Saab 9000 - Page 41

38 Technical Description
Ignition discharge module (contd.)
346
589
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__________
...
CYL 1
CYL 1 CYL2
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u
u
- 17
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The ionization current is measured in pairs, cylinders
1 +2 and cylinders 3+4. If combustion takes place in
cylinder
1 or
2, the ignition discharge module sends
a pulse (B+) to pin 17 of the Trionic's engine control
module. Similarly, the ignition discharge module
sends a pulse (B+) to pin 18 of
the
engine
control .
module if combustion takes place in cylinder 3 or 4.
If the crankshaft position sensor indicates that cylin-
ders
1
and
4 are at top dead centre and a pulse (B+)
is applied to pin 17 of the engine control module at the
same time, then the engine control module knows
that cylinder 1 has fired.
When the engine is started, the ECM does not know
which of cylinders
1
and
4 is in the compression posi-
tion, and ignition consequently takes place on both
cylinders 1 and 4 simultaneously. In the same way
ignition takes place on cylinders 2 and 3. As soon as
combustion signals arrive on pins 17 and 18 of the
ECM, both ignition and fuel injection are synchro-
nized to the engine firing order.
On M96 and later models (On-Board Diagnostic II),.
combustion signals are also used to detect misfiring.
In the event of an open circuit in either of the leads
connected to pins 17 and 18 of the electronic control
module, no synchronization of ignition timing or fuel
injection will occur. Knock control will take place at
the same time on cylinders 1 +4 and 2+3.
+30
230
Saab 9000
CYL4 .CYL4
CYL3 CYL3
JULJ118
229
G7P
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