Fiat 1998 Marea Weekend, Service Manual

Page: 224 / 846

Engine
Fuel system
Marea- Marea Weekend
99 range
10.
MANAGEMENT OF THE IGNITION
The management of the ignition mainly consists of determining the desired ignition advance according to
the engine operating conditions and implementing it by operating the power transistor, located inside the
control unit.
The "basic" advance value, calculated according to the engine load and speed conditions, is then cor-
rected on the basis of the different engine operating conditions.
The primary winding for each coil is supplied by the battery voltage via the relay and is connected to the
power transistor manifold incorporated in the engine control unit whose emitter is connected to earth,
whilst the base receives the operating voltage from the control unit.
According to the engine rotation speed and the ignition advance to be implemented, the engine control
unit establishes the moment for the start of the conduction for the primary winding so that the desired
current intensity (saturation) is reached in the primary winding just before this current is cut off.
This moment obviously varies in terms of the angle in relation to TDC for the explosion stroke of each
cylinder and the greater the engine rotation speed, the greater the advance, since the time required to
saturate the current in the coil primary winding is virtually constant: it is determined using appropriate co-
efficients stored in the memory during the mapping stage (dwell management).
The moment for the start of conduction is also corrected according to the battery voltage.
The engine control unit therefore determines the moment the current in the primary winding is cut off,
transforming the advance degrees into the time needd for the engine to travel through this angle: this time
constitutes the advance in relation to TDC for the explosion stroke with which the current at the base of
the transistor is interrupted.
The moment at which the current at the base of the power transistor is interrupted, the connection to earth
of the primary winding is interrupted and, as a result, the high tension is discharged at the secondary
winding.
1 0 B 1 9 B
A
J T J ~ U ~ L _ T L 3
5
JxnP
5
IT© @
28B
3 8 B
1. Engine rpm and TDC sensor and flywheel
2. Control unit input signal
3. Signal converted in control unit (square wave)
4. Internal power module
5. Coil
6. Spark plugs
10
Print n° 506.763/14