If the task is to find a malfunction in the electrical system or mount an additional device, then you cannot do without a circuit diagram, because. it shows the current flow and therefore the cable connections. This or that electrical circuit must be closed, otherwise the current will not flow. For example, it is not enough that there is voltage at the positive terminal of the terminal, if at the same time the current circuit is not closed through the earth connection.
Therefore, one of the battery cables is connected to the body. However, sometimes this connection is not enough, and another wire connected to the mass goes to one or another consumer, the insulation of which, as a rule, is painted brown. Individual circuits may include switches, relays, fuses, meters, motors, or other items. So that these elements can be connected correctly, the individual contacts are labeled.
In order to be able to understand the interweaving of cables at least in the diagram, individual conductors are placed vertically next to each other and numbered.
The vertical lines end in the most often underlined gray field. This field symbolizes the relay board with fuse holders and the positive lines of the current circuit. Of course, the relay board has a wire connected to ground (terminal 31). At the bottom, the current circuit closes on a horizontal line, symbolizing mass. Ground connection is usually made by direct connection to the body or to a conductor connected to the body.
If the circuit is interrupted by a rectangle with a number inside it, then that number indicates the number of the wire that continues the circuit.
To read the circuit diagram, you need to know the designation of the parts.
Letter designations of the most important details:
| Letter | Detail |
| ? | Battery |
| ? | Starter |
| ? | Three-phase current generator |
| ? | ignition switch |
| ? | Manual switch |
| ? | mechanical switch |
| ? | Sensor, control device |
| ? | horn, sirens |
| ? | Relay, control device |
| L, M, X | Control lamps, lamps, lighting devices |
| ? | Solenoid valves, resistors, switching devices |
| O | Distributor |
| P, Q | Spark plug, spark plug plug |
| R | Radio |
| S | Fuse |
| T | Plug connector |
| V | electric motor |
For more accurate identification, numbers are added to the letters.
Relays and electronic controls are usually greyed out. Lines passing through this background are inner connections. They show how relays and other electrical and electronic components are connected.
The number in the black square indicates the relay number on the relay board with fuse holders. Next to the relay is the designation of the contacts.
For example, if next to the relay in the diagram is 17/87, then 17 indicates the terminal on the relay board, and 87 indicates the terminal of the relay or control element.
The terminals are marked in accordance with the German industrial standard. The most important are:
Terminal 30. Battery voltage is always applied to this terminal. The cable is usually red or red with colored stripes.
Terminal 31 leads to ground. Ground wires are usually brown.
Terminal 15 is powered by the ignition switch. Current flows through the conductors only when the ignition is on. The cables are mostly green or green with colored stripes.
The X terminal also conducts current when the ignition is on, but it is interrupted when the starter is actuated. This ensures that the ignition system is supplied with full battery power at start-up. All powerful current consumers are connected to this circuit. The main beam lamps are also supplied via terminal X. Thus, when the main beam is on and the ignition is off, the sidelight is automatically switched on.
On all circuit diagrams, the same elements have the same designations, for example, A for a battery:
| A | battery | 4 |
| IN | starter | 7-8 |
| WITH | alternator | 1-2 |
| C1 | voltage regulator | 1-2 |
| T1f | 1-pin plug connector, engine compartment on the right | 1 |
| T1r | 1-pin plug connector, behind the relay board | 2 |
| 1 | earth connection, battery-body | 5 |
| 2 | ground connection, transmission-body | 6 |
| 30 | ground, point 1, next to the relay board | 2 |
| 119 | connection to housing 3, in wiring harness headlight | 4 |
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