General information about the ignition system and precautions (BMW 5 Series E28)

The ignition system includes the ignition switch, battery, distributor, primary (low voltage or LT) and secondary (high voltage or HT) chains, spark plugs, and spark plug wires. Models with a carburetor or L-Jetronic fuel injection system are equipped with a solid-state ignition system (TCI). Models with a Motronic fuel injection system have an ignition system integrated with the Motronic system (Digital Engine Electronics or DME).

Transistor ignition system (TCI)

This system has four main components: a pulse sensor, an ignition control unit, an ignition coil, and spark plugs. The pulse sensor provides the ignition system with a timing signal. Equivalent to the cam switch in a standard distributor, the pulse sensor produces an alternating voltage signal whenever a rotor lobe passes the sensor lobe. When the ignition control unit (capacitive discharge unit) receives this voltage signal, then by interrupting the primary circuit it creates a spark discharge.

Circuit closed state time (ignition coil charge duration) is installed in the ignition control unit based on the calculation of the most intense spark formation.

Air gap (distance between rotor protrusions and sensor protrusions) can be adjusted (see section Checking and adjusting the air gap (tCI system)).

The ignition timing is adjusted mechanically (see section Checking the operation and adjusting the ignition timing (tCI system)). A centrifugal governor, consisting of spring-loaded rotating weights, advances the ignition timing as engine speed increases. A vacuum governor changes the ignition timing to compensate for changes in engine load.

Motronic ignition system

Also known as Digital Engine Electronics (DME), this system combines all ignition and fuel injection functions into one central unit or ECU (computer). Ignition timing is determined based on input signals to the ECU about engine load, engine speed, coolant temperature and intake air temperature. The sole function of the distributor is to supply high-voltage signals to the spark plugs. The distributor is mounted directly to the cylinder block. There are no mechanical ignition advance devices in these systems.

The ignition timing is set electronically and is not adjustable in Motronic systems. During engine start-up, the crankshaft position sensor (position sensor) transmits a signal to the ECU and thus determines the starting point for the ignition timing. During engine operation, the ignition timing is continuously varied according to various input signals to the ECU. The engine speed is measured by a speed sensor. In early Motronic systems, the position sensor and the speed sensor are located on the clutch housing above the flywheel on the left side. In later Motronic systems, a single sensor is installed (pulse sensor), located above the crankshaft pulley. This sensor functions simultaneously as a speed and position sensor.

For questions on checking and replacing ignition system sensors, please refer to Section Checking and replacing ignition sensors (motronic system).

Some models are equipped with a TDC sensor located at the front of the engine. This sensor is part of the BMW Test Service Unit and is not part of the Motronic ignition system.

Precautions

When working with a semiconductor ignition system, certain precautions must be taken:

• Do not disconnect the battery cables while the engine is running.
• Make sure the ignition control unit (tCI ignition system) always reliably grounded (see section Checking and replacing the pulse sensor and ignition control unit (tCI system)).
• Do not allow water to come into contact with the distributor and high-voltage wires.
• If you need to connect a tachometer to the engine, always connect the positive (+) lead of the tachometer to the negative (-) terminal of the ignition coil and never to the distributor.
• Do not allow the ignition coil contacts to be connected to the housing, as this may damage the pulse sensor or the coil.
• Do not leave the ignition on for more than ten minutes with the engine off, otherwise the engine will not start.