Note. Before starting work, read the warnings given in paragraph "Safety first!" and in paragraph 1 this chapter. The procedures described in this chapter are for electrical circuits only and should not be used to test the electronic circuits of systems (such as anti-lock braking system), and especially those associated with electronic control units.
Warning. The electrical system of a BMW 3 Series is very complex. Most control units are integrated into the system "common bus", from which they receive signals from various sensors and can exchange information with each other. For example, when a certain gear is engaged in an automatic transmission, this signal is transmitted through "common bus" to the engine control unit. Despite. that the gear is shifted by the transmission control module, the engine control module changes the ignition timing, reducing the engine output to ensure smooth changes in vehicle speed during gear changes Because the control modules are combined "common bus", these blocks cannot be checked with a multimeter in the usual way. To facilitate troubleshooting in the circuits, the control units have a built-in self-diagnosis function. The control units store fault codes that help determine the location of the fault. Special equipment is required to read these moves (fault code reader or scanner).
Basic information
1. A typical electrical circuit consists of electrical components - switches. relays, motors, fuses, fuse links or circuit breakers connected by wiring and connectors to each other, and to the battery and chassis. To assist in troubleshooting electrical circuits, wiring diagrams are provided at the end of this manual.
2. Before troubleshooting an electrical target, study the appropriate wiring diagram and get a complete understanding of the components included in the circuit. Troubleshooting is easier if you make sure that the other components in the circuit are working properly. If several components or circuits fail at the same time, the fault may be due to their common fuse or ground connection.
3. Malfunctions in electrical circuits usually occur for simple reasons, such as: loose or corroded connection contacts, unreliable grounding, fuse or fuse failure, or relay failure (For a detailed description of the relay test, see paragraph 3). Before starting to check the details of the circuit, inspect the condition of all fuses, wires and connectors in the failed circuit. Use the wiring diagrams to determine which connections you need to check to pinpoint the location of the problem.
4. The list of basic tools for troubleshooting electrical circuits is quite simple and contains: a tester or a voltmeter (a 12 volt test light with wires may also be used for some tests): battery powered indicator (also called break indicator); ohmmeter (for measuring resistance); battery with a set of wires for control; jumper wire, preferably with a fuse, which can be used to bypass wires being tested or suspect electrical components. Before you start troubleshooting with the help of devices, determine the points for their connection according to the electrical diagram.
5. To find the source of an intermittent fault in the wiring (usually caused by a bad connection, or damaged insulation), you can just move the wires. If at the same time the contact in the circuit either appears or disappears, then the source of the malfunction is located in this place of the wiring. This verification method may be used along with any of the others described in the following subparagraphs.
6. In addition to bad contacts, there are two more main types of malfunction: open circuit and short circuit.
7. If the target breaks in any of its places, the current supply to the component stops, and therefore its operation, although a break in the circuit does not lead to failure of the fuse.
8. Circuit short circuits are usually caused by broken wiring insulation. when there is a mutual touch of the wires or the wire with the body. A short circuit usually destroys the fuse in the corresponding circuit.
Open Circuit Detection
9. To check for an open circuit, connect one of the tester wires to the negative battery terminal or to a good ground.
10. Connect the other tester lead to a connector on the circuit under test, preferably one closest to the battery or fuse.
11. Turn on the circuit. Note that some circuits are only closed when the ignition switch is in a certain position.
12. Presence of voltage (as evidenced by the light of a control light or a voltmeter reading), means that the section of the circuit between the corresponding connector and the battery is good.
13. Continue checking the rest of the circuit in the same way, gradually moving away from the battery.
14. If the point at which there is no voltage is found, then the section between this point and the previous checked point is faulty. Check it in more detail to find the problem. Most failures are caused by corrosion, broken or loose contacts.
Circuit short detection
15. Before checking the circuit for a short circuit, disconnect the load from the circuit (load - components that consume power, such as light bulbs, motors, heating elements, etc).
16. Remove the appropriate circuit fuse and connect the tester or voltmeter wires to its terminals.
17. Turn on the circuit, bearing in mind that some circuits are closed when the ignition switch is in a certain position.
18. Presence of voltage (as evidenced by the light of a control light or a voltmeter reading) confirms a short circuit.
19. If no voltage is present during this test, but the fuse blows when the load is connected, this indicates an internal fault in the load.
Ground Fault Detection
20. The negative battery terminal is connected to "grounding" - engine-transmission-body block. For most consumers of electricity, only the positive wire is suitable, and the negative conductor is the metal of the body (pic. 2.20). This means that the mass of the body is part of the chain. In this regard, poorly tightened connections can lead to partial or complete breakage of the chain. And this can cause a dim glow of lighting devices (especially if another circuit is switched on using the same ground point), deceleration of motor rotation (such as a wiper motor or a cooling fan motor) and the influence of one circuit on the operation of another. Please note: many vehicles use flexible ground bars to connect to the body of the units. which do not have direct metallic contact with it, such as. for example, an engine-transmission unit mounted on rubber pads.
21. To check if the component is properly grounded, disconnect the battery and connect one ohmmeter lead to a good ground. Connect the second wire of the ohmmeter to the component to be tested. The ohmmeter should register zero resistance; otherwise, check the component ground connection as follows.
22. If you suspect that the ground connection is broken, disassemble this connection, clean both the contact point on the body and the wire tip (or mounting surface of the assembly housing) to metal. Remove all traces of dirt and corrosion. scrape off the paint with a knife to ensure a clean metal-to-metal connection. Assemble the connection and carefully tighten the fasteners. If the wire ends have lugs, install serrated washers between the lugs and the housing to ensure a secure connection. In order to prevent corrosion of the connection after its restoration, lubricate it from the outside with technical vaseline or silicone grease (or periodically spray on it with special moisture-protective agents).