Four key points in reading automotive circuit schematics

Due to the differences in drawing methods, symbol markings, text markings, and technical standards of automobile circuit diagrams in different countries, there are great differences in automobile circuit diagrams drawn by various automobile manufacturers. Therefore, before reading different series of automobile circuit diagrams, you need to understand the characteristics of the circuit diagrams and master the basic methods of reading automobile circuit diagrams.

1. Be familiar with the rules of drawing automobile circuits
In the full circuit diagram of the car, the electrical appliances are arranged from left to right (power supply on the left, electrical equipment on the right, in the schematic diagram of the local circuit, the signal input end is on the left, the signal output end is on the right), from top to bottom (live wire on the top, ground wire on the bottom), and the circuits of each electrical system 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, clarify 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 the car are represented by electrical symbols in the circuit diagram. Electrical symbols are simple graphic symbols that only roughly represent the appearance of the electrical appliances, and the names of the electrical appliances are explained in text on or next to the graphic symbols. The electrical symbols drawn by each automobile manufacturer are different, some are simple, and some are complex. Follow the public account Circuit One-stop Example:
Figure 1 is the symbol of Volkswagen/Audi/Skoda car series, which is the most common symbol of the engine electronic control unit.

Figures 1
, 2 and 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
and Figure 4 are the symbols of 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.

Figures 4
and 5 are 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
and Figure 6 are the symbols of Toyota car series, and English letters are used to indicate the function of the terminal at the electronic control unit.

Figures 6
and 7 are the symbols of Honda cars. A simple internal circuit is drawn at the electronic control unit and the terminals are marked with English letters.

Some electrical symbols in Figure 7
also simply express the working principles and circuits inside the electrical appliance, such as the symbol of the starter shown in the figure below. From the figure, we 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. Understanding the markings of these terminals can accurately find the wires and the corresponding wiring terminals.

The marking methods of terminals of automobile manufacturers in different countries are not the same. The following table shows some marking instructions for terminals of German automobile circuit equipment.
03 Abbreviations in automobile circuits
Due to the limited size of circuit diagrams, 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 connected to the terminals and INJ (Injector) for injectors connected to the terminals.
Only by correctly understanding the abbreviations in the circuit diagram can we correctly read the circuit diagram. The abbreviations in the circuit diagram can be understood by consulting the English-Chinese automobile 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 controlling the on and off of the circuit. The main switches in the circuit often gather many wires, such as the ignition switch and the headlight control switch. When reading the circuit diagram related to the switch, you should pay attention to the following analysis:
■ Among the many terminals of the switch, find out which ones are connected to the power supply and which ones are connected to the electrical appliances, and what the wiring symbols next to the terminals mean.
■ What path does the current of the battery or generator take to reach this switch, whether it passes through other switches and fuses in the middle, and whether the control switch is a manual button or automatic control.
■ How many gears does the switch have, what is the function of each gear, and in each gear, which terminals are energized and which are de-energized.
■ What electrical appliances are controlled by each switch, and what are the functions and functions of the controlled electrical appliances.
■ Among the controlled electrical appliances, which electrical appliances are normally on, which circuits are temporarily connected, which ones should be connected first, which ones should be connected later, which ones work alone, and which ones should work at the same time.
02
Relay Acts as a switch. It uses electromagnetic or other methods (such as thermoelectric or electronic) to control the connection or disconnection of a certain circuit, so as to control large currents with small currents, thereby reducing the current load of the control switch contacts. When analyzing a circuit with a relay, it is important to distinguish between the main circuit and the control circuit. The following figure shows the working circuit of the FAW-Volkswagen Jetta NF power supply relay, with the control circuit and the main circuit marked with arrows of different colors.

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

FAW-Volkswagen Magotan throttle circuit
When analyzing the sensor circuit, the elimination method can be used to judge the circuit, that is, to exclude 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 actuator needs three signals to work properly, namely power, ground and control signals. The control signal is mainly sent by the control unit. In the automotive circuit, you will see that the actuator shares the power line, ground line and even the control line. 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 the automotive electronic control system, the ECU of the electronic control system should be the center, because this is the control center of the entire system, and all electrical components must be related to this.
■ Have a general impression of the various pins of the ECU, and figure out how many areas are divided and the rules of the pin arrangement in each area.
■ Find out which power lines the system uses to power the ECU. Note that generally ECUs have more than one power line, and find out the power supply status of each power line (such as normal live wire or switch control).
■ Find out which grounding wires the system has, and pay attention to distinguish which ones are grounded inside the ECU, which ones are grounded on the frame, and which ones are grounded on each assembly body.
■ Find out which are the signal input sensors of the system, whether each sensor needs power, and find out the corresponding power lines and where the sensor is grounded. Follow the public account Circuit One.
■ Find out which actuators the system has, figure out the power supply and grounding conditions, and how the computer controls the actuators (control the grounding end 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 is the circuit of the Honda Accord electric rearview mirror. There are three ways of thinking when reading a circuit diagram:
Idea 1: Follow the direction of the circuit current, starting from the positive pole of the power supply, to the fuse, switch, control device, electrical equipment, etc., and 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), through the electrical equipment, control device, switch, fuse, etc. and return to the positive pole of the power supply.
Idea 3: Starting from the electrical equipment, find its control switch, connection, control unit in turn, and reach the positive pole of the power supply and ground (or negative pole of the power supply).
The above figure is used as an example to illustrate the three ideas. Simplify the circuit diagram and get the following circuit: