System Description

The Marelli Digital Ignition System is a fully digital microprocessor controlled system comprising:

An Electronic Control Unit (ECU).
A Distributor.
Two HT coils (A and B banks).
Two amplifier units (A and B banks).
A Crankshaft (TDC) sensor.
A Flywheel sensor.
A Coolant temperature sensor.
An Air temperature switch.
A Throttle idle switch.
One 6K8 resistor.
An Ignition map select link.

The microprocessor (located within the Electronic Control Unit) controls the timing of the low tension voltage, via the two amplifier units located on the front cross member, to two ignition coils which are located between the two cylinder banks. The coils in turn supply the high tension voltage via the distributor (located just in front of the ignition coils) to the sparking plugs.

The upper coil supplies 'B' bank and the lower coil supplies 'A' bank.

The ECU (located in the front passenger footwell) controls the ignition timing according to the inputs from sensors mounted on the engine.

The control parameters fall into two groups; Primary, consisting of intake manifold vacuum and engine speed. Secondary, consisting of throttle position, engine coolant temperature and intake air temperature.


Engine Starting

The ignition timing during cranking and start up is dependant on engine speed and engine coolant temperature. The engine speed is sensed by an inductive pick up sensor (located at the rear of the engine), the speed signal is obtained by the rotation of the starter gear ring, with segments inducing a signal frequency proportional to the engine speed.

The engine coolant temperature is sensed by a sensor located on the front of 'A' bank engine coolant rail. The timing information is stored in memory and permits an early establishment of a good idling condition.


Closed Throttle Running

The ignition timing for closed throttle running, idling and overrun conditions is programmed independently according to engine speed and engine coolant temperature, the timing information being stored in memory. This function is controlled by a throttle mounted switch located on the end of the 'B' bank bell-crank lever rod.


Open Throttle Running

The ignition timing for open throttle running is controlled primarily by intake manifold pressure and engine speed. The information for the ignition requirements is stored in a pre-programmed memory, so that for any given manifold pressure and engine speed the memory will generate a number which relates to the timing required. The timing thus computed is then subject to adjustment dependent on prevailing engine coolant and inlet air temperature.

The inlet air temperature is determined by a sensor located on the back plate of the 'A' bank air cleaner. Intake manifold pressure is sensed by an absolute pressure aneroid capsule located in the ECU and ported to the intake manifold. The crankshaft position is determined by an inductive pick up sensor sensing the position of a three toothed wheel mounted at the front of the crankshaft.