The distributor ignition (DI) 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. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system consists of a single ignition coil and ignition control module (ICM). Spark energy is delivered via a distributor cap, rotor, and secondary spark plug wires. The driver module within the ICM is commanded to operate the coil by the engine control module (ECM), that has complete control over spark timing. The DI system consists of the following components:
Crankshaft Position (CKP) Sensor
The CKP sensor is a three wire sensor based on the magneto resistive principle. A magneto resistive sensor uses two magnetic pickups between a permanent magnet. As an element such as a reluctor wheel passes the magnets the resulting change in the magnetic field is used by the sensor electronics to produce a digital output pulse. The ECM supplies a 5-volt, low reference, and signal circuit to the CKP sensor. The sensor returns a digital ON/OFF pulse 3 times per crankshaft revolution for the V6 engine, 4 times for the V8 engine. The CKP sensor reads the crankshaft mounted reluctor wheel to identify pairs of cylinders at top dead center (TDC).
Camshaft Position (CMP) Sensor
The CMP sensor is a hall-effect sensor located in the ignition distributor base. The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor information is used by the ECM to determine the position of the valve train relative to the CKP.
Ignition Coil and ICM
The ICM is connected to the ECM by an ignition control (IC) circuit. The ICM also has a ground circuit and shares an ignition 1 voltage supply with the ignition coil. The coil driver in the ICM controls current through the ignition coil based on signal pulses from the ECM. There is no back-up or by-pass function in the ICM.
Secondary Ignition Components
Distributor
The distributor contains the Camshaft Position (CMP Sensor), cap, rotor and shaft. The Distributor is splined by a helical gear to the camshaft and rotates providing a spark to each spark plug wire. When servicing the Distributor, it is critical to ensure proper cap sealing to the Distributor body and correct installation to the camshaft. If the Distributor is installed a tooth off in relation to the camshaft, a DTC sets.
Ignition Wires
The spark plug wires are a carbon-impregnated cord conductor encased in a silicone rubber jacket. Silicone wiring will withstand very high temperature and is an excellent insulator for the higher voltages. The silicone spark plug boots provide a tight seal on the spark plug.
Engine Control Module (ECM)
The ECM controls all ignition system functions, and constantly corrects the basic spark timing. The ECM monitors information from various sensor inputs that include the following:
| • | The throttle position (TP) sensor |
| • | The engine coolant temperature (ECT) sensor |
| • | The manifold air temperature (MAT) sensor |
| • | The vehicle speed sensor (VSS) |
| • | The transmission gear position or range information sensors |
| • | The engine knock sensors (KS) |
Modes of Operation
There is one normal mode of operation, with the spark under ECM control. If the CKP pulses are lost the engine will not run. The loss of a CMP signal may result in a longer crank time since the ECM cannot determine which stroke the pistons are on. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.
