General information and precautions (BMW 3 Series E46)

General information

The engine electrical system consists primarily of the starting and battery charging systems. Because the functions of these devices are closely related to the engine, they are described separately from other electrical devices in the vehicle, such as lights, instrument panel, etc. (which are described in Chapter 12). A description of the engine ignition system is given in Chapter 5B.

The car's electrical system is 12-volt, with a negative ground.

All models are equipped with a maintenance-free or low-maintenance battery, which is charged by an alternator with a rectifier. The alternator is driven by a belt from the crankshaft pulley.

The engine starter has a pre-engagement mechanism, which is achieved by means of a built-in electromagnetic traction relay. When starting, the traction relay engages the starter drive pinion with the flywheel ring gear before the starter is energized. Once the engine is running, the overrunning clutch prevents the starter rotor from rotating away from the engine until the drive pinion disengages from the flywheel.

The negative terminal of the battery is connected to the body. To increase the reliability of the electrical circuits, the engine and the electrical assembly unit are connected to the body by additional grounding buses (fig. 1.1. a, b).

Precautionary measures

More details about the design and maintenance of various systems will be given in the relevant paragraphs of this chapter. Although the book provides a description of some repair procedures, the usual method of repair is to replace the faulty element. An owner who does not want to limit himself to simple replacement of elements is recommended to purchase the book "Automobile Electrical & Electronic Systems Manual" ("Electrical and Electronic Equipment of the Automobile") published by Haynes Publishing. A Russian translation of this book is available for sale.

Caution: When working on the engine's electrical system, care must be taken to avoid damaging the semiconductor devices (diodes and transistors) and avoid causing injury to yourself. In addition to the precautions listed in the Safety First section, observe the following rules when working on the electrical system.

Always remove rings, watches, etc. before working on the electrical system. Even with the battery disconnected, a capacitor may discharge through a metal object you have to the ground. This may cause shock or severe burns.

Do not frighten the battery terminals: you can seriously damage components such as the generator, electronic circuits and other elements that include semiconductor devices.

If the engine needs to be started from an external battery, use the terminals specially provided for this purpose to connect to the external battery (see "Starting the engine from an external battery"). You also need to connect an external charger to the battery.



Never disconnect the battery terminals, alternator, any wires or control equipment while the engine is running.

Do not allow the engine to turn the generator when the generator is not connected.

Never "test" the generator by shorting its output to ground.

Never use a hand-held ohmmeter to test electrical circuits.

When working on the electrical system, always disconnect the negative battery terminal from ground.

Before using electric welding equipment on a vehicle, disconnect the battery, alternator and components such as the electronic fuel injection/ignition control unit to protect them from the risk of damage.

If your vehicle is equipped with a car radio with anti-theft protection, please note the following: If the power supply to the car radio is interrupted, the protection system is activated. Even if the power supply is immediately restored, the unit will not operate until the correct protection code is entered. Therefore, if you do not know the protection code, do not disconnect the battery or remove the radio from the vehicle. To avoid surprises, read the section "Anti-theft Audio System" in the Appendix.