Table of contents: General information ↓ Search for a broken chain ↓ Short circuit detection ↓ Detection of a break in the "mass" ↓
Note: Before you begin, please refer to the Preconceptions section "Safety first!" (at the beginning of this guide). The following tests relate to general electrical circuit tests and should not be applied to sensitive electronic circuits (such as ABS), in particular where electronic control units are used.
General information
1. A typical electrical circuit consists of electrical components - switches, relays, motors, fuses, fuse links, etc., connected by wires to the positive and negative terminals of a battery as a power source. To facilitate troubleshooting electrical circuits, electrical circuit diagrams are provided at the end of this chapter.
2. Before you begin troubleshooting, carefully examine the circuit in question in the diagram to gain a complete understanding of its components. The possible source of the fault can be determined by making sure that other components in the circuit are functioning properly. If several components or circuits fail at the same time, the fault is most likely in the corresponding fuse or ground/negative connection.
3. Electrical problems usually occur due to simple reasons such as corrosion of contacts, poor ground connection due to oxidation, a blown fuse or fusible link, a faulty relay (please refer to paragraph 3 for information on testing the relay). Therefore, before you begin a detailed inspection of the components of the problematic circuit, visually inspect the condition of all its fuses, wires and connections. Follow the diagram to see which contacts and connections need to be checked to determine the fault.
4. For most checks you only need the following basic tools; tester or voltmeter (for some tests, a 12-volt test light with two wires will also work); sampler (self-powered indicator light), ohmmeter (for measuring resistance), battery and set of wires; "cigarette lighter" wires with an automatic switch or built-in fuse, which can be used to bypass suspicious circuit elements. Before attempting to find a fault with tools, study the diagram to find out which points it is advisable to connect the device to.
5. To find an "intermittent" fault in a circuit, which is usually caused by a poor connection or insulation, wiggle the wiring to see if the fault disappears or reappears. This troubleshooting method can be combined with the methods described below.
6. In addition to problems associated with poor contacts, two main types of faults can appear in an electrical circuit - open circuits and short circuits.
7. An open circuit interrupts the electrical current, stopping the operation of system components, but does not cause the fuse to blow.
8. A short circuit (SC) allows electric current to leave the circuit by a "bypass" path, in most cases to the housing. SC usually occurs due to aged or damaged wire insulation, which allows the "bare" wire to short-circuit to another wire or the housing. SC usually causes the fuse in the corresponding circuit to blow.
9. Fuses are designed to protect circuits from overload. A blown fuse indicates a problem in the protected circuit - before replacing a fuse, find the cause of its blown fuse and eliminate it. Replace a blown fuse with a new one of the prescribed rating. Installing "bugs" and fuses designed for increased current will lead to overheating of the circuit or a fire.
Search for a broken chain
10. To check for an open circuit, connect one test lead or the negative lead of a voltmeter to the negative battery terminal or a good ground connection (body). Connect the other lead of the meter to a terminal in the circuit being tested, preferably the one closest to the positive battery terminal or fuse. Apply power to the circuit. Note that some circuits receive power only when the ignition key is turned to a certain position. Voltage Presence (what does the glow of the control lamp or the deviation of the instrument arrow indicate?) means that the section of the circuit between the connection point and the positive battery terminal is OK. Continue checking the circuit in the same way. If a point is found where there is no voltage, then the section of the circuit between this point and the previous point where voltage was present is faulty. Check it to determine the fault. Most faults are caused by poor contacts - breakage, corrosion or weakening.
Short circuit detection
11. Before checking a circuit for short circuit, disconnect the load from that circuit (components - current consumers - lamps, motors, heating elements, etc.).
12. Turn off the ignition. To de-energize the circuit, remove the appropriate circuit fuse and connect the tester, voltmeter, or test lamp leads to its terminals.
13. Apply power to the circuit, keeping in mind that some circuits only receive power when the ignition key is in a certain position. Voltage presence (the control lamp lights up or the instrument needle deflects) confirms the presence of a short circuit in the circuit. If there is no voltage, but the fuse still burns out when the load is connected, then the fault is in the load itself.
Detection of a break in the "mass"
14. The negative terminal of the battery has electrical contact with the "ground" - the engine block, gearbox, car body. Only the positive wire goes to most current consumers, the negative conductor is the car body. This means that the ground is a section of the circuit. Thus, loose connections can lead to a break in the circuit - partial or complete. This can cause dim lighting (especially if another consumer is included, using the same ground connection point), slowing down the rotation of engines (windshield wiper or heater fan motor) and the influence of one circuit on the operation of another. Many cars use flexible braided copper tapes (busbars) to connect masses that are not directly electrically connected to each other, such as the engine/gearbox block to the body, which is mounted on the body on rubber insulating support pads.
15. To check the reliability of the ground connection of a particular component, disconnect the battery, then connect one lead of the ohmmeter to the body and the other lead to the point of the component being tested. The ohmmeter should indicate zero, otherwise check the component's ground connection as follows.
16. If there is a suspicion that the ground connection is broken, disassemble this connection, clean the contact point on the body and the wire tip (or the mating surface of the component body) to the metal. Remove all traces of dirt and corrosion, scrape off paint with a knife to ensure a clean metal-to-metal connection. Assemble the connection and tighten the fasteners. If the ends of the wires have loop tips, install serrated washers between them and the housing. To prevent corrosion of the connection, lubricate the outside with technical petroleum jelly or silicone grease (do this regularly).
[The original source of the article can be found on the website: AudiManual]