How to check the diode bridge of a Bosch alternator

Another way of how to check a removed alternator is to diagnose the voltage regulator. To do this, connect a 12 volt light bulb to its brushes, and apply voltage to the mass of the device and its plus terminal. If everything is normal, the light will come on and go out when the voltage increases to 15 V.

Using a multimeter in resistance measurement mode, check the field winding (on the rotor). To do this, attach the feeler gauges to the contact rings. Resistance of reliable winding should be within ohms. If there is no resistance reading at all, then there is a break in the winding.

Checking of alternator on the machine

The first thing to do is to check if the alternator belt is intact. If it is not broken, then check the belt tension. Then turn to the battery. Use a tester (multimeter) to measure voltage on the terminals. It should be in the region of 12-12.7 volts. If everything is normal, start the engine. If the battery is discharged, charge it and restart the engine.

Measure the voltage at the battery terminals (battery). It should be within the specified range, usually 13.2 to 14.5 volts. But on modern vehicles these limits can be different. If you have the manual you can read it. Deviation from the set values in any direction is a malfunction. These deviations can be of three kinds:

  • No charging current. the generator does not work.
  • Charging current is present, but below the minimum value. insufficient charging of the battery.
  • If the battery voltage is above the maximum value, it is overcharged.

All three cases indicate there is a problem with the vehicle’s electrical system. Perform a complete inspection of the alternator.

But before you do so, conduct a visual inspection of all wires and cables that go from the alternator to the battery. There should be no visible damage, breaks or oxidation of the wiring. Be sure to check the terminals on the battery, starter, and alternator. They should be clean and dry. Any oxidation, rust and dirt should be cleaned off. It is often possible to restore the lost contact and the vehicle begins to work as it should. If this does not help, proceed to the detailed check.

Using a Multimeter

It is best to remove the alternator from the vehicle for further inspection. First of all remove a voltage regulator from an alternator and check it. To test the voltage regulator you will need a multimeter and a battery charger with voltage regulation. It would be better to use the power supply instead of the charger. A voltage adjustment of 0 to 16 volts is sufficient.

Connect the plus end of the power supply to the regulator. usually a male connector. Connect the minus to the minus line, it is usually located at the relay’s mounting ear. Connect the red wire of the tester to the plus wire of the power supply, and the black wire to the minus wire. Connect two stripped wires to the brushes, one for each. Connect light bulb to other ends, previously stripped (it can be removed from the rear lights of the car for the test). The test stand is ready.

Testing of voltage regulator

Connect power unit to mains and carefully with regulator knob start to increase voltage. At the same time, watch the multimeter reading. The light bulb at the beginning should not burn, but as you raise the voltage should light up, first at half voltage and as you add the brightness should increase.

See also  How to make a cardan on a single axle tractor

When it reaches the 14.5 volt mark, the regulator should trip, cutting off the voltage. The light bulb should go out after that. It is generally assumed that the regulator works, if it shuts down the current at values from 14.2 to 14.8 volts. If it happens at lower or higher values, the voltage regulator is defective. And also the relay is defective if there is no current cutoff at all.

In the case of the relay failure replace it with a new one. If it is faulty, continue checking.

Rectifier failures

Since the rectifier assembly of an alternator consists of several semiconductor devices, and in 90% of cases is protected by a cover, for the diagnosis you will need electrical devices and partial disassembly of the alternator. However, in some cases, the driver can hear the signs of diode bridge failure:

  • when ripples appear (alternating voltage instead of direct voltage is fed into the onboard network) electric motors of some consumers may produce sounds similar to those of a loudspeaker;
  • most often “squeaks” the drive of windows and the heater, and the tone changes with the change of speed of these devices, but not the speed of the crankshaft.

In all other cases, malfunctions of the car alternator in the rectifier unit are diagnosed solely by devices. This will require a wiring diagram of the diode bridge in a particular modification of the alternator, since the symptoms of violation of the mechanical part are completely similar to the breakdown of electrical parts.

Failure diagnostics

Rectifier unit is assembled according to different technologies. some parts are mechanically fastened, small diodes are soldered into the circuit, large-size diodes are usually pressed. Therefore, it may be necessary to repair the rectifier, not only when the semiconductor elements fail, but also when they are installed incorrectly on the “horseshoe” of the heat sink plate.

Before testing a circuit or an individual semiconductor, you should visually inspect the design. Even in the absence of a tester, ohmmeter, voltmeter you can use a light bulb and a special connection diagram of the battery to understand whether the diode is faulty or works correctly.

The diagnostic technique is as follows:

  • the back cover is removed from the alternator to provide access to the diodes;
  • the “-” wire from the battery is fed to the plate, it is pressed to the housing on the alternator, one wire of the lamp touches the diode at the point of connection of the stator winding, the second. to the “” of the battery, at breakdown the lamp will light up;
  • the tester is set in ohmmeter mode to 1 kOhm, if you swap the multimeter probes, the reading should change from 0 to 400. 800 ohms in different directions.

In most cases, the diode bridge burns when moisture penetrates.

Symptoms of malfunction

The following signs may indicate a malfunction of the alternator:

  • on the “dashboard” the indicator light is constantly on;
  • the battery began to discharge quickly, and recharging does not save the situation;
  • There are failures in the functioning of electrical equipment (ventilation and heating, multimedia devices, alarm system and lighting), even if the motor will work properly;
  • there is a smell of something burning in the car interior;
  • The alternator started whistling or rustling.

Read also: Lead shielded cable marking

If you notice these symptoms, you should immediately go to the car service center for a thorough diagnosis. However, you can check the functionality of the alternator on your own, especially if you have the skills to use an autotester.

See also  Installation of a reduction gearbox on a single-axle centaur tractor

Testing of diode assembly

The tricky thing about the diode assembly is that we don’t see diodes separately. But there is nothing complicated here, we have a diode bridge diagram to help us. To make it clear we place it near ourselves and start testing. We will check as in the first point of the article. one diode at a time. In a diode assembly each pin is signed, so it is not difficult to find the diode we need.

  • Diode 1: minus assembly. anode, one of the variable leads. cathode;
  • Diode 2: the minus of the assembly is the anode, one of the variable leads is the cathode;
  • Diode 3: alternating lead is the anode, plus assembly is the cathode;
  • Diode 4: AC lead. anode, plus assembly. cathode.

Knowing the pin designations, check each diode in two directions. If any of these have a breakdown or breakage, the whole diode assembly has to be replaced. The pictures are for illustration:

Checking diodes 1 and 2 in direct connection:

Checking diodes 1 and 2 when turned on in reverse:

How to Make a Blinking LED?

Checking diodes 3 and 4 when directly on:

Checking Diodes 3 and 4 in reverse connection:

If you still don’t understand something, you might want to see a video on checking your diode bridge.

Principle of an oscillator and a diode

The generator is a device that serves to convert the mechanical energy supplied by the engine into electrical energy. When it malfunctions, the following malfunctions occur:

  • headlight glow becomes dim;
  • the battery charge signal lights up continuously;
  • an unusual sound is heard in the area where the generator is located;
  • The analog voltmeter pointer goes into the red zone.

Since the generator is directly connected to the charging circuit, the primary symptoms of rectifier malfunction will be observed as overcharging or complete lack of charge of the battery. If these problems appear, the alternator needs to be checked.

The alternator outputs a sinusoidal signal, but the appliances in the vehicle require constant voltage. A rectifier unit is built into the alternator to form a constant signal. It consists of six power diodes.

A diode is a semiconductor element with a pair of leads, the electrodes. Its principle of operation is based on the ability to carry current in one direction. If positive polarity is applied to p area of the device and negative polarity to n area, it is called direct connection. If polarity is reversed, it is reversed.

In forward connection the resistance of p-n junction is negligible and does not affect the passage of electric current, while in reverse connection it practically does not pass through.

Main malfunctions

It is diodes most often turn out to be the main cause of malfunctioning of the alternator. It is necessary to check this device after the appearance of some signs. The following are considered to be the main ones:

How to check Alternator Diode bridge working perfectly or not

  • The alternator cannot produce voltages over 13.5 volts.
  • After starting the power plant, the battery warning light on the dashboard lights up.
  • The analog voltmeter hand moves into the red zone.
  • Battery light is not lit before or after starting the engine.

But we should remember that similar signs are in a faulty voltage regulator, and it is this device that we should check first of all. The question how to check the diode bridge with multimeter can also arise after the appearance of other symptoms, for example, when using the receiver or CD.The sound is distorted if the machine’s power plant is running.

See also  How to check the electromagnetic clutch of air conditioner compressor

Since the car has two sources of direct current to ensure the operation of the onboard electrical circuit. the battery and the generator, the failure of the diode bridge will certainly affect its performance.

There can be many reasons for this, but the most common are

  • Dampness got on the board due to leakage of the alternator body.
  • Dirt or dust mixed with oil has penetrated and shorted the bridge.
  • Battery contacts are polarized.

The rectifier unit (another name of the device) is an integral part of the generator and serves to modify the alternating current produced by the unit into direct current, needed to charge the battery and power all electrical appliances of the car.

It is located on the oscillator frame and consists of 4, 6, 9 or more diodes placed on two insulated aluminum or steel horseshoe-shaped plates which act as cooling radiators.

The main components of the bridge are semiconductor diodes (most often silicon diodes), conducting the current in the necessary direction. They are the rectifiers that convert alternating current into direct current. The capacitor, which mitigates the inevitable surges, is the battery.

check, diode, bridge, alternator

Instead of diodes any types of valves can be used in the circuit. for example, selenium poles, the principle of the circuit will not change from this

How to check without disassembling

It is possible to diagnose on site without disassembling the alternator and without unsoldering the parts. To do this, unscrew all the wires on the alternator and voltage regulator, set the tester to ohmmeter mode, connect the lamp to the battery as described above.

This method allows the whole bridge and individual groups to be checked for faultless operation.

Short circuit

To do this test, the plus side of the electrode is attached to generator output 30 (“plus” of the power rectifier), and the minus side is attached to the housing. Resistance value equal to 1 indicates the operability of the bridge.

check, diode, bridge, alternator

Connect one end of the lamp to the alternator minus terminal and the other end to the plus. If lamp is on, this indicates a short circuit.

Negative group

Connect the minus electrode of the tester to the alternator carcass, the plus electrode. to one of the screws, securing the diode bridge. Negative pole is intact if its resistance is infinity.

To test it with a lamp, its minus should be connected to the cover of the generator, plus. to the axle mounting bolt. Lamp illuminates or flashes if negative band is faulty.


Connect the plus probe of the multimeter to the plus terminal, and the minus one. to the bridge fixing bolt. When the group is in good condition, the resistance is infinite.

Connect the minus of the lamp to the mounting bolt of the bridge, the plus is applied to the terminal of the plus. Illuminated lamp indicates a short circuit in positive semiconductors.

Related Posts

| Denial of responsibility | Contacts |RSS