Ignition Control Module: Description and Operation
Ignition Discharge ModuleIgnition timing
The ignition discharge module is mounted on the camshaft cover over the spark plugs. There are 4 ignition coils in the ignition discharge module, the secondary windings of which are connected directly to the spark plugs.
The ignition discharge module is supplied with battery positive voltage (B+) from the main relay and connected to ground at grounding point G7.
When the main relay operates, battery positive voltage (B+) is supplied to the ignition discharge module which convents 12 V to 400 V d.c. and stores it in the capacitor. The 400 V current is connected to one of the poles of the 4 ignition coils' primary windings.
Connected to the ignition discharge module are 4 trigger leads from DI/APC control module pins 38 (cyl. 1), 19 (cyl. 2), 37 (cyl. 3) and 18 (cyl. 4).
When the control module grounds pin 38, the other pole of the primary winding of the ignition coil for cylinder 1 is connected to ground (via the ignition discharge module's battery positive (B+) input) and 400 V is stepped up to a maximum of 40 000 V in the ignition coil for cylinder 1. Ignition for cylinders 2, 3 and 4 takes place in the same way.
If there is an open circuit in the power supply or a ground connection to the ignition discharge module, the engine will not start.
Ignition control
On starting, ignition timing is set at 10 degrees BTDC. To facilitate starting before engine speed reaches 150 rpm or when it drops below 420 rpm for more than 2 5, the control module will ground the relevant trigger lead 210 times a second from 1O°BTDC to 2O°ATDC, bringing the "multispark" function into play. This function ceases when engine speed exceeds 850 rpm.
In the case of a closed throttle, a special ignition control curve is used. Normal ignition advance is about 6-8.
As the throttle opens, ignition control changes over to normal control depending on load and engine speed
Combustion signals
The DI/APC system has no camshaft position sensor. A camshaft position sensor is normally essential for sequential knock control.
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.