Note. The procedures described below provide a general diagnosis of the condition of the main electrical circuits, but should not be used to test easily vulnerable electrical systems (such as ABS), especially those that include electronic control modules (ECU).
General information
A typical electrical circuit consists of an electrical consumer (working component), control switches (keys), relays, electric motors, fuses, fuses / circuit breakers related to the operation of this component, as well as connecting wiring, its terminals and connectors. For the purpose of simplification of performance of diagnostic procedures at the end of this Chapter diagrams of electric connections of various systems of electric equipment of the car are given.
Before you start troubleshooting a failed electrical appliance, carefully study the appropriate electrical diagram. Try to imagine as clearly as possible the principle of operation of the components that make up this circuit. The list of possible causes of failure can be minimized by excluding from it unambiguously properly functioning components that are relevant to the operation of the circuit under test. If several components or circuits fail at the same time, the most likely cause of the failure is the failure of a fuse common to these 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 (a description of the procedures for checking the correct operation of the relay is given in Section Fuses, relays and breakers - general information) and so on. Before starting to search for internal defects of the failed component itself, carefully check the condition of all fuses, connectors and wires related to its operation. To determine the list of units and terminal connections to be checked, study the corresponding electrical diagram.
The main tools needed when searching for electrical equipment failures include:
- a) Circuit meter or voltmeter (for some tests, a regular 12-volt lamp with a set of connecting wires will also work);
- b) Probe lamp with individual power supply (sometimes also called a conductivity meter);
- c) Ohmmeter;
- d) Battery;
- e) Set of connecting wires;
- f) Set of jumper wires equipped with various types of connection terminals and preferably equipped with a circuit breaker or fuse (for shunting suspicious sections of a circuit or electrical components).
Before proceeding with the actual performance of the checks, carefully study the relevant (from those given at the end of this Chapter) wiring diagrams.
To find the cause of an intermittent failure (violations of this kind are usually associated with oxidation or loosening of the wiring terminal connections) a simple check of the circuit can be made, performed by twitching various sections of the wiring of the corresponding circuit. As a result of such manipulations, it is often possible to localize the defective segment of the chain. This check can be performed in conjunction with any of the following in the relevant subsections.
In addition to the problems associated with a violation of the quality of electrical connections, breaks and short circuits are among the most probable and common failures of electrical circuits.
An open circuit is usually determined by mechanical damage to the conductive wires or disconnection of the contact terminals, which leads to the opening of the circuit and the cessation of the circulation of electric current in it. An open circuit will stop the working component from 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, usually going to ground. Short circuits are usually associated with a violation of the integrity of the insulation of the electrical wiring and without fail lead to the failure of the circuit fuses / fuses.
Finding open circuits
To test for an open circuit, connect one of the leads from a circuit meter or voltmeter to the negative battery terminal or any of the positively grounded points on the vehicle's chassis or engine.
Connect the second test lead to the terminal connection of the circuit under test, preferably the one closest to the battery or fuse.
Turn on the power - be aware that some of the 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.
Continue testing, alternately moving from one terminal connection of the circuit to another (moving away from the battery/fuse).
The faulty section of the circuit will lie between the point at which the device does not register the presence of voltage and the last of the tested good terminal connections. Most often, the cause of the violation is a broken electrical wiring, or oxidation / loosening of the terminal fastening.
Search for the causes of a short circuit
Disconnect the consumer first (And) electric power of the tested circuit (consumers of electricity, or the payload of the circuit, are components whose operation consumes the main share of the energy circulating in the current circuit, such as lamps, electric motors, heating elements, etc.).
Remove the fuse protecting the circuit under test and connect a circuit meter or voltmeter to its installation terminals.
Turn on the power - remember that some of the circuits are only energized in certain positions of the ignition switch.
If voltage is present at the fuse terminals, then there is a short circuit in the circuit.
If there is no voltage, but the fuse still blows when power is applied to the circuit, then there is an internal consumer defect (to her) circuit electricity.
Search for ground faults
The negative terminal of the battery is grounded to the "weight" 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 connecting wiring is used only to supply power to the consumer from the positive terminal of the battery, while the return of current to the battery is carried out through the mass metal. This means that the fasteners of the electrical consumers form the return part of the circuit. In view of the foregoing, the weakening of the fastening or the development of corrosion of the supporting elements of the working component of the circuit entails a malfunction of the circuit from the complete failure of the last failure to the partial failure of various sections of the circuit. In particular, as a result of loosening fasteners, the brightness of lighting fixtures may decrease (especially when common grounding with another circuit) or motor speed (e.g. wiper drive or cooling fan). In this case, the failure of one circuit can cause a disruption in the functioning of another, externally 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 elastic rubber pads and bushings (as, for example, in the suspension bearings of the power unit).
To test for proper component grounding, disconnect the battery and connect one of the ohmmeter leads to a known good ground point on the chassis or power package. Connect the second test lead to the ground point of the component under test. The device should fix zero resistance, otherwise you should check the connection.
If there is a suspicion of a violation of the quality of the ground terminal connection, disassemble the 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. Between the wiring terminals and the ground contacts, to ensure the quality of the electrical connection, lay knurled washers. To prevent future corrosion, coat the connected terminal assemblies with acid-free Vaseline or silicone grease. An aerosol used to seal the components of the ignition system and a moisture-repellent lubricant are also good tools.