General information (Audi A4 B7)

Table of contents: Engine starting system ↓ Charging system ↓

The electrical system of the vehicles covered in this manual operates at 12 V. The ground connection is via the negative terminal of the battery. The battery is located under the cover (1 in the illustrations at the beginning of Chapter 1) in the gutter in front of the windshield.

Engine starting system

The sole purpose of the engine starting system is to ensure that the engine rotates at a speed sufficient to start it.

The starting system consists of a starter, battery, ignition switch, traction relay and connecting electrical wiring.

The starter is a DC electric motor with mixed excitation and an externally mounted electromagnetic traction relay. The starter consists of a housing (stator) with excitation windings, an anchor with an overrunning clutch, a cover with brush holders and a traction relay.

The starter switch is an integral part of the ignition switch and is designed to excite the traction relay installed on the starter. When the starter is turned on, current from the battery begins to flow through the winding of the traction relay. The relay anchor is pulled in and the contacts close. At the same time, the relay anchor, through the drive lever, ensures the extension of the gear with the overrunning clutch, while the clutch hub rotates on the helical splines of the starter anchor shaft and also rotates the gear, which facilitates the engagement of the latter with the flywheel ring gear. Current flows through the closed contacts of the traction relay, feeding the stator and anchor windings, and the starter anchor begins to rotate together with the hub and overrunning clutch.

After the engine is started, the rotation speed of the pinion exceeds the rotation speed of the starter anchor. In this case, the overrunning clutch rotates freely, and the torque is not transmitted from the engine flywheel to the starter anchor shaft. After releasing the ignition key, the power supply circuit of the traction relay windings through the ignition switch is opened, the anchor of the traction relay is pressed back to its original position by the spring, the relay contacts are opened, and the drive pinion disengages from the flywheel ring gear.

The starter with a solenoid relay is located parallel to the engine crankshaft and is secured to the power unit.

When performing any work on the engine starting system, observe the following requirements:

a) to avoid failure of the starter due to overheating, do not turn it on for more than 15 seconds - wait at least 1 minute before turning it on again. Long cranking of a non-starting engine with the starter leads to the accumulation of unburned fuel in the catalytic converter, which is fraught with failure of the latter due to ignition of the fuel during starting;
b) the starter is connected directly to the battery and careless handling may cause a fire due to a short circuit;
c) Always disconnect the negative cable from the battery before servicing the starter.

Charging system

The vehicles in question use an alternator. When installing additional electrical equipment, check that the generator capacity is sufficient to supply new consumers with electricity.

The alternator is driven by a belt from the engine crankshaft. On some models, alternators are equipped with a pulley with a built-in freewheel clutch. Such a pulley allows the alternator anchor to freewheel when the engine slows down sharply, so that a smooth decrease in the charging current occurs. In addition, this reduces changes in the load placed on the auxiliary drive belt.

The generator is a three-phase synchronous electric machine with electromagnetic excitation. To convert alternating current into direct current, a diode rectifier is built into the generator. Output voltage is adjusted by a built-in regulator. The generator has grounding by mass.

When the generator is operating, the electric current flowing through the excitation winding creates a magnetic flux around the rotor poles. When the rotor rotates, its magnetic poles periodically change under each stator tooth. As a result, the magnetic flux passing through the teeth continuously changes in magnitude and voltage. This variable magnetic flux creates an electromotive force (EMF) in the stator winding. The wedge-shaped rotor pole pieces are selected in such a way that it allows for an EMF curve shape close to sinusoidal.

At high rotor speed, when the generator output voltage begins to exceed 13.6÷14.6 V, the voltage regulator is locked, and the current does not pass through the excitation winding. When the voltage decreases, the regulator is unlocked again, providing freedom of current flow through the excitation winding. The higher the rotor speed, the longer the regulator remains locked and, accordingly, the more the voltage at the generator output decreases. The process of locking and unlocking the regulator occurs at a high frequency, so the output oscillations remain almost unnoticeable and the generator voltage can be considered constant.

The generator voltage depends on the operating state, which is transmitted as signals from the engine management system (ECM) via the BSD interface. Therefore, the charging voltage at the battery terminals can be up to 15.5 V, depending on the temperature and charge of the battery (the ECM receives these values from the power module). This improves the charging balance of the battery.

The charging system does not require periodic maintenance, however, the condition of the alternator drive belt, battery and its wiring should be checked and replaced on a regular basis in accordance with the maintenance schedule (see sections 17 and 18 Chapters 1).

When the ignition is turned on, the charge indicator on the instrument cluster should light up briefly (see chapter "Controls and operating techniques"). If the lamp does not turn off after starting the engine or lights up during its operation, check the condition of the charging system components. Failure to activate the lamp when the ignition is turned on indicates that it is out of order, the corresponding electrical wiring is damaged, or the generator is not functioning properly (including a broken drive belt).

When servicing the generator, take the following safety precautions:

do not disconnect the battery or voltage regulator while the engine is running;
do not short-circuit the generator excitation terminal or the cable attached to it to ground;
do not confuse the order of connecting the voltage regulator wiring;
remember that turning on a voltage regulator shorted to ground will cause it to fail immediately;
never remove the alternator while the battery is connected;
do not confuse the polarity of the battery connection;
never use voltage meters or test lamps connected to the household network (110/220 V) when checking on-board electrical equipment;
when checking the condition of the diodes, do not apply a voltage higher than 12 V to them and do not use megohmmeters that also have a high output voltage - a breakdown of the diodes will lead to a short circuit. Remember that when checking the insulation of the wiring with a megohmmeter, it is necessary to disconnect all the wiring from the generator;
when charging the battery without removing it from the car, make sure that both wires are disconnected from it; do not confuse the polarity of the charger connection;
before carrying out any electric welding work on the vehicle, do not forget to disconnect the electrical wiring from the generator and battery;
any checks of circuits and components of the on-board electrical wiring should be carried out with the engine off and the battery disconnected;
remember that reversing the polarity of any connections carries the risk of irreversible failure of the rectifier and generator voltage regulator.