Getting to know the electrical circuit (BMW 5 Series E39)

If you want to detect a fault in the electrical system or install additional electrical components, you cannot do without an electrical diagram. Using the diagram, you can detect the presence of electric current and wire connectors. The corresponding electrical circuit must be closed, otherwise there will be no electric current in it. It is not enough, for example, to apply voltage to the positive terminal (+) of the headlight if the electrical circuit is not simultaneously closed through the ground (-).

Therefore, the ground (-) wire of the battery is also connected to the car body. However, sometimes this connection to the ground is not sufficient, and the corresponding consumer is connected to the ground directly by a wire, the insulation of which is usually brown. A switch, relays, fuses, measuring instruments, electric motors or other electrical components can be connected in a separate electrical circuit. In order to be able to connect these components correctly, the individual connecting wires have different colors and the individual contacts have corresponding numbers.

To visually organize the wiring, at least in the electrical diagram, the entire electrical system of the car is divided into individual circuits. Electrical components that are interconnected are shown on the combined diagram. As a rule, the positive (+) connectors of the electrical circuit are shown at the top, while the connections to the ground (-) are indicated at the bottom. The connection to the ground is usually made directly through the body or through a wire from the ground connector located on the body.

Note: The diagrams of nodes and wires are not drawn to scale. For example, a wire longer than 1 meter is shown in the same way as a wire only a few centimeters long.

The most important terminal designations:

Terminal 15 is powered via the ignition switch. Current flows through the wires only when the ignition is on. The wires are usually black or black with colored stripes.

Terminal 30. Battery voltage is always applied to this terminal. The wires are mostly red or red with colored stripes.

Terminal 31 is connected to ground. Ground wires are usually brown.

In the electrical diagram, all consumers and switches are shown in a resting state. The changing current flow after the switch is turned on is explained using the example of a two-position switch:



If the switch is pressed into the first position, the current coming from terminal B2 flows through terminal B3. The second jumper also moves one position, but the switch is not turned on. Only after the switch is pressed into the second position does the second jumper connect the internal wire B2 to the wire B4. and now the current continues to flow through the wire B4. At the same time, the current continues to flow through the internal connection in the switch through the wire B3.

The following example explains how to read an electrical diagram:

• 1) Terminal designation, here: terminal 30.
• 2) The dashed line between contacts B and 2 on the X293 plug indicates that both contacts belong to the X293 plug.
• 3) The dashed line from fuse F1 to contact 2 of plug X293 shows the operating power supply of relay K36 with positive (+) voltage.
• 4) Permissible current through the fuse, here: 30 amperes.
• 5) The switch and relay are always shown in the idle state. The circuit element enclosed in a solid line is shown in the circuit diagram in full, here: K36.
• 6) Designation of the circuit element, here: K36.
• 7) Name of the circuit element, here: windshield wiper relay I.
• 8) The dashed line from fuse F26 to contact B of plug X293 shows the control supply of relay K36 with positive (+) voltage.
• 9) The circuit element enclosed in the dotted line is not fully represented in the circuit diagram, here: main module A1.
• 10) Contact designation.
• 11) The double line from contact 2 of plug XI07 to ground connection X490 indicates the ground connection of the windscreen washer pump motor M4.
• 12) Connection to ground X490.
• 13) Wire color, here: BR/VI - brown/violet.
• 14) Designation of the plug connector, here: X295.

Letter designations of the most important parts:

• A - fuse holder / control unit / module
• B - temperature sensor
• E - lamp
• F - fuse
• H - horn/turn signal/brake signal
• K - relay
• M - electric motor
• N - radio
• R - potentiometer
• S - switch
• V-valve
• X - plug connector or ground connection
• Y - outside rear view mirror

Decoding the colors of the wires:

• BL - blue
• BR - brown
• GE - yellow
• GN - green
• GR - grey
• RT - red
• SW - black
• VI - purple
• WS - white