Contents: General information ⤓ Checking for voltage in the circuit ⤓ Finding the Causes of a Short Circuit ⤓ Search for ground faults ⤓ Search for chain breaks ⤓
Caution! The procedures described below allow you to perform a general diagnosis of the condition of the main electrical circuits, but should not be used to check vulnerable electrical systems (such as, for example, ABS), especially those that include electronic control modules (ECM).
General information
A typical electrical circuit consists of an electrical consumer (the working component), a set of switches, relays, actuator motors, fuses, fuse links/circuit breakers related to the operation of that component, and the connecting electrical wiring, its terminals and connectors. To facilitate diagnostic procedures, the last Section of this Chapter provides electrical wiring diagrams of the various vehicle electrical systems.
Before you start looking for the reasons for the failure of a failed power consumer, carefully study the corresponding electrical circuit, try to imagine as clearly as possible the operating principle of the components included in the suspected circuit. The list of possible reasons for failure can be minimized by excluding from it properly functioning components related to the operation of the circuit being checked. In case of simultaneous malfunction of several components, the most likely cause of failure is the failure of a fuse/fuse link common for the corresponding circuits, or a grounding fault.
Most often, electrical equipment failures are explained by simple reasons, such as damage from corrosion, or loose terminal connections, failure of a fuse or fuse link, failure of a relay, etc. Before you start looking for internal defects of the actual failed component, carefully check the condition of all fuses, connectors and connecting wires related to its operation.
To determine the list of components and terminal connections to be checked, study the relevant electrical connection diagrams.
Diagnostic equipment needed when troubleshooting electrical equipment failures includes a multi-function circuit meter/voltmeter (a 12-volt light bulb with jumper leads will also work for some tests), a test lamp with its own power supply (sometimes called a continuity meter), an ohmmeter, a power supply with jumper leads, and a set of jumper leads equipped with various types of connecting terminals and, preferably, a built-in circuit breaker or fuse (for bypassing suspect circuits or electrical components). Before using diagnostic equipment, carefully study the electrical connection diagram of the components in the circuit in question (see Section Single lock with ETACS, Galant models 1990-1993.).
To find the cause of an unstable failure (failures of this kind are usually associated with oxidation of contact terminals or loosening of the terminal connections of the electrical wiring), a simple circuit test can be performed by twitching various sections of the electrical wiring of the corresponding circuit, as a result of which the defective section of the circuit is localized. This test can be performed together with any of those listed below in the relevant subsections.
In addition to problems associated with poor electrical connections, the most likely and frequently occurring failures of electrical circuits include breaks and short circuits in the circuit.
(The article is based on data from the website www.mitsubishiman.ru)
A circuit break 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 circulation of electric current in it. As a result of a circuit break, its working component stops functioning, but the corresponding fuses/fuse links do not fail.
A short circuit is a short circuit in the 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, usually going to ground. Short circuits are most often associated with a violation of the integrity of the insulation of the electrical wiring and inevitably lead to the failure of the corresponding fuses/fuse links.
Checking for voltage in the circuit
Checking for voltage is a standard check when any electrical consumer fails. Connect one lead of a circuit tester or voltmeter to the negative battery terminal or any reliably grounded point on the vehicle's chassis/engine. Connect the other lead of the meter to the terminal connection of the circuit being tested, preferably the one closest to the battery or fuse. Apply power to the circuit. Remember that some circuits are powered only in certain ignition switch positions. If voltage is present (the meter lamp lights or the voltmeter indicator registers a reading), the section of circuit between the terminal connection being tested and the battery is good.
Continue testing in this manner, moving from one terminal connection in the circuit to the next, moving away from the battery/fuse. The faulty section of the circuit will be located between the point where the meter registers no voltage and the previously tested and working terminal connection. The most common cause of failure is a broken wiring or oxidation/loosening of the terminal connection.
Finding the Causes of a Short Circuit
First of all, disconnect the electrical consumer(s) of the circuit being tested (electric consumers, or the useful load of the circuit, are the components whose operation consumes the current circulating in the circuit, such as lamps, electric motors, heating elements, etc.). Remove the fuse protecting the circuit being tested and connect a test lamp or voltmeter to its installation terminals. Apply power to the circuit. Remember that some circuits are supplied with power only in certain positions of the ignition switch. If voltage is present at the fuse terminals, then there is a short circuit in the circuit (pull the wiring, since a short circuit can be caused by rubbing through its insulation and be unstable). If there is no voltage, but the fuse continues to burn out when power is supplied to the circuit after replacement, then there is an internal defect in the electrical consumer(s), switch, or wiring insulation.
Search for ground faults
The negative terminal of the battery is grounded to the "mass", which is the metal of the power unit, chassis and body elements of the car. The electrical circuits of most electrical equipment are designed in such a way that the 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 metal of the mass. This means that the fastening elements of the power consumers form the return part of the electrical circuit. In view of the described situation, loosening of the fastening or corrosion of the supporting elements of the working component of the circuit entails a violation of the proper functioning of the circuit (from the complete failure of the latter to partial failure of various sections of the circuit). In particular, as a result of loosening of the fasteners, the brightness of the glow of lighting devices (especially if there is a common ground with another circuit) or the rotation speed of the electric motor (for example, the windshield wiper drive or the cooling system fan) may decrease. In this case, the failure of one circuit can cause a malfunction of another, outwardly unrelated to the failed one. Please note that on many vehicles, certain units are connected to each other by special grounding buses. Such buses are used in cases where there is no direct contact between the metal parts of the units due to the support being equipped with flexible rubber bushings (as, for example, in the supports for attaching the power unit to the vehicle chassis).
To check the integrity of the component grounding, disconnect the battery and connect one of the ohmmeter leads to a known reliable ground point on the vehicle. Connect the other lead of the meter to the ground point of the component being tested. The device should record zero resistance, otherwise, you should check the integrity of the connection (see below).
If you suspect that the quality of the terminal connection is compromised, disassemble the grounding contact unit 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 contact of the metal surfaces. When assembling the unit, ensure that the fasteners are tightened securely. To ensure the quality of the electrical connection, place knurled washers between the wiring terminals and the ground contacts. To prevent future corrosion, coat the mated terminal connections with acid-free petroleum jelly or silicone grease. Good products include an aerosol for sealing ignition system components and a water-repellent grease.
Search for chain breaks
Intermittent failures of electrical consumers are most often associated with poor quality terminal connections due to oxidation or loosening of fasteners. Often, to bring a component into working condition, it is enough to simply tug on the corresponding wiring harness/electrical connector. The simplest way to find a break in a circuit is to check its working sections for conductivity. Disconnect the power supply to the circuit and use a measuring device equipped with an independent power source. Connect the measuring device wires to both outputs of the circuit being tested (the power supply terminal and a well-grounded point). If the device detects conductivity (zero resistance/trigger of the test lamp), then the section of the circuit being tested is in good condition. Otherwise, there is a break. The proper functioning of switches can be checked in a similar way.