GM Service Manual Online
For 1990-2009 cars only

The electronic ignition (EI) system consists of the following components and wiring circuits:

    • The ignition control (IC) module
    • The ignition coils
    • The powertrain control (PCM) module
    • The crankshaft position (CKP) sensor
    • The camshaft position (CMP) sensor

The EI wiring circuits listed here use service common names, they are as follows:

Between the IC module and the PCM.

    • Medium resolution engine speed signal
    • Low resolution engine speed signal
    • Camshaft position signal
    • IC timing control
    • IC timing signal
    • Low reference

Between the CKP sensor and the IC module:

    • CKP sensor 1 signal
    • CKP sensor 2 signal

Between the CMP and the IC module:

    • CMP sensor signal
    • Low reference
    • 10 volt reference

The CMP voltage and low reference circuits are shared with the CKP sensor.

The IC module is also supplied with the following:

    • Ignition (1) voltage
    • Ground

Purpose

The EI system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time to provide optimal performance, fuel economy, and control of exhaust emissions.

Operation

In this EI system there is one coil for each pair of cylinders. Each pair is at top dead center (TDC) at the same time, and are known as companion cylinders. The cylinder that is at TDC of its compression stroke is called the event cylinder. The cylinder that is at TDC of its exhaust stroke is called the waste cylinder. When the coil is triggered both plugs fire at the same time, completing a series circuit. Because the lower pressure inside the waste cylinder offers very little resistance the event cylinder uses most of the available voltage producing a very high energy spark. This is known as waste spark ignition.

EI Component Description

The following is a list of ignition system components and their functions.

Crankshaft Position (CKP) Sensor System


Object Number: 12303  Size: SH

The 18x crankshaft position (CKP) sensor contains two hall-effect switches in one housing. A hall-effect switch is a solid state switching device, which produces an ON-OFF pulse when a rotating element passes the sensor pick-up and interrupts the magnetic field.


Object Number: 12358  Size: SH

The rotating element is called an interrupter ring or blade. In this case there are two interrupter rings (2) built into the crankshaft balancer. The outer ring and outer switch provides the PCM with the 18x signals, or 18 pulses per crankshaft revolution. The inner ring and inner switch provide the PCM with a sync pulse three times per crankshaft revolution. The IC module uses the 18x and sync pulses to determine the engine position, by counting how many ON-OFF 18x pulses occur during a given sync pulse. Each of the 3 sync pulses represent a pair of cylinders both at top dead center at the same time, which are called companion cylinders. With this dual interrupter ring arrangement the IC module can identify the correct pair of cylinders to fire within as little as 120 degrees of crankshaft rotation.

Camshaft Position (CMP) Sensor


Object Number: 12305  Size: SH

The camshaft position (CMP) sensor is a hall effect sensor which is triggered by a magnet pressed into the camshaft sprocket. The CMP sensor provides a signal pulse once every camshaft revolution, known as the CMP sensor signal. The IC module uses this signal to identify the position of the #1 cylinder at top dead center during its power stroke.

Ignition Coils and Ignition Control (IC) Module


Object Number: 12299  Size: SH

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

    • Supplies both a power and low reference circuit to the CMP and CKP sensors.
    • Receives and processes the signals generated by the CMP and CKP sensors.
    • Determines the correct coil triggering sequence, based on how many 18x ON-OFF pulses occur during a sync pulse. This coil sequencing occurs at start-up, and is remembered by the IC module. After the engine is running, the module will continue to trigger the coils without the CKP input.
    • Determines the correct direction of the crankshaft rotation, and cuts spark and fuel delivery to prevent damage from backfiring if reverse rotation is detected.
    • Sends 18x and 3x reference signals to the PCM.

The 3x reference signal is also known as the low resolution engine speed signal. This signal is generated by the IC module using an internal divide-by-six circuit. This circuit divides the 18x signal pulses by six. This divider circuit will not begin operation without a sync pulse present at start-up, and without 18x and 3x reference signals the fuel injection will not occur.

Powertrain Control Module (PCM)


Object Number: 159242  Size: SH

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 and ignition dwell. To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating ignition control (IC) spark timing:

    • The ignition control (IC) module
    • The engine coolant temperature (ECT) sensor
    • The intake air temperature (IAT) sensor
    • The mass air flow (MAF) sensor
    • The Internal Mode or PNP inputs from Internal Mode switch or Park/Neutral position switch
    • The throttle position (TP) sensor
    • The vehicle speed (VSS), or transmission output speed (TOSS) sensors

The following describes the PCM to IC module circuits:

    • Low resolution engine speed (3x reference)--PCM input -- From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position above 1,200 RPM. The PCM also uses the pulses on this circuit to initiate injector operation.
    • Medium resolution engine speed signal (18X reference)-- PCM input -- The 18X reference signal is used to accurately control spark timing at low RPM and allow IC operation during cranking. Below 1,200 RPM, the PCM is monitoring the 18X reference signal and using the signal as the reference for ignition timing advance. When engine speed exceeds 1,200 RPM, the PCM begins using the 3X reference signal to control spark timing.
    • Camshaft position PCM input -- The PCM uses this signal to determine the position of the cylinder #1 piston during its power stroke. This signal is used by the PCM to calculate true Sequential Fuel Injection (SFI) Mode of operation. The PCM compares the number of CAM pulses to the number of 18X and 3X reference pulses. If the number of 18X and 3X reference pulses occurring between CAM pulses is incorrect, or if CAM pulses are not received while the engine is running, the PCM will set a DTC. If the cam signal is lost while the engine is running the Fuel Injection System will shift to a calculated Sequential Fuel Injection Mode based on the last CAM pulse, and the engine will continue to run. The engine can be re-started and will run in the calculated Sequential Mode as long as the condition is present with a 1 in 6 chance of being correct.
    • 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 assures there is not a ground drop between the PCM and IC module.
    • IC timing signal --PCM output -- The IC module controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives low resolution engine speed signals from the IC module, the PCM applies 5 volts to the IC timing signal circuit allowing the IC module to switch spark control to PCM control.
    • IC timing control-- PCM output -- The IC output circuitry of the PCM sends out timing signals to the IC module on this circuit. When in the Bypass Mode, the IC module grounds these signals. When in the IC Mode, the signals are sent to the IC module to control coil dwell and spark timing. Proper sequencing of the ignition coils is always controlled by the IC module.

Ignition System Modes of Operation

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

When the PCM begins receiving 18x 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 dwell and 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 mode will stay in default until the ignition is cycled OFF to ON, or the fault is not present.

Ignition system service

Special care must be taken when diagnosing and handling EI systems. The secondary voltage output can exceed 40,000 volts. Refer to the applicable diagnosis or repair section for more information.