Diagnostics of faults of on-board electrical equipment (BMW 7 Series E38)

The procedures and codes for diagnostics of electronic control systems are discussed in Chapter Engine electrical systems.

A typical electrical circuit may include: an electrical component, various switches, relays, motors, fuses, fuse links, or circuit breakers associated with the component, and the wiring and electrical connectors that connect the component to the battery and body ground. To help troubleshoot electrical circuits, wiring diagrams are included at the end of this manual.

Before you start troubleshooting any of the electrical circuits, carefully read the corresponding diagram in order to have as clear an idea of its functional purpose as possible. Narrowing the scope of troubleshooting is usually achieved by gradually identifying and eliminating normally functioning components of the same circuit. If several components or circuits fail at the same time, the most likely cause of failure is a blown fuse or a ground fault (different circuits in many cases can be shorted to one fuse or ground terminal).

Electrical equipment failures are often due to simple causes such as corroded terminals or a failed fuse or relay. Visually inspect all fuses, wiring, and electrical connections in the circuit before attempting more specific component checks.

When using diagnostic tools to locate a fault, carefully plan in accordance with the attached electrical diagrams where and in what sequence the tool should be connected in order to most effectively identify the fault.

Basic diagnostic tools include an electrical circuit tester or voltmeter (a 12-volt test light with a set of connecting wires can also be used), a circuit continuity indicator (probe) that includes a light bulb, its own power source, and a set of connecting wires. In addition, you should always have a set of jumper cables in your vehicle, equipped with alligator clips and, preferably, a circuit breaker, which can be used to bypass and connect various electrical components during circuit diagnostics. As mentioned above, before you start testing a circuit with diagnostic equipment, determine from the diagrams where it is connected.

Checking for voltage

Voltage checks are performed when a circuit malfunctions. Connect one of the tester leads to either the negative battery terminal or a well-grounded point on the vehicle body. Connect the other tester lead to a terminal on the circuit's electrical connector, preferably the one closest to the battery or fuse. If the tester's indicator light comes on, voltage is present on that section of the circuit, confirming that the circuit between that terminal and the battery is functioning properly. Continuing in the same manner, check the remainder of the circuit. If no voltage is detected, there is a malfunction between that point in the circuit and the last point checked (where the voltage was present). In most cases, the cause of failure is loose electrical connections and poor contact quality.

Please note that power is supplied to some of the on-board electrical circuits only in certain ignition switch positions.

Short circuit detection

One method of finding a short circuit is to remove the fuse and connect a test lamp or voltmeter instead. There should be no voltage in the circuit. Pull the wiring while watching the test lamp. If the lamp starts to blink, there is a short to ground somewhere in the harness, possibly caused by a wire insulation failure. A similar check can be made for each of the components in the circuit, including the switches.

Checking the grounding serviceability

This test is performed to determine the reliability of the component grounding. Disconnect the battery and connect one of the wires of the self-powered test lamp to a known good ground point. Connect the other wire of the lamp to the harness or terminal being tested. If the lamp lights, the grounding is OK (and vice versa).

Conductivity checks

The test is performed to detect breaks in the electrical circuit. After disconnecting the circuit power, test it using a test lamp equipped with an independent battery. Connect the test lamp wires to both ends of the circuit (or to the "power" end (+) and a well-grounded point of the body); if the test lamp lights up, there is no break in the circuit. Failure of the lamp to turn on indicates a violation of the circuit's conductivity. In a similar way, you can check the serviceability of the switch by connecting the test lamp to its terminals. When you move the switch to the "On" position, the test lamp should light up.

Localization of the break

When diagnosing a suspected circuit break, it is quite difficult to visually detect the cause of the malfunction, since inspecting the terminals for corrosion or damage to the quality of their contacts is difficult due to limited access to them (usually the terminals are covered by the connector housing). A sharp jerk of the connector housing on the sensor or its wire harness in many cases leads to the restoration of conductivity. Do not forget about this when trying to localize the cause of failure of a suspected open circuit. Unstable failures may be caused by oxidation of terminals or poor quality of contacts.

Diagnosing electrical circuit faults is not a difficult task at all, provided that there is a clear understanding that current is flowing to all electrical loads (lamp, electric motor, etc.) from the battery through wires via switches, relays, fuses, fuse links, and then returns to the battery through the vehicle's ground. Any problems associated with electrical equipment failure can only be caused by the cessation of the supply of electric current to them from the battery or its return to it.

[The original text can be read on the website www.bmwman.ru]