Electrical equipment checks (Mercedes-Benz C-Class W202)

When troubleshooting electrical equipment, a systematic approach is important. This is true both when checking failed incandescent lamps and in the case of non-working electric motors.

The first step is always to check the fuse, as it protects an electrical component of the structure. The actual location of the fuses can be seen on the printout located on the fuse box cover.

If necessary, replace the faulty fuse and after connecting the electrical consumer, check whether it has blown again. In this case, first of all, it is necessary to detect the fault and eliminate it, here, as a rule, we are talking about a short circuit.

Second check step: If the incandescent lamp does not light up with a working fuse, or, accordingly, the electric motor does not work, then the power supply system should be checked.

Checking an incandescent lamp

Remove and inspect the bulb. If the filament is burnt out or the glass bulb is loose in the socket, the bulb should be replaced.

To determine if an incandescent light bulb is working properly, follow these steps:



Connect the positive (+) and ground cables directly to the battery terminals (on the car body) wire (–) and connect them to the lamp. It does not matter which wires are connected to the lamp. Connect one wire to the current lead, the other to the body of the incandescent lamp. If the lamp still does not light, replace it.

It is necessary to be sure that the contacts on the lamp and lamp socket are not covered with corrosion. If necessary, clean the correlated or bent leads with sandpaper and make a normal contact.

If the bulb is OK, insert it and turn it on. If the bulb does not light, use a test light to check the power supply. To do this, connect the test light to the vehicle's ground. That is, one wire of the test light must be connected to a good point on the engine housing (polished metal) or directly to the negative pole of the battery. Keep the other probe of the test lamp (+) either on the current-carrying plug or connect it to the wire that is under voltage. If the test lamp now lights up, but the bulb still does not light, then the line connecting the car body to the incandescent bulb is broken. To check this, attach an auxiliary wire connected to the car body to the bulb socket. The incandescent bulb should now light up.

Note: There are lamps that are equipped with only one live supply line, such as parking lights, interior lights of a car. These lamps are directly connected to the vehicle mass via their housing.

If there is no voltage in the current-carrying wire to the lamp and the control lamp does not light, then it is very likely that the switch is faulty. Check the switch for current flow.

Checking Electric Motors

In a car, ever-increasing comfort is provided by low-power electric motors. These include, for example, the window lift, the sliding roof panel, the centrally controlled door locking system or the electric antenna. If necessary, each electric motor is switched on by a single switch, usually manually. In the case of the electric antenna, the switch is automatically controlled via the radio.

Check the resistance of the corresponding electric motor, replace it if necessary.

Caution! Before reactivating, eliminate the cause of the overload of the electric window lift and central locking system. This could be, for example, frozen windows and locks or dirty window channel grooves.

If the fuse immediately blows again, this indicates a short circuit.

To definitely determine whether there is a fault in the engine, run two auxiliary wires with a diameter of about 2 mm directly from the car battery to the electric motor. Connect the positive wire to the positive pole, and the wire connected to the body to the ground pole. In case of doubt, the distribution of the poles is determined from the basic diagram of the electrical equipment. If necessary, you need to disassemble the electric motor for this. All electric motors in the car are powered by the on-board voltage (from 12 to 14 Volts). If the electric motor now operates normally, the power supply system was faulty.

Caution! If the motor rotates too slowly or works intermittently, it may mean that the sliding electrographite contacts are worn out. In this case, replace these contacts.

If the engine is running, use the electrical diagram to determine which supply wire supplies voltage to the electric motor when the switch is activated with the ignition previously on.

Use a test light to check the live wire in the electric motor. Since electric motors carry a lot of current, you can replace the regular test light with an incandescent light bulb. These lights have pointed test needles that you can use to prick the connecting wire. This is a simple way to check the voltage. Motors that alternately rotate left/right, such as window lift motors, have two positive terminals.

Attention! Check the wiper motor.

If there is no voltage supplied to the electric motor, then the power supply system is faulty; find the defect in the supply line according to the basic diagram of the electrical equipment and eliminate it. As a rule, due to high current consumption, electric motors have an additional switching relay.

If no fault is found, check the switch.

If a wire is damaged, it is often more rational to lay a new wire, since it is difficult to determine the location of the break under the insulation.

Checking switches for current flow

Most electrical consumers are switched on and off using a manually pressed switch. In addition, there are switches with an automatic drive. Such switches include, for example, a switch with a hydraulic drive (brake pressure drop sensor) and brake fluid level sensor.

Basically, a switch has the task of closing or breaking an electrical circuit. There are switches that break the ground line and switches that break the positive pole line.

Checking lamp switches and electric motors

Remove the corresponding switch.

Simple switches have only two terminals for connecting the wire. In this case, voltage (+) should be supplied to one terminal, and after switching on/off – to the other terminal. There are switches with several terminals. These switches are explained on the basis of the circuit diagram to which terminal voltage should be applied. If necessary, turn on the ignition in advance.

Check with a test light to see if voltage is reaching the switch. If the test light comes on, turn on the switch and check if voltage is also reaching the output terminal. If so, the switch is found to be functional.

If there is no voltage at the output terminal, there is a break in the power supply wire. Based on the electrical circuit diagram, check the voltage supply and, if necessary, install a new wire.

Checking switches-sensors

Switch-sensors, for example, are a switch with a hydraulic drive, brake fluid level sensors and coolant level sensors.

Connect the probe (a test lamp or ohmmeter) to the inlet and outlet wires of the switch, to do this, remove the wire from the switch.

Caution! Switches that are screwed into the engine block usually do not have a wire connecting them to the housing, since the switch housing on top of the engine block serves as a ground pole.

The probe should indicate "current flow" when the switch is on. The best probe is an ohmmeter: when the switch is on, it shows zero Ohms, and when it is off, it shows infinity.



The quickest way to check the coolant and brake fluid level warning switches is to pull the lead from the switch with the ignition on and press it to a well-grounded place, such as the engine block. If the warning light on the instrument panel comes on, the fault is in the switch. A special case is a switch with a hydraulic drive (brake voltage drop sensor): when the engine stops, the contact is closed (the signal light is on), the switch opens only at a certain oil pressure.

Checking the relay

Many electrical circuits have a relay installed. A switching relay works like a switch. Example

: if the high beam headlights are turned on using the manual switch, the relay receives a command to switch the current to the high beam headlights. Of course, it would be possible to supply the current directly from the battery, using the light switch. But all elements with high current consumption (high beam headlight, windshield wiper, fog light) a relay is connected to the circuit to avoid overloading the switch, providing shorter current paths. Next to these combination relays, there are also functional relays, for example, for windscreen cleaning and washing systems or a warning sound relay for external lighting.

Checking the switching relay

When the lighting element is switched on, the relay is activated, i.e., with the help of the switching current, the electromagnetic coil pulls the relay contact inward and thus closes the electric circuit of the "working current". The working current goes on to the consumer through the relay. The easiest way to check the functionality of the relay is to replace it with a working one. This is done even in a car repair shop. Since the car owner rarely has a new relay at his disposal right away, then in such cases it is recommended to carry out the following operation in the case of so-called switching relays, which are used, among other things, to switch on fog lights and main lights. The marking of the terminals indicated here, especially for serially installed relays, can be presented differently.



Remove the relay from the holder.

Turn on the ignition and the corresponding switch.

First, use a voltage indicator to determine whether voltage is supplied to terminal 30 (+) in the relay holder. To do this, connect the voltage indicator to the housing (–), and carefully insert the other contact into terminal 30. If the LED flashes, there is voltage. If the voltage indicator does not show voltage, then use the circuit diagram to find a break in the wire running from the positive pole of the battery (+) to terminal 30.

Make a conductive jumper from a piece of insulated wire, stripping it until shiny.

Using this jumper in the relay holder, connect terminal 30 (+ battery is always live) to the output of the normally open contact at terminal 87. This operating procedure is practically the same as installing a working relay.

If, for example, the high beam lights up with a jumper, then it can be assumed that the relay is faulty

If the high beam does not turn on, find out if the connection to the housing at the headlights is OK. In this case, find and eliminate the break in the line from terminal 87 to the main headlight using the wiring diagram. If necessary, install a new relay.

Attention! If a defect in an electrical circuit equipped with one relay appears only periodically, then in this case, as a rule, the fault lies in the relay: the contact in the relay disappears or the contact sticks too much, while the relay functions normally the rest of the time. If a defect appears, lightly tap the relay housing. If the relay still switches, replace it.

Checking the windshield wiper motor

The windshield wiper motor is installed in the water collector, under the windshield. To check it, you need to remove the corresponding casing.

Terminal markings

The terminals on the motor are typically designated as follows:

• terminal 31 is the terminal for connection to the housing (in general in electrical equipment of vehicles);
• terminal 53 - receives voltage for the first speed of the windshield wiper;
• terminal 53a - supplies positive current (+) for resetting the windscreen wipers: the motor receives voltage through the sliding contact until the windscreen wipers are in the reset position, if the driver switches off the windscreen wipers;
• terminal 53v - supplies voltage for the second speed of the windshield wiper (parallel winding);
• terminal 53e brakes the windscreen wiper motor when moving backwards after switching off, so that the windscreen wipers do not leave their stop position;
• terminal 53c (from the relay) - supplies voltage to the electric windshield wiper pump.

Checking the windshield wiper motor

First, find out if the windshield wiper motor is working properly and if the electrical system is OK. To find out, do the following:

• disconnect the multi-pin connector from the windshield wiper motor;
• using two auxiliary wires, supply voltage (+) and ground (-) from the battery to the windshield wiper motor;
• run one wire from the positive pole of the battery to terminal 53 or 53v;
• run the second wire from the negative pole of the battery to terminal 31 of the engine;
• now, depending on the terminal used, the motor should run in stage I or II. If the motor does not run, then it or its corresponding stage is faulty. Remove the wiper motor.

Checking the flashing light device

The cycles for the flashing light device and the hazard warning light system are generated by a relay, the so-called flashing light sensor. The hazard warning light system is connected to a relay without a fuse. The direction indicator system is protected by a fuse located in the fuse box.

If the flashing rhythm is faster on one side than the other, the bulb on the "fast" side is faulty or there is an open circuit.

If other defects occur, in most cases the cause is the turn signal switch (flashing light sensor).

The following terminal positions are indicated on the turn signal switch:

• terminal 31 is the vehicle ground (minus in the electrical equipment of all vehicles);
• terminal 49 - relay input (positive voltage is constantly supplied);
• terminal 49a - relay output;
• terminal C leads to a control lamp on the instrument panel; if a towing device is present, there may be an additional terminal C2 to signal that the trailer direction indicators are on.

If you do not have a new relay, carefully insert a thin wire between terminals 49 and 49a in the relay plug connector.

Caution! Be careful not to damage the sensitive relay contacts. Before inserting, bend the ends of the wire so that there are no sharp edges. Re-attach the faulty turn signal flasher. Extend the connecting busbars so that you can re-attach the flasher despite the bypass.

Switch on the ignition. If the turn signal lever is now activated, the corresponding side of the flashing light device lights up for a long time. By switching this device on and off, a flickering rhythm can be formed.

If, despite the bypass, the flashing light does not light, then the turn signal switch in the electrical supply line is faulty.

Checking the brake light

If the brake light does not come on, first check the fuse in the fuse box.

If the fuse is OK, check the brake light bulbs and replace any burnt out bulbs if necessary.

If the bulbs are OK, check the brake light switch, which is mounted on top of the brake pedal on its stand. When you press the brake pedal, the push pin is pulled out of the switch. The switch contact closes and the brake light comes on.

Check the brake light switch. To do this, remove the protective cover on top of the pedal. Remove the wire tip from the brake light switch.

Turn off the ignition.

Shunt both contacts at the brake light switch wire end with a short jumper wire. If the brake lights now come on, the brake light switch is faulty and needs to be replaced.

[Original article is on the portal MercedesMan]

Checking the rear window defroster

When the rear window heating system is turned on, after a certain period of time the area with visible tracks should be free of plaque or ice.

If there is a problem, first check the fuse in the fuse box.

If the fuse is good, check the tight fit of the wire tip by turning it left and right on the rear window, if necessary, clean it from corrosion products.

If the rear window heating system still does not function, check the integrity of the supply wires and the functionality of the switch, as well as the switching relay.

If the heating lines are broken, conductive silver varnish helps to restore their connection. To do this, clean the damaged area with thinner or ethyl alcohol.

Cover the torn area on both sides with adhesive tape and apply silver varnish with a small brush.

Leave the varnish to dry at a temperature of about +25 degrees for 24 hours. You can warm it up with hot air. At a temperature of 130°C, the varnish dries in about 30 minutes.

Warning! Do not turn on the rear window heating system until the varnish is completely dry. Do not use gasoline or other solvent to clean the damaged part.