9000 L4-2290cc 2.3L DOHC Turbo EFI (1992)
The DI/APC control module has to decide whether cylinder 1 or 4 fires when the crankshaft position sensor indicates that cylinders 1 and 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 present 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).
The ionization current is measured in pairs of cylinders, 1+2 and 3+4. If combustion occurs in cylinder 1 or cylinder 2, the ignition discharge module
sends a 5 V pulse to pin 25 of the DI/APC control module. Similarly, the ignition discharge module sends a 5 V pulse to pin 5 of the control module if
combustion occurs in cylinder 3 or cylinder 4.
If the crankshaft position sensor indicates that cylinders 1 and 4 are at top dead centre and a 5 V pulse is applied to pin 25 of the control module at the
same time, the control module knows that cylinder 1 has fired.
On starting, the control module does not know whether cylinder 1 or cylinder4 is under compression and ignition consequently takes place in both
cylinders. Ignition in cylinders 2 and 3 occurs in the same way. As soon as combustion signals are received on control module pins 25 and 5, ignition
and fuel injection are both synchronized with the engine's firing order.
In the event of an open circuit in any of the leads connected to control module pins 25 and 5, ignition timing will not be synchronized. knock control
will take place in parallel on cylinders 1+4 and 2+3.
Spark plug burn-off
M90 and earlier
If the ignition switch is allowed to return to the ON position after an attempt to start has been made, spark plug burn-off will take place. The DI/
APC control module grounds all trigger leads 210 times a second for 5 seconds. Impurities on the electrodes of each spark plug are then burnt off
with more than 1000 sparks.
M91 and later
When the ignition has been switched oft and the engine has stopped, the main relay remains operated for an additional 6s This when the control
module activates the burn off function.
