900 S Coupe L4-1985cc 2.0L DOHC Turbo EFI (1998)
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 cylinders1 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 position, 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 synchronized 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 engine 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.
