Fiat 1996 Brava Service Manual page 422

Engine
Fuel system
10.
Bravo-Brava ^
'98 range
The leading edge of the signal (graph B-figure on preceding page) thus represents the moment when
alternative conduction of the primary windings "could" occur. This point is established solely by the
control unit's power module. The conduction time required by the H.T. coils for storing energy is also
defined by the DWELL management strategy. It depends on the time taken by the current in the primary
circuit of the coil to reach 6A, on the battery voltage and the calculation algorithm carried out by the mi­
croprocessor which uses coefficients stored in an appropriate memory; all this naturally depends on the
engine rpm.
With reference to the figure on the preceding page, the falling edge of the signal, (B) end-of-conduction
point (or fall to zero of the current), is instead a "categoric order" to stop the current circulating in the primary
winding, and it represents the point of ignition advance (a) calculated by the computer (the advance - (a) -
varies depending on the rpm).
The control unit thus manages both the advance of the spark to the various cylinders in relation to top
dead centre, and the conduction time required by the coil for storing energy, alternately controlling the
two power stages which permit current to circulate in the primary windings (pins 59 and 66) of the coils
(6) for sufficient time to guarantee a nominal 6 A.
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66
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Functional diagram of the ignition
1. RPM and TDC sensor.
2. Phonic wheel.
3. Signal picture taken from phonic wheel
(60-2) teeth.
The TDCs correspond to teeth 20 and 50.
4. Succession of rectangular wave form sig­
nals of constant amplitude.
5. Ignition power module (inside the control
unit).
6. Ignition coils.
7. Spark plugs
NOTE The numbers in squares indicate the corresponding control unit pins.
In the instant when the control unit stops driving one of the two power stages, the passage of current is
interrupted. This causes, by induction, an increase in voltage (up to 30kV without load) on the sec­
ondary winding.
10
Publication no. 506.670/14