Ignition System: Description and Operation
Transistorized Ignition SystemsThe principal difference between the previous ignition system, i.e. the standard system found on earlier BMW's, and the electronic system on current models, is the ignition primary circuit. Instead of points and a condenser we use an impulse generator and electronic switching device.
The Primary Circuit consists of the battery, the ignition switch, the ballast resistors and/or starter the primary winding in the coil, an impulse generator and an electronic control unit.
Some BMW's do not have a ballast resistor in the primary circuit. A circuit in the control unit has been designed to replace the function of the ballast resistor.
The control unit receives signals from the impulse generator. These signals are processed by the control unit to switch current flow in the primary circuit on and off.
The impulse generator in the distributor consists of a coil surrounded by a stator and a permanent magnet, and a rotor attached to the distributor's shaft.
The permanent magnet creates a magnetic field which goes through the stator. The magnetic circuit is closed when the pole teeth are opposite each other, open when the pole teeth are separated. This means that the armature closes and opens the magnetic field while it is rotating, inducing a varying voltage in the coil. This is the signal received by the control unit.
The control unit then acts like a switch, turning primary current on and off Like a breaker point system, when current flow in the primary winding is interrupted, the magnetic field in the coil collapses, a high voltage is induced in the secondary winding, and the spark plug fires. The secondary circuit operates just like a breaker point ignition system.
But unlike a breaker point system, the control unit can vary the "on-off" time of the primary circuit, resulting in varying dwell. In a point-controlled ignition system, dwell is set to a "happy" medium. In our electronic system, dwell varies with engine speed. For example, it increases at higher engine speeds to allow sufficient time for complete coil saturation. This results in improved operation throughout the entire speed range of the engine.
And of course, with no points to wear out, the system provides much greater reliability.
WARNING: Voltages in electronic ignition systems can be hazardous! Always use caution when servicing these systems.
TSZI-S Transistorized Ignition System
As of 12/84 production, the 318i is equipped with a TSZi-S transistorized ignition system, developed with the following innovations:
Studies have shown that, for optimum starting on a four (4) cylinder engine, the ignition timing angle should be close to top dead center. Therefore a special starting function has been integrated into the TSZi-S control unit to ensure good starting behavior over the entire temperature range.
The operation of the circuit is shown graphically here where:
1. Track of ignition timing curve with starting function (broken line)
2. Normal timing curve (solid line)
Starting is activated via terminal 50 as long as the starter is operated. As soon as the engine has started and the ignition lock goes back to terminal 15, the timing angle is moved into the area of normal engine idle timing. Above idle speed normal centrifugal and vacuum timing takes place.
The control unit processes the signal of terminal 1 into a square wave as opposed to the previous primary circuit spike.
The TD signal is used for:
rpm - tachometer service indicator, instruments
rpm - switch
idle - control
L - Jetronic fuel shut-off (overrun)
The TD connection has been introduced on the TSZi-S system for interference suppression. The square-wave signal shown here is easier to process electronically and emits no interference as opposed to the signals of terminal 1.
In the current version terminal 1 only connects the TSZi-S control unit with the ignition coil and the diagnosis plug. L-Jetronic control units with TD signal cannot be replaced by standard units which are designed to use the spike type of signal of coil terminal 1.