Mercedes M-Class W163

since 1997 release

Repair and operation of the car

Search of causes of failures of electric equipment

The procedures described below allow to make the general diagnostics of a condition of the main electric contours, however should not be applied to check of the electric systems open to injury, in particular, which are turning on electronic modules of management in the structure!

General information

The typical electric contour consists of the consumer of the electric power (a working element), a set of switches, the relay, executive electric motors, safety locks, fusible inserts / breakers of a chain concerning work of this element, and also connecting electrical wiring, its contact plugs and sockets. For the purpose of simplification of implementation of diagnostic procedures schemes of electric connections of systems of onboard electric equipment are provided in the last Section of the present Chapter.

Before starting search of causes of failure of the failed consumer of the electric power, attentively study the corresponding electric circuit, try as it is possible to imagine more clearly the principle of functioning of the elements which are a part of the suspected contour. The list of possible causes of failure can be minimized by an exception of it of regularly functioning components concerning work of the checked contour. At simultaneous violation of functioning at once of several components, failure of the general for the corresponding chains, or grounding violation is safety lock/melting of an insert the most probable cause of refusal.

Most often failures of electric equipment are explained by the elementary reasons, such as oxidation, or weakening of fastening of terminal connections, failure of a safety lock or fusible insert, failure of the relay, etc. Before starting search of internal defects of actually refused component, attentively check a condition of all safety locks concerning its functioning, sockets and connecting wires (see the Section the Digital tire of data of CAN).

For definition of the list of the knots which are subject to check and contact sockets, study the corresponding schemes of electric connections (see Schemes of electric connections).

It is necessary to carry to number of the diagnostic equipment most often used by search of failures of electric equipment the universal measuring instrument the chain/voltmeter (the 12-voltny lamp with a set of connecting wires will also be suitable for some checks), a lamp sampler with the individual power supply (sometimes called also by the conductivity measuring instrument), an ohmmeter, the power supply with a set of connecting wires, and also a set of the wires crossing points equipped various type with connecting plugs and, - it is desirable, - the built-in breaker of a chain or a safety lock (for shunting of suspicious sites of a chain or electric components). Before resorting to use of the diagnostic equipment attentively study the scheme of electric connections of components of the corresponding contour.

At identification of the reason of the refusal having unstable character (violations such usually are connected with oxidation of contact plugs, or weakening of fastening of terminal connections of an electrical wiring) the simplest check of a chain which is carried out by twitching of various sites of an electrical wiring of the corresponding contour as a result of which the defective piece of a chain is localized can be made. This check can be made together with any of listed below in the corresponding subsections.

Except the problems connected with violation of quality of electric connections, it is necessary to refer breaks and short circuits in a chain to number of the most probable and often happening refusals of electric contours.

The break of a chain usually is caused by mechanical injury of conducting veins or a detachment of contact plugs that leads to disconnection of an electric contour and the termination of circulation in it electric current. As a result of break of a chain its working component ceases to function, however the corresponding safety locks / fusible inserts do not fail.

Short circuit is called unforeseen a chain design short circuit of its electrical wiring. At the same time current begins to circulate on the shortest way, in most cases leaving on weight. Short circuits most often are connected with violation of integrity of isolation of an electrical wiring and without fail lead to failure of the corresponding safety locks / fusible inserts.

Verification of tension in a chain

PERFORMANCE ORDER

Verification of tension is among standard checks in case of refusal of any consumer of the electric power. Connect one of wires ("-") of the chain/voltmeter measuring instrument to the negative plug of the battery, or any of reliably grounded points on the car chassis/engine. Connect the second wire of the device to terminal connection of the checked contour, preferably to the next to the battery or a safety lock. Give food to a contour. Do not forget that some chains are powered only in certain provisions of the switch of ignition. If tension takes place (the measuring instrument lamp joins, or on the indicator of the voltmeter the corresponding indication is fixed), the chain piece between the checked terminal connection and the battery means it is serviceable. Continue check in the same spirit, serially passing from one terminal connection of a chain to another, moving in the direction from a battery/safety lock. The faulty site of a contour will be located between a point on which the device will not register presence of tension and the next tested correct terminal connection. Most often the break of an electrical wiring, or oxidation/weakening of fastening of a contact element appears a cause of failure.

Search of the reasons of short circuit

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First of all disconnect the consumer(s) of the electric power of the checked contour (consumers of the electric power, or contour payload, components for which functioning the current circulating in a chain, such as lamps, electric motors, heating elements, etc. is spent are called). Take the safety lock protecting the checked contour, and connect a lamp sampler or the voltmeter to its adjusting plugs. Give power supply to a contour, - you remember that some of contours are powered only in certain provisions of the switch of ignition. If tension on plugs of a safety lock takes place, therefore, in a chain there was a short circuit, - pull an electrical wiring as short circuit can be caused by destruction of its isolation and have unstable character. If tension is absent, however the safety lock after replacement continues to fuse when giving food in a chain, so internal defect of the consumer(s) of the electric power, the switch or isolation of an electrical wiring takes place. Alternatively check can be made by means of an ohmmeter: disunite all contact sockets of the suspect of the site of a chain and, begin to connect an ohmmeter between each of sockets and weight, registration by the conductivity existence measuring instrument (the zero indication) demonstrates short circuit on the mass of the electrical wiring connected to the corresponding plug of the checked socket.

Search of violations of grounding

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The negative plug of the battery is grounded on "weight" as which metal of the power unit, chassis and body elements of the car acts. Electric contours of the most part of electric equipment are constructed in such a way that the electrical wiring is used only for giving of power supply to the consumer from the positive plug of the battery, return of current to the battery is carried out on weight metal. Told means that fasteners of consumers of the electric power form themselves a returnable part of an electric chain. In view of the described situation, weakening of fastening or corrosion of basic elements of a working component of a chain involves violation of serviceability of functioning of a contour (from a full exit of the last out of operation before partial refusal of various sites of a chain). In particular, as a result of weakening of fixture brightness of a luminescence of lighting fixtures (in particular in the presence of the general grounding with other contour), or the speed of rotation of an electric motor can decrease (for example, the drive of screen wipers or the fan of the cooling system). At the same time the refusal of one contour can cause violation of functioning of another, at first sight not connected with failed in any way. Pay attention that on many cars certain knots are connected among themselves by special tires of grounding. Such tires are used when there is no direct contact of metal parts of blocks in view of the equipment of support flexible rubber plugs (as, for example, in support of a suspension bracket of the power unit). For check of serviceability of grounding of a component switch-off the battery and connect one of ohmmeter wires to obviously reliably grounded car point. Connect the second wire of the measuring instrument to a point of grounding of the checked component. The device has to record zero resistance, otherwise it is necessary to check serviceability of electric connection (see further). In the presence of suspicions on violation of quality of terminal connection, sort contact knot of grounding and smooth out the interfaced surfaces of plugs to pure metal. Try to remove completely all traces of corrosion and dirt, then scratch out a knife paint, trying to obtain unambiguous contact of metal surfaces. At assembly of knot take care of durability of tightening of fixture. Between plugs of an electrical wiring and contacts of weight for a quality assurance of electric connection lay washers with a notch. In order to avoid corrosion development in the future cover the joined terminal connections with acid-free vaseline or silicone lubricant. Good means are also an aerosol for sealing of components of system of ignition and water-repellent lubricant.

Search of breaks of a chain

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Unstable refusals of consumers of the electric power most often are connected with violation of quality of terminal connections due to oxidation or weakening of fixture. Often for reduction of a component in the working condition, it is rather simple to pull the corresponding plait of an electrical wiring / the electric socket. The easiest way of search of break of a chain is check of its working sites on conductivity existence. Disconnect power supply of a contour and use the measuring instrument equipped with the independent power supply. Connect measuring instrument wires to both exits of the checked chain (the plug of giving of food and well grounded point). If the device fixes conductivity existence (zero resistance / operation of a lamp sampler), therefore, the checked site of a chain is serviceable. Otherwise the break takes place. Serviceability of functioning of switches can be checked by a similar way.

Electric sockets - general information

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The majority of contact sockets of chains of onboard electric equipment are manufactured of plastic and are multicontact. Reliability of a joint of half of such sockets is provided with a zashchelkivaniye of lock uvulas of the clamps which are built in in plugs. Large sockets, such as some from located under the car dashboard, are most often fastened with the through bolts passed through the central part of plugs. For a rassoyedineniye of the sockets equipped with plastic clamps the small screw-driver which should wring out accurately lock uvulas usually is used (previously attentively study a design of the joined socket - often it is not simple to measure a way of fixing of its half by eye at all; some sockets are equipped with several lock knots). Pull only for the plug, and not for an electrical wiring plait, in order to avoid casual damage of the contact plugs which are built in in the socket at all. Sockets always consist of two sections, plugs of one of which enter in plugs another. When studying the schematic image of the socket try to define first of all which of its sections it is presented on an illustration, - connected to a plait, or fixed on a component. Remember that plugs of one section of the socket are always placed specularly in relation to plugs another. At verification of conductivity between plugs of the socket or voltage measurement between any of plugs and "weight" always connect measuring instrument probes to plugs from the back (cordlike) party of the corresponding section of the socket. In case of need use probes of a tiny design, or enter into the socket the unbent office paper clip to which then it is possible to connect a measuring instrument wire by means of a clip like "crocodile". Voltage measurement on contact plugs of the joined electric socket is performed by introduction of probes of the measuring instrument (1) to plugs from the back party of the plug; at a tight design of the socket the tester can be connected from the terminal party of the plug by means of probes of a tiny design (2) If the socket has a tight design, the measuring instrument can be connected from the terminal party of the socket, however, in such situation it is necessary to observe extra care in order to avoid damage of plugs.