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GM 3.4L (3400) Electronic Ignition system Description and Operation
•  Camshaft position (CMP) sensor

The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the
camshaft. The CMP sensor does not directly affect the operation of the ignition
system. The CMP sensor information is used by the PCM to determine the position of
the valve train relative to the crankshaft position. By monitoring the CMP and CKP
signals the PCM can accurately time the operation of the fuel injectors. The PCM
supplies the sensor with a 12-volt reference, low reference, and signal circuit.

•  Ignition control module (ICM) and ignition coils

Three dual tower ignition coils are mounted to the ICM, and are serviced individually.
The ICM performs the following functions:

-  The ICM receives and processes the signals from the CKP sensor B.

-  The ICM determines the correct direction of the crankshaft rotation, and cuts spark
and fuel delivery to prevent damage from backfiring if reverse rotation is detected.

-  The ICM determines the correct coil triggering sequence, based on the 7X CKP
signal. This coil sequencing occurs at start-up, and is remembered by the ICM. After
the engine is running, the ICM will continue to trigger the coils in the correct
sequence.

-  The ICM produces and inputs 3X reference signals to the PCM.

-  The ICM contains the coil driver circuits that command the coils to operate.
•  The powertrain control module (PCM)

The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions. Ignition
control (IC) spark timing is the method the PCM uses to control spark advance. To provide optimum driveability
and emissions, the PCM monitors input signals from the following components in calculating ignition spark
timing:

-  The ignition control module (ICM)

-  The throttle position (TP) sensor

-  The engine coolant temperature (ECT) sensor

-  The mass air flow (MAF) sensor

-  The intake air temperature (IAT) sensor

-  The vehicle speed sensor (VSS)

-  The transmission gear position or range information sensors

-  The engine knock sensors (KS)

•  The following describes the PCM to ICM circuits:

-  Low resolution engine speed, 3X reference--PCM input--From the ICM, the PCM uses this signal to calculate
engine RPM and CKP. The PCM also uses the pulses on this circuit to initiate injector operation.

-  Low reference--PCM input--this is a ground circuit for the digital RPM counter inside the PCM, but the wire is
connected to engine ground only through the IC module. This circuit creates a common ground plane and
assures there is no ground drop between the PCM and IC module.

-  IC timing signal --PCM output--ICM controls spark timing while the engine is cranking, this is called bypass
mode. Once the PCM receives 3X reference signals from the ICM, the PCM applies 5 volts to the IC timing
signal circuit allowing the ICM to switch spark advance to PCM control.

-  IC timing control--PCM output--The IC output circuitry of the PCM sends out timing signals to the ICM on this
circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode, the signals are sent
to the ICM to control spark timing.

•  Modes of operation

Anytime the PCM does not apply 5 volts to the IC timing signal circuit, the ICM controls ignition by triggering
each coil in the proper sequence at a pre-calibrated timing advance. This is called Bypass Mode ignition used
during cranking or running below a certain RPM, or during a default mode due to a system failure.  

When the PCM begins receiving 24X reference and 3X reference pulses, the PCM applies 5 volts to the IC
timing signal circuit. This signals the IC module to allow the PCM to control the spark timing. This is IC Mode
ignition. During IC Mode, the PCM compensates for all driving conditions. If the IC mode changes due to a
system fault, the system will stay in default until the ignition is cycled OFF/ON, or the fault is no longer present.
Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.