the automotive circuit diagram, you must first understand the basic functions and characteristics of each electrical component that makes up the circuit. After roughly understanding the basic principles of the entire diagram, separate the unit system circuits one by one, so that it is easy to grasp the main functions and characteristics of each part.

When demarcating each system, you must follow the loop principle. Be careful not to miss any components in each system, and do not demarcate too many components of other systems. The general rule is:

The power supply and the main switch are the only common components of each electrical system. Any other system should be a complete and independent electrical circuit, including power supply, switch (fuse), electrical appliance (or electronic circuit), wire, etc. From the positive pole of the power supply, through the wire, switch, fuse to the electrical appliance, then to the ground, and finally back to the negative pole of the power supply.

7. Remember the internal connections and relationships between each local circuit

In terms of the vehicle circuit, except for the power supply circuit, all other unit circuits are relatively independent, but there are also internal connections between them (such as signal sharing). Therefore, when reading the diagram, you should not only be familiar with the composition, characteristics, working process and current flow path of each local circuit, but also understand the connection and mutual influence between each local circuit. This is a necessary condition for quickly finding the fault location and troubleshooting.

8. Understand the functions of various switches in circuits

For switches with multiple layers and multiple gears, the functions of each layer and gear should be analyzed step by step by layer, gear, and wiring. Some electrical equipment is controlled by more than two single-gear switches (or relays), and some are controlled by more than two multi-gear switches, and their working conditions are relatively complicated. When there are many switch terminals, first grasp one or two terminals from the power supply, and then analyze one by one what gear the electrical equipment connected to the other terminals is in, so as to find out the control relationship.

For combination switches, the actual circuits are together, but in the circuit diagram they are drawn in their respective local circuits according to their functions. When encountering this situation, you must study and read it carefully.

9. Comprehensively analyze the initial state and working state of switches and relays. In the circuit diagram, various switches and relays are drawn according to the initial state. That is, the button is not pressed, the switch is not turned on, the relay coil is not energized, and its contacts are not closed (referring to the normally open contact). This state is called the original state. When reading the diagram, you cannot analyze it completely according to the original state, otherwise it will be difficult to understand the working principle of the circuit, because most electrical equipment changes the circuit through the changes of switches, buttons, and relay contacts, thereby realizing different circuit functions. Therefore, it is necessary to analyze the working state.

10. Understand the location of electrical devices in the circuit diagram

A large number of electrical devices are electromechanical. When representing them on circuit diagrams, manufacturers often use centralized or separate representation methods based on actual conditions in order to make the drawing simple (easy to draw) and easy to read.

The concentrated representation method is a method of drawing all the components of an electrical device together on a diagram. This method is only applicable to relatively simple circuits.

Separate representation method, such as drawing the coil and contacts of the relay separately in different circuits, and connecting the separate parts with the same text symbol or numerical symbol.

11. Start with the easy and then move on to the difficult

Some local circuits in some automobile circuit diagrams may be complex and difficult to understand at first. You can put them aside temporarily. After you understand other local circuits, you can further read this part of the circuit in combination with the relevant information related to the circuit in the diagram.

12. Pay attention to collecting information and accumulating experience

For circuits that you don't understand, you should be good at asking relevant personnel for help, and at the same time be good at finding and collecting relevant information; pay attention to in-depth research on typical automotive circuits so that you can apply what you have learned to other similar situations; pay special attention to the accumulation of practical work experience, and the application and innovation of new technologies and new processes.

13. How to read the diagram of automotive electronic control system

There are more and more automotive electronic control systems. In addition to the above-mentioned methods, the following methods and steps are very helpful for reading automotive electronic control systems.

1. The ECU of the electronic control system should be the center, because it is the control center of the entire system, and all electrical components must be related to it.

2. Have a general impression of the various pins of the ECU, and understand how they are divided into several areas and the arrangement rules of the pins in each area.

3. Find out which power lines the system uses to power the ECU. Note that generally ECUs have more than one power line. Find out the power supply status of each power line (such as live wire or switch control).

4. Find out which grounding wires the system has, and make sure to distinguish which are grounded inside the ECU, which are grounded on the frame, and which are grounded on each assembly body.

5. Find out which are the signal input sensors of the system, whether each sensor needs power, and find the corresponding power lines and ground the sensors.

6. Find out which actuators are in the system, clarify the power supply and grounding conditions, and how the computer controls the actuators (controls the grounding end or the power end).

Automotive wiring harness

Automobile wire

Classified by the voltage they can withstand: high-voltage wires and low-voltage wires.

⑴ Correct selection of conductor cross-sectional area

Select the cross-sectional area of ​​the wire according to the load current of the electrical equipment. The general principles are:

For electrical equipment that works for a long time, wires with a current carrying capacity of 60% of the actual current can be selected; for electrical equipment that works for a short time, wires with a current carrying capacity between 60% and 100% of the actual current can be selected. At the same time, the voltage drop in the circuit and the heating of the wire should also be considered to avoid affecting the electrical performance of the electrical equipment and exceeding the allowable temperature of the wire. In order to ensure a certain mechanical strength, the cross-sectional area of ​​the low-voltage wire is generally not less than 0.5mm2.