Fiat 1996 Brava Service Manual page 261

Engine
Fuel system
Bravo-Brava igi i
6v
98 range
10.
M A N A G E M E N T OF THE I G N I T I O N
The management of the ignition basically consists of determining the desired ignition advance accord­
ing to the engine operating conditions and its implementation through the operation of the power tran­
sistor located inside the control unit.
The "basic" advance value, calculated on the basis of the engine load and speed conditions, is then cor­
rected according to the different engine operating conditions.
The primary w i n d i n g for each coil receives the battery voltage via the relay and is connected to the p o w ­
er transistor incorporated in the engine control unit, whose transmitter is connected to earth, whilst 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 condution of the primary winding, in such a way as to
reach the desired current intensity (saturation) in the primary w i n d i n g immediately before this current is
interrupted.
This moment obviously varies in terms of the angle in relation to TDC for the explosion stroke for each
cylinder and the greater the advance, the greater the engine rotation speed because the time required to
saturate the current in the coil primary w i n d i n g is more or less constant: it is determined using suitable
coefficient stored in the memory (dwell management).
The moement for the start of conduction is also corrected on the basis of the battery voltage.
The engine control unit therefore determines the moment the current in the primary coil is interrupted
transforming the advance degrees into the time required by the engine to cover this angle: this time c o n ­
stitutes the advance in relation to TDC for the explosion stroke w i t h w h i c h the current is interrupted at
the base of the transistor.
The moment the current at the base of the power transistor is interrupted, the connection to earth for the
primary coil is interrupted and consequently there is a high tension discharge at the secondary w i n d i n g .
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1. Engine rpm and TDC sensor and flywheel
2. Control unit input signal
3: Signal converter in the control unit (square
wave)
4. Internal power module
5. Coil
6. Spark plugs
10
Print no. 506.670/16