Semiconductor ignition system (TCI)
This system has four main components: an impulse sensor, an ignition control unit, an ignition coil and spark plugs. The impulse sensor generates a signal for the ignition timing for the ignition system. Equivalent to a cam breaker in a standard ignition distributor, the pulse sensor generates an AC voltage signal whenever the rotor lip passes the sensor lip. When the ignition control (capacitive discharge node) receives this voltage signal, it creates a spark by interrupting the primary circuit.
Closing time (duration of the ignition coil charge) installed in the ignition control unit based on the most intense sparking.
Air gap (distance between rotor lobes and sensor lobes) can be adjusted (see Section Checking and adjusting the air gap (TCI system)).
Ignition timing is mechanically adjustable (see Section Function test and ignition timing adjustment (TCI system)). A centrifugal governor, consisting of spring-loaded rotating weights, advances the ignition as the engine speed increases. The vacuum regulator changes the ignition timing to compensate for changes in engine load.
Motronic ignition system
This system, also known as Digital Engine Electronics (DME), combines in one central unit or ECU (computer) all ignition and fuel injection control functions. Ignition timing is determined based on engine load, engine speed, coolant temperature, and intake air temperature inputs to the ECU. The sole function of the distributor is to supply high voltage signals to the spark plugs. The distributor is mounted directly on the cylinder block. These systems do not have mechanical ignition timing devices.
The ignition timing is set electronically and is not adjustable in Motronic systems. During engine start, crankshaft position sensor (position sensor) transmits a signal to the ECU and thereby determines the starting point for the ignition timing. While the engine is running, the ignition timing changes continuously according to various inputs to the ECU. Engine RPM is measured by a RPM sensor. In early Motronic systems, the position sensor and RPM sensor are located on the clutch housing above the flywheel on the left side. Late Motronic systems have a single sensor (pulse encoder), located above the crankshaft pulley. This sensor works simultaneously as a speed and position sensor.
On the issue of checking and replacing sensors of the ignition system, 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 belongs to the BMW test unit and is not part of the Motronic ignition system.
Precautions
When working with a solid-state ignition system, certain precautions must be observed:
- Do not disconnect the battery wires while the engine is running.
- Make sure the ignition control unit (TCI ignition system) always securely grounded (see Section Checking and replacing the pulse sensor and the ignition control unit (TCI system)).
- Keep water away from the distributor and high-voltage wires.
- If you want to connect a tachometer to the engine, always connect the positive wire (+) tachometer to the negative terminal (-) ignition coils and never to the distributor.
- Do not allow the contacts of the ignition coil to be connected to the housing, as the pulse sensor or coil may be damaged.
- Do not leave the ignition on for more than ten minutes with the engine off, otherwise the engine will not start.