Solved: car charging system

The automobile charging system provides electrical power for operation of the vehicle's ignition system, starting system and all electrical accessories. The battery serves as an electrical surge or storage tank, storing (in chemical form) the energy originally produced by the engine driven generator. The system also provides a means of regulating output to protect the battery from being overcharged and to avoid excessive voltage to the accessories.

The storage battery is a chemical device incorporating parallel lead plates in a tank containing a sulfuric acid/water solution. Adjacent plates are slightly dissimilar, and the chemical reaction of the two dissimilar plates produces electrical energy when the battery is connected to a load such as the starter motor. The chemical reaction is reversible, so that when the alternator is producing a voltage (electrical pressure) greater than that produced by the battery, electricity is forced into the battery, and the battery is returned to its fully charged state.

Newer automobiles use alternating current alternators, because they are more efficient, can be rotated at higher speeds, and have fewer brush problems. In an alternator, the field usually rotates while all the current produced passes only through the stator winding. The brushes bear against continuous slip rings. This causes the current produced to periodically reverse the direction of its flow. Diodes (electrical one way valves) block the flow of current from traveling in the wrong direction. A series of diodes is wired together to permit the alternating flow of the stator to be rectified back to 12 volts DC for use by the vehicle's electrical system.

The voltage regulating function is performed by a regulator. The regulator is often built in to the alternator; this system is termed an integrated or internal regulator.

An alternator differs from a DC shunt generator in that the armature is stationary, and is called the stator, while the field rotates and is called the rotor. The higher current values in the alternator's stator are conducted to the external circuit through fixed leads and connections, rather than through a rotating commutator and brushes as in a DC generator. This eliminates a major point of maintenance.

The rotor assembly is supported in the drive end frame by a ball bearing and at the other end by a roller bearing. These bearings are lubricated during assembly and require no maintenance. There are six diodes in the end frame assembly. These diodes are electrical check valves that also change the alternating current developed within the stator windings to a Direct Current (DC) at the output (BAT) terminal. Three of these diodes are negative and are mounted flush with the end frame while the other three are positive and are mounted into a strip called a heat sink. The positive diodes are easily identified as the ones within small cavities or depressions.

The alternator charging system is a negative (-) ground system which consists of an alternator, a regulator, a charge indicator, a storage battery and wiring connecting the components, and fuse link wire.

The alternator is belt-driven from the engine. Energy is supplied from the alternator/regulator system to the rotating field through two brushes to two slip-rings. The slip-rings are mounted on the rotor shaft and are connected to the field coil. This energy supplied to the rotating field from the battery is called excitation current and is used to initially energize the field to begin the generation of electricity. Once the alternator starts to generate electricity, the excitation current comes from its own output rather than the battery.

The alternator produces power in the form of alternating current. The alternating current is rectified by 6 diodes into direct current. The direct current is used to charge the battery and power the rest of the electrical system.

When the ignition key is turned ON , current flows from the battery, through the charging system indicator light on the instrument panel, to the voltage regulator, and to the alternator. Since the alternator is not producing any current, the alternator warning light comes on. When the engine is started, the alternator begins to produce current and turns the alternator light off. As the alternator turns and produces current, the current is divided in two ways: part to the battery (to charge the battery and power the electrical components of the vehicle), and part is returned to the alternator (to enable it to increase its output). In this situation, the alternator is receiving current from the battery and from itself. A voltage regulator is wired into the current supply to the alternator to prevent it from receiving too much current which would cause it to put out too much current. Conversely, if the voltage regulator does not allow the alternator to receive enough current, the battery will not be fully charged and will eventually go dead.

The battery is connected to the alternator at all times, whether the ignition key is turned ON or not. If the battery were shorted to ground, the alternator would also be shorted. This would damage the alternator. To prevent this, a fuse link is installed in the wiring between the battery and the alternator. If the battery is shorted, the fuse link melts, protecting the alternator.

An alternator is better that a conventional, DC shunt generator because it is lighter and more compact, because it is designed to supply the battery and accessory circuits through a wide range of engine speeds, and because it eliminates the necessary maintenance of replacing brushes and servicing commutators.

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The procedure is as follows :----

Alternator

TESTING

Fig. The A4 and Passat alternators are similar to this BOSCH unit

Fig. Remove the connections from the rear of the alternator

Voltage Drop Test

NOTE

These tests will show the amount of voltage drop across the alternator output wire from the alternator output ( B+ ) terminal to the battery positive post. They will also show the amount of voltage drop from the ground ( - ) terminal on the alternator.

A voltmeter with a 0-18-volt DC scale should be used for these tests. By repositioning the voltmeter test leads, the point of high resistance (voltage drop) can easily be found. Test points on the alternator can be reached by either removing the air cleaner housing or below by raising the vehicle.

Before starting the test, make sure the battery is in good condition and is fully charged. Check the conditions of the battery cables.

Start the engine, let it warm up to normal operating temperatures, then turn the engine OFF .

Connect an engine tachometer, following the manufacturer's directions.

Make sure the parking brake is fully engaged.

Start the engine, then place the blower on HIGH, and turn on the high beam headlamps and interior lamps.

Bring the engine speed up to 2,400 rpm and hold it there.

To test the ground ( - ) circuitry, perform the following:

Touch the negative lead of the voltmeter directly to the positive battery terminal.

Touch the positive lead of the voltmeter to the B+ output terminal stud on the alternator (NOT the terminal mounting nut). The voltage should be no higher than 0.6 volts. If the voltage is higher than 0.6 volts, touch the test lead to the terminal mounting stud nut, and then to the wiring connector. If the voltage is now below 0.6 volts, look for dirty, loose or poor connections at this point. A voltage drop test may be performed at each ground ( - ) connection in the circuit to locate the excessive resistance.

To test the positive ( + ) circuitry, perform the following:

Touch the positive lead of the voltmeter directly to the negative battery terminal.

Touch the negative lead of the voltmeter to the ground terminal stud on the alternator case (NOT the terminal mounting nut). The voltage should be no higher than 0.3 volts. If the voltage is higher than 0.3 volts, touch the test lead to the terminal mounting stud nut, and then to the wiring connector. If the voltage is now below 0.3 volts, look for dirty, loose or poor connections at this point. A voltage drop test may be performed at each positive ( + ) connection in the circuit to locate the excessive resistance.

This test can also be performed between the alternator case and the engine. If the test voltage is higher than 0.3 volts, check for corrosion at the alternator mounting points or loose alternator mounting.

Output Voltage Test

Fig. Typical charging system output voltage test connections

Fig. VAT-40 charging system tester. Similar testers are available that perform as well

Most automotive repair facilities purchase specialized test equipment to check a vehicle's charging system. The test equipment is expensive, however it is fast and easy to use and able to pinpoint the cause of the problem. This type of test equipment is a good investment for a repair shop, however an alternator can be checked with the use of a reasonably priced volt-ohm (VOM) meter. For additional information, please refer to the following topic(s): General Information and Maintenance, Tools and Equipment, Chassis Electrical.

A volt/amp tester, such as the VAT-40, or an equivalent, which is equipped with a battery load control (carbon pile rheostat), full field tester and an inductive-type pickup clamp (ammeter probe) can be used to test an alternator's diodes, amperage and voltage output following the directions supplied with the tester.

This information is useful for someone who may take it upon himself or herself to open up an alternator and replace the defective internal part. Almost all alternators used in today's vehicles have internal voltage regulators. Thus when replacing the alternator the voltage regulator no longer must be purchased separately. Because the voltage regulators have no moving parts and are electronically controlled, the only wear related item found in an alternator would be the brushes. More often than not, the alternator brushes are part of the voltage regulator, so in order to replace the brushes, the voltage regulator must be replaced.

The only drawback is that at times the cost of the voltage regulator/brush assembly may be almost as expensive as the alternator itself. Therefore it makes more sense to replace the alternator assembly rather than just one failed component. One common symptom of a failed voltage regulator/brush assembly is that the battery warning light does not operate when the ignition switch is first turned to the ONposition without the engine being started. The battery must be disconnected and the alternator removed to replace the voltage regulator.

To test the voltage output of the alternator, use a VOM meter capable or reading at least 20 volts DC and proceed as follows:

Before starting the test, make sure the battery is in good condition and is fully charged. Check the conditions of the battery cables.

Perform the voltage drop test to ensure clean and tight alternator/battery electrical connections.

Be sure the alternator drive belt is properly tensioned. For additional information on drive belt tension, please refer to the following topic(s): General Information and Maintenance.

Set the parking brake, and then place the transmission in Park or Neutral.

Start the engine and let it run until it reaches normal operating temperature.

Connect the VOM meter to the battery following the proper polarity for the leads. Connect the positive lead to the positive terminal of the battery and attach the negative lead of the VOM meter to the negative terminal of the battery or a known good ground on the vehicle.

Raise the engine speed to 1,500 rpm and check the voltage. The voltage should be 13.5-14.2 Volts. If the voltage is not within specification replace the alternator.

NOTE

If the voltage exceeds 14.2 Volts the battery could be damaged. Higher than recommended charging system voltages can warp the battery casing, boil the fluid out of the battery, and warp the internal plates. If battery damage has occurred, neutralize the battery mounting area with a ⁵⁰ / ₅₀ solution of baking soda and water, then replace the battery.

REMOVAL & INSTALLATION

Before purchasing a replacement alternator, read the specification plate on the housing. The number "14V" will appear to indicate maximum voltage rating. On the same line will be 2 more digits followed by the letter "A". This is the maximum amperage output. Be sure to purchase an alternator with the same rating. If the back of the alternator is readily accessible, the voltage regulator can be replaced without removing the alternator.

Note the radio security code and disconnect the negative battery cable.

If additional work space is needed, on A4 and 1998-00 Passat models, place the lock carrier in the Service Position. For additional information, please refer to the following topic(s): Body and Trim, Lock Carrier, Service Position.

Disconnect the wiring from the alternator. If necessary, mark the wires with tape or other means to ensure they are connected properly upon installation.

If equipped, remove the cooling ducts and shroud as necessary.

Loosen the belt tension, and remove the drive belt from the alternator. For additional information, please refer to the following topic(s): General Information and Maintenance, Belts.

If necessary, remove the belt tensioner.

If equipped, remove the alternator adjustment bolt.

Remove the mounting/pivot bolt(s).

Carefully lift the alternator from the bracket.

To install:

Hold the alternator in position on the mounting bracket, and install the mounting/pivot bolt(s).

If equipped, install (but Do NOT tighten) the alternator adjustment bolt.

Place the alternator drive belt on the pulley.

Adjust belt tension and tighten the mounting and adjustment bolts as necessary. For additional information, please refer to the following topic(s): General Information and Maintenance, Belts