Saab 9000 Service Manual page 40

Technical Description 37
Ignition discharge module (contd.)
346
589
..-----------«
CYL 1
CYL 1 CYL2
r----t--:::.i.-i--___.;._~
17--===
~
M o
~
~-•
--17
I
I
-=- - - - - - - - - - - - - - - -
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Combustion signals
The Trionic system does not have a camshaft posi-
tion sensor. A camshaft position sensor is normally
required for sequential knock control and fuel injec-
tion.
The Saab Trionic must decide whether cyl. 1 or ~yl.
4 is firing when the crankshaft position sensor indi-
cates that cyl. 1 and cyl. 4 are at top dead centre.
This is done as follows:
One of the poles of the secondary windings of the 4
ignition coils is connected in the usual manner to the
spark plugs. The other pole is not connected directly
to ground but to 80 V. As a result, 80 V is always pres-
. ent across the spark plug electrodes except at the
very moment a spark is produced.
When combustion takes place, the temperature in
the combustion chamber is very high. The gases are
ionized and start to conduct current. This means that
a current passes across the spark plug gap (without
producing a spark).
+30
230
Saab 9000
CYL4 CYL4
CYL 3 CYL 3
JULJULa
346
589
a---------- ....... --.
I
µp
229
u-D-µp
G7P
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