Easily understand the schematic diagram of the car circuit

1. Be familiar with the rules of drawing automobile circuits

In the car's full circuit diagram, the electrical appliances are arranged from left to right (the power supply is on the left, the electrical equipment is on the right; in the schematic diagram of the local circuit, the signal input terminal is on the left, and the signal output terminal is on the right), and from top to bottom (the live wire is on the top, the ground wire is on the bottom), and the circuits of the electrical systems are drawn together as much as possible.

2. Be familiar with the symbols and meanings of automotive circuit components

Be familiar with the names of automotive circuit diagrams, understand the meanings of electrical symbols, text annotations, codes and abbreviations, and establish a one-to-one correspondence between components and graphic symbols.

01 Electrical symbols

All electrical appliances in a car are represented by electrical symbols in the circuit diagram. Electrical symbols are simple graphic symbols that only roughly represent the appearance of the appliance, and the name of the appliance is explained in words on or next to the graphic symbol. The electrical symbols drawn by each car manufacturer are different, some are simple, and some are complex. For example:

Figure 1 is the symbol of Volkswagen/Audi/Skoda car series, which is the most common symbol of the engine electronic control unit.

Figure 1

Figure 2 and Figure 3 are the symbols of GM and BMW car series respectively, with simple internal circuits drawn at the electronic control unit.

Figure 2

Figure 3

Figure 4 shows the symbols for the Mercedes-Benz series. English letters are used to indicate the function of the terminal at the electronic control unit, and arrow symbols are used to indicate whether the signal is input or output.

Figure 4

Figure 5 shows the symbols of Beijing Hyundai car series. The name and type of the signal are marked at the electronic control unit. From the figure, it can be seen whether it is power supply, grounding, input signal or control signal.

Figure 5

Figure 6 shows the symbols for Toyota cars, with English letters used to indicate the functions of the terminals at the electronic control unit.

Figure 6

Figure 7 shows the symbols for Honda cars. A simple internal circuit is drawn at the electronic control unit and the terminals are labeled with English letters.

Figure 7

Some electrical symbols also simply express the internal working principles and circuits of electrical appliances, such as the symbol of the starter shown in the figure below. From the figure, you can see the connection relationship between the starter, electromagnetic switch coil, electromagnetic switch contacts and the circuits between them.

02 Electrical terminal marking

In order to facilitate the search and maintenance of automobile circuits, the wiring terminals of electrical appliances are marked with certain numbers and letters in the circuit diagram. By understanding the markings of these terminals, you can accurately find the wires and the corresponding wiring terminals.

Automobile manufacturers in different countries have different terminal marking methods. The following table shows some marking instructions for German automobile circuit equipment terminals.

03 Abbreviations in automotive circuits

Due to the limited space of the circuit diagram, a large number of abbreviations are used in the annotations of various components. Some abbreviations are abbreviations of the English names of the systems, such as ABS (Anti-lock Braking System) for anti-lock braking system and AT (Automatic Transmission) for automatic transmission. Some abbreviations use the English abbreviations of the electrical appliances connected to the terminals as abbreviations of the terminals, such as BAT (battery) for batteries and INJ (Injector) for injectors.

Only by correctly understanding the abbreviations in the circuit diagram can you read the circuit diagram correctly. The abbreviations in the circuit diagram can be understood by consulting the English-Chinese automotive abbreviation dictionary or by referring to the instructions in the circuit diagram.

3. Familiar with the role of components

01 Switch

The switch is the key to control the on and off of the circuit. The main switches in the circuit often gather many wires, such as the ignition switch and the light control switch. When reading the circuit diagram related to the switch, you should pay attention to the following points:

■ Among the many terminals on the switch, find out which ones are connected to the power supply and which ones are connected to electrical appliances, and what the wiring symbols next to the terminals mean.

■ What path does the current from the battery or generator take to reach this switch? Does it pass through other switches and fuses in the middle? Is the control switch a manual button or automatic control?

■ How many positions does the switch have? What is the function of each position? In each position, which terminals are energized and which are de-energized.

■ What electrical appliances does each switch control, and what are the functions and effects of the controlled electrical appliances?

■ Among the electrical appliances being controlled, which ones are normally on, which circuits are temporarily connected, which ones should be connected first, which ones should be connected later, which ones work individually, and which ones should work simultaneously.

02 Relay

It acts as a switch. It uses electromagnetic or other methods (such as thermoelectric or electronic) to control the connection or disconnection of a circuit, so as to control a large current with a small current, thereby reducing the current load of the control switch contacts. When analyzing a circuit with a relay, it is necessary to distinguish between the main circuit and the control circuit. The figure below shows the working circuit of the FAW-Volkswagen Jetta NF power supply relay. The control circuit and the main circuit are marked with arrows of different colors.

03 Sensor

Sensors in automotive circuits often share power and ground lines, but never signal lines.

FAW-Volkswagen Magotan throttle circuit

When analyzing sensor circuits, the method of elimination can be used to judge the circuit, that is, to eliminate its impossible functions to determine its actual function. For example, when analyzing a sensor circuit with three wires, if its power circuit and ground circuit have been analyzed, the remaining circuit must be a signal circuit.

04 Actuator

The most common actuators in automotive circuits are mainly injectors, ignition coils, shift solenoid valves, idle stepper motors, air conditioning compressors, etc. The actuators need three signals to work properly, namely power, ground and control signals. The control signal is mainly sent by the control unit. In automotive circuits, you will see that the actuators share power lines, ground lines and even control lines. In the circuit shown in the figure below, the ignition coils N70, N127, N291, and N292 share the power line and ground line.

New Passat ignition coil circuit

05 Electronic Control Unit (ECU)

There are more and more automotive electronic control systems. When reading the circuit diagram of an automotive electronic control system, the focus should be on the ECU of the electronic control system, because this is the control center of the entire system and all electrical components are inevitably related to it.

■ Have a general impression of each pin of the ECU, and understand the areas into which it is divided and the pattern of pin arrangement in each area.

■ Find out which power lines the system uses to supply power to 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).

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

■ Find out which are the signal input sensors of the system, whether each sensor requires power, and find out the corresponding power lines and where the sensor is grounded.

■ Find out which actuators are in the system, understand the power supply and grounding conditions, and how the computer controls the actuators (controls the grounding terminal or power supply).

4. Principles of using loops

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.

The above figure shows the Honda Accord electric rearview mirror circuit. There are three ways of thinking when reading the circuit diagram:

Idea 1: Follow the flow of circuit current, starting from the positive pole of the power supply, to the fuse, switch, control device, electrical equipment, etc., and back 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, control devices, switches, fuses, 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).

Take the above figure as an example to illustrate three ideas. Simplify the circuit diagram and get the following circuit: