Saab 9000 Service Manual page 39

36 Technical Description
Ignition discharge module (contd.)
BTDC BTDC
10(!)
20~~(!)
Ignition control
On starting, ignition timing is 10° BTDC. To facilitate
starting when the coolant temperature is below 0°c,
the electronic control module will ground the relevant
trigger lead 21 0 times a second from 10° BTDC to
20° ATDC, which gives rise to the "multispark" func-
tion. This function is discontinued at engine speeds
above 850 rpm.
A special ignition control curve is used at idling
speed. Normal ignition advance is about 0-8°, de-
pending on the engine version.
If engine speed drops, as may happen when the ra-
diator fan cuts in, the ignition timing will be advanced
to a maximum of 20° BTDC to increase engine
torque and restore engine revs. Similarly, the ignition
will be retarded if engine speed increases.
Ignition control during idling compensates for rapid
changes in engine idling speed.
When the throttle position moves away from idling,
ignition control changes over to normal control de-
pending on load and engine speed.
Saab 9000
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