Attention! The procedures described below allow a general diagnosis of the condition of the main electrical circuits, but should not be used to test easily vulnerable electrical systems, especially those that include electronic control modules!
General information
A typical electrical circuit consists of an electrical consumer (working element), a set of switches, relays, executive electric motors, fuses, fuses / circuit breakers related to the operation of this element, as well as connecting wiring, its contact terminals and connectors. In order to facilitate the implementation of diagnostic procedures, at the end of the Guide, Wiring diagrams.
Before starting to search for the reasons for the failure of a failed consumer of electricity, carefully study the corresponding electrical circuit, try to imagine as clearly as possible the principle of operation of the elements that make up the suspect circuit. The list of possible causes of failure can be minimized by excluding properly functioning components from the list that are relevant to the operation of the circuit under test. If several components fail at the same time, the most likely cause of the failure is the failure of a fuse / fuse common to the corresponding circuits, or a ground fault.
Most often, electrical equipment failures are explained by the simplest causes, such as oxidation or loosening of terminal connections, failure of a fuse or fuse, relay failure, etc. Before starting to search for internal defects of the failed component itself, carefully check the condition of all fuses, connectors and connecting wires related to its operation.
To determine the list of components and connectors to be checked, study the corresponding wiring diagrams (see Wiring diagrams).
The most commonly used diagnostic tool for troubleshooting electrical equipment is the universal circuit meter/voltmeter (for some tests, a regular 12-volt lamp with a set of connecting wires is also suitable), probe lamp with individual power supply (sometimes also called a conductivity meter), ohmmeter, power supply with a set of connecting wires, as well as a set of jumper wires equipped with various types of connecting terminals and, preferably, a built-in circuit breaker or fuse (for shunting suspicious sections of a circuit or electrical components). Before resorting to the use of diagnostic equipment, carefully study the wiring diagram of the components of the corresponding circuit (see Wiring diagrams).
When identifying the cause of an unstable failure (violations of this kind, as a rule, are associated with the oxidation of the contact terminals, or loosening of the fastening of the terminal connections of the electrical wiring) a simple circuit check can be performed by twitching various sections of the wiring of the corresponding circuit, as a result of which a defective section of the circuit is localized. This check can be performed in conjunction with any of the following in the relevant subsections.
In addition to problems associated with a violation of the quality of electrical connections, open circuits and short circuits in the circuit should be attributed to the most probable and frequently occurring failures of electrical circuits.
An open circuit is usually caused by mechanical damage to the conductive wires or disconnection of the contact terminals, which leads to the opening of the electrical circuit and the cessation of the circulation of electric current in it. An open circuit will cause the working component to stop functioning, but the related fuses/fuse links will not fail.
A short circuit is a short circuit of its electrical wiring that is not provided for by the design of the circuit. In this case, the current begins to circulate along the shortest path, in most cases going to ground. Short circuits most often turn out to be associated with a violation of the integrity of the insulation of the electrical wiring and without fail lead to the failure of the corresponding fuses / fuses.
Checking the presence of voltage in the circuit
1. Checking the presence of voltage is one of the standard checks in case of failure of any consumer of electricity. Connect one of the wires («-») circuit meter/voltmeter to the negative battery terminal, or any of the positively grounded points on the chassis/engine of the vehicle. Connect the second wire of the instrument to the terminal connection of the circuit under test, preferably closest to the battery or fuse. Apply power to the circuit. Be aware that some circuits are only energized in certain positions of the ignition switch. If tension is present (the meter lamp turns on, or the corresponding reading is fixed on the voltmeter indicator), then the segment of the circuit between the terminal connection being checked and the battery is working.
2. Continue the test in the same vein, alternately moving from one terminal connection of the circuit to another, moving in the direction from the battery / fuse. The faulty section of the circuit will be located between the point at which the device does not register the presence of voltage and the nearest tested good terminal connection. Most often, the cause of the failure is a break in the wiring, or oxidation / loosening of the fastening of the contact element.
Search for the causes of a short circuit
1. Disconnect the consumer first (And) electric power of the tested circuit (consumers of electricity, or the payload of the circuit, are the components for the operation of which the current circulating in the circuit is consumed, such as lamps, electric motors, heating elements, etc.). Remove the fuse that protects the circuit under test and connect a test lamp or voltmeter to its installation terminals. Apply power to the circuit - remember that some of the circuits are only energized in certain positions of the ignition switch. If there is voltage at the fuse terminals, therefore, a short circuit has occurred in the circuit, pull the electrical wiring, as the short circuit may be caused by the destruction of its insulation and be unstable. If there is no voltage, but the fuse after replacement continues to blow when power is applied to the circuit, then there is an internal defect of the consumer (to her) electricity, switch or electrical wiring insulation.
2. Alternatively, the check can be made using an ohmmeter: disconnect all the connectors of the suspect circuit section and, start connecting an ohmmeter between each of the connectors and ground, registering the presence of conductivity with the meter (zero reading) indicates a short to ground in the electrical wiring connected to the corresponding terminal of the connector under test.
Search for ground faults
The negative battery terminal is grounded at «mass», which is the metal of the power unit, chassis and car body elements. The electrical circuits of most electrical equipment are constructed in such a way that the electrical wiring is used only to supply power to the consumer from the positive terminal of the battery, while the current is returned to the battery through the mass metal. The foregoing means that the fasteners of electricity consumers form the return part of the electrical circuit. In view of the situation described, loosening or corrosion of the supporting elements of the working component of the circuit entails a malfunction of the circuit (from complete failure of the last failure to partial failure of various sections of the circuit). In particular, as a result of loosening fasteners, the brightness of lighting fixtures may decrease (especially if there is a common ground with another circuit), or the speed of rotation of the electric motor (e.g. wiper drive or cooling fan). In this case, the failure of one circuit can cause a disruption in the functioning of another, at first glance, in no way connected with the failed one. Please note that on many vehicles certain nodes are connected to each other by special grounding buses. Such tires are used in cases where there is no direct contact between the metal parts of the blocks due to the equipment of the supports with flexible rubber bushings (as, for example, in the suspension bearings of the power unit).
1. To test for proper component grounding, disconnect the battery and connect one of the ohmmeter wires to a known good ground point on the vehicle. Connect the second test lead to the ground point of the component under test. The device should fix zero resistance, otherwise, the electrical connection should be checked (see below).
2. If there is a suspicion of a violation of the quality of the terminal connection, disassemble the ground contact assembly and clean the mating surfaces of the terminals to bare metal. Try to completely remove all traces of corrosion and dirt, then scrape off the paint with a knife, achieving unambiguous metal-to-metal contact. When assembling the assembly, take care of the fastener tightening strength. Place knurled washers between the wiring terminals and ground contacts to ensure the quality of the electrical connection. Coat mated terminal connections with acid-free Vaseline or silicone grease to prevent future corrosion. Ignition sealant spray and water-repellent lubricant are also good tools.
Finding open circuits
Unstable failures of electricity consumers most often turn out to be associated with a violation of the quality of terminal connections due to oxidation or loosening of fasteners. Often, simply tugging on the associated wiring harness/electrical connector is enough to bring a component to working order. The easiest way to find an open circuit is to check its working areas for conductivity. Turn off the power supply to the circuit and use the meter equipped with an independent power supply. Connect the meter leads to both outputs of the circuit under test (power supply terminal and a well-grounded point). If the device detects the presence of conductivity (zero resistance/probe lamp operation), therefore, the tested section of the circuit is working. Otherwise, a break occurs. In a similar way, the correct functioning of the switches can be checked.
Electrical connectors - general information
Most of the onboard electrical circuit connectors are made of plastic and are multi-contact. The reliability of the articulation of the halves of such connectors is ensured by snapping the locking tabs of the latches mounted in the plugs. Large connectors, such as some of those located under the dashboard of a car, are most often held together by through-bolts threaded through the center of the plugs.
To disconnect connectors equipped with plastic latches, a small screwdriver is usually used, which should be carefully pressed out of the locking tabs (first carefully study the design of the docked connector - it is often not easy to determine the method of fixing its parts by eye; some connectors are equipped with multiple locking elements). Pull only on the plug, and in no case on the wiring harness, in order to avoid accidental damage to the contact terminals mounted in the connector.
Connectors always consist of two sections, the terminals of one of which go inside the terminals of the other. When studying the schematic representation of the connector, first of all try to determine which of its sections is shown in the illustration - connected to the harness, or fixed to the component. Remember that the terminals of one section of the connector are always mirrored in relation to the terminals of the other.
When checking for continuity between the connector terminals or measuring voltage between any of the terminals and «weight» always connect the meter probes to the terminals on the rear (bundled) side of the corresponding connector section.
Note. If necessary, use probes of a miniature design, or insert an unfolded paper clip into the connector, to which you can then connect the meter wire using a clamp of the type «crocodile».
Measurement of voltage at the contact terminals of the docked electrical connector is carried out by inserting the probes of the meter (1) into the terminals on the back of the plug; with a sealed connector design, the tester can be connected to the terminal side of the plug using probes of miniature design (2).
If the connector is sealed, the meter can be connected to the terminal side of the connector, however, in such a situation, special care must be taken to avoid damaging the terminals.