System Operation - Ignition

SYSTEM OPERATION 

Courtesy of CHRYSLER GROUP, LLC

The Powertrain Control Module (PCM)

does not 

directly control energizing the ignition coils. The Ignition System Control Module (ISCM) is used for direct control and diagnostics of the ignition system.

The operation of the Ignition Coils occurs as follows: 

1. The PCM sends command signals to the ISCM for ignition coil on/off control.
2. The ISCM provides the supply voltage and replicates the command signals from the PCM to the ignition coils through the coil control circuits.
3. The ISCM controls (charge interruption) to fire each ignition coil.
4. The ISCM monitors the secondary circuit current and ionization of the coils over the ionization signal circuits (secondary signal feedback).
   NOTE:

   The Crankshaft Position Sensor (CKP) signal is hard wired to the PCM. The ISCM receives a replicated CKP Signal from the PCM on a dedicated hard wired circuit.

Courtesy of CHRYSLER GROUP, LLC

The on time (dwell) for each ignition coil is typically about 3.0 ms (point between callout #2 and #3). The ISCM decides based on engine conditions, how much dwell time is needed. The ISCM may delay the timing of the when the dwell period starts by delaying when the control driver for the given channel is turned on (2). The ISCM makes the determination based on several inputs received over the private CAN bus from the PCM. These inputs include:

• Engine status - Off_On_Crank_Shutdown
• Engine load percentage
• Engine coolant temperature sensor signal
• All intake air temperature sensor signals
• The PCM also sends a replicated buffered CKP signal to the ISCM on a dedicated circuit. 

At the end of the dwell window, upon the falling edge of the PCM command signal for each coil (2), the ISCM will simultaneously turn off the control driver for the given ignition coil allowing the ignition coil to fire.

Courtesy of CHRYSLER GROUP, LLC

The ISCM monitors the ignition secondary circuit through the ion signal  circuit. After firing each coil, the ISCM uses ion current sensing across the spark plug gap to analyze several aspects of each combustion event. The ISCM can establish whether there was spark across the electrode of the spark plug and whether there was combustion in the cylinder. This allows the ISCM to detect misfire detection, knock detection, Air Fuel Ratio cylinder balancing/correction, pre-ignition, and spark plug fouling.

The ISCM reports faults that are detected in the control circuit, ion signal circuit and misfire detection to the PCM over the private CAN bus. The PCM performs diagnostics on the command signals to the ISCM. All DTCs are stored and reported to the scan tool through the PCM.

There is also a Pulse Width Modulated (PWM) Liveness Signal circuit that is connected between the ISCM and the PCM. The Liveness Signal is used for backup communication in the case of a loss of communication over the Private Engine CAN bus. The Liveness Signal typically has a 10ms pulse width. In the case of a loss of power supply to the Ignition Coils on either bank, the Liveness signal is set to a 20ms pulse width in order to communicate the Ignition Coil Supply Fuse failure as quickly as possible to the PCM.