Saab 9000 - Page 42

Technical Description 39
Ignition discharge module (contd.)
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Knock control
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230
The Saab Trionic does not have a conventional
knock sensor. Instead the ignition discharge module
analyzes the ionization currents for all the cylinders
and sends signals to pin 44 on the Trionic ECM. This
function is adaptive with respect to upsetting fuel ad-
ditives.
From the combustion signals the engine control
module knows which cylinder has fired and if at the
same time the knocking signal is above a definite lev-
el, the control module records knocking on that cylin-
der. The ignition timing is then retarded 1.5° on that
cylinder.
In the event of persistent knocking, the timing will be
retarded additionally but not more than a maximum
of 12°. If ignition timing is retarded by more than a
certain amount on all cylinders, the quantity of fuel in-
jected will be slightly increased.
229
346
G7P
589
pp
pp
PP
pp
µp
µp
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µp
If knocking occurs when intake manifold pressure is
higher than about 140 kPa, knock control will take
place in a different way, as follows:
first the fuel injection and ignition matrices are
changed and if this does not help then boost pres-
sure will be reduced.
The aim is to achieve good performance even in the
event of knocking.
In the event of a break in the lead connected to pin
44 of the electronic control module, basic boost pres-
sure will be obtained and ignition timing retarded by
12° when the engine load is so high that knocking
could occur.
Saab 9000
I