Contents: General information ⬇ Search for chain breaks ⬇ Finding the Causes of a Short Circuit ⬇ Search for ground faults ⬇
Note: The procedures described below allow for general diagnostics of the condition of the main electrical circuits, but should not be used to check vulnerable electrical systems (such as ABS), in particular, those that include electronic control modules (ECUs).
General information
A typical electrical circuit consists of an electrical consumer (working component), control switches (keys), relays, electric motors, fuses, fuse links/circuit breakers related to the operation of the component, as well as the connecting electrical wiring, its terminals and connectors. To facilitate the diagnostic procedures, the electrical connection diagrams of the various vehicle electrical systems are provided at the end of this Chapter.
Before you start troubleshooting a failed electrical appliance, carefully study the corresponding electrical circuit. Try to imagine as clearly as possible the operating principle of the components included in this circuit. The list of possible causes of failure can be minimized by excluding from it clearly functioning components related to the operation of the circuit being tested. If several components or circuits fail simultaneously, the most likely cause of failure is the failure of the fuse common to these circuits or a ground fault.
Most often, electrical equipment failures are explained by simple reasons, such as oxidation or loosening of terminal connections, failure of a fuse or fusible link, or relay failure (a description of the procedures for checking the proper functioning of a relay is given in Section Fuses, Relays and Circuit Breakers - General Information) etc. Before you start looking for internal defects of the actual failed component, 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 basic tools needed to troubleshoot electrical equipment include:
- a) Circuit meter or voltmeter (for some tests, a regular 12-volt lamp with a set of connecting wires will also work);
- b) Test lamp with individual power supply (sometimes also called a conductivity meter);
- c) Ohmmeter;
- d) Battery;
- e) Set of connecting wires;
- f) A set of jumper wires equipped with different types of connecting terminals and, preferably, equipped with a circuit breaker or fuse (to bypass suspected circuit sections or electrical components).
Before you actually start performing the checks, please carefully study the relevant (from those given at the end of this Chapter) electrical connection diagrams.
To find the cause of an unstable failure (faults of this kind are usually associated with oxidation 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 such manipulations, it is often possible to localize the defective section of the circuit. This test can be performed together with any of those listed below in the corresponding subsections.
In addition to problems associated with poor electrical connections, the most likely and common electrical circuit failures include breaks and short circuits.
A circuit break is usually determined by mechanical damage to the conductive wires or disconnection of the contact terminals, which leads to the circuit being opened and the cessation of the circulation of electric current in it. As a result of a circuit break, its working component ceases to function, 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 usually associated with a violation of the integrity of the insulation of the electrical wiring and inevitably lead to the failure of the fuses/fuse links of the circuit.
Search for chain breaks
To check the circuit for an open, connect one of the leads of a circuit tester or voltmeter to the negative battery terminal or any well-grounded point on the vehicle's chassis or engine.
Connect the second lead of the device to the terminal connection of the circuit being tested, preferably the one closest to the battery or fuse.
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Turn on the power - remember that some of the circuits are only powered in certain positions of the ignition switch.
If tension is present (the meter lamp turns on, or the corresponding reading is recorded on the voltmeter indicator), this means that the section of the circuit between the tested terminal connection and the battery is intact.
Continue testing by moving from one terminal connection in the circuit to the next (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 checked serviceable terminal connections. Most often, the cause of the malfunction is a break in the electrical wiring, or oxidation/loosening of the terminal fastening.
Finding the Causes of a Short Circuit
First of all, disconnect the electrical consumer of the circuit being tested (consumers of electrical energy, or the useful load of the circuit, are components whose operation consumes the bulk of the energy of the current circulating in the circuit, such as lamps, electric motors, heating elements, etc.).
Remove the fuse for the circuit being tested and connect a circuit tester or voltmeter to its installation terminals.
Turn on the power - remember that some of the circuits are only powered in certain positions of the ignition switch.
If voltage is present at the fuse terminals, there is a short circuit in the circuit.
If there is no voltage, but the fuse still blows when power is supplied to the circuit, then there is an internal defect in the circuit's power consumer.
Search for ground faults
The negative terminal of the battery is grounded to the so-called "mass" metal of the power unit, chassis and body elements of the car. The electrical circuits of most electrical equipment are constructed in such a way that the connecting 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 metal of the mass. This means that the fastening elements of the power consumers form the return part of the circuit. In view of the above, loosening of the fastening or the development of 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 the partial failure of various sections of the circuit. In particular, as a result of loosening of the fasteners, the brightness of the lighting devices may decrease (especially when grounded in common with another circuit) or the speed of rotation of the electric motor (for example, a windshield wiper drive or a cooling system fan). In this case, the failure of one circuit can cause a malfunction of another, which is not outwardly connected with the failed one. Please note that on many cars, 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 blocks due to the equipment of the supports with elastic rubber cushions and bushings (such as in the powertrain suspension supports).
To check the integrity of the component grounding, disconnect the battery and connect one of the ohmmeter leads to a known good ground point on the chassis or powertrain. Connect the other lead of the meter to the ground point of the component being tested. The device should register zero resistance, otherwise, check the integrity of the connection.
If you suspect that the quality of the ground terminal connection is compromised, disassemble the 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. Place knurled washers between the wiring terminals and the ground contacts to ensure the quality of the electrical connection. To prevent future corrosion, coat the connected terminal units with acid-free petroleum jelly or silicone grease. Aerosol used to seal ignition system components and water-repellent grease are also good options.