Circuits Affecting Ignition Control

In order to properly control ignition timing, the PCM relies on the following information:
^ Engine load (manifold pressure or vacuum)
^ Atmospheric (barometric) pressure
^ Engine temperature
^ Intake air temperature
^ Crankshaft position
^ Engine speed (RPM)

The Ignition Control (IC) system consists of the following components:
^ Ignition coils
^ Ignition control module
^ 7X crankshaft position sensor
^ 24X crankshaft position sensor
^ Powertrain control module
^ All connecting wires

The electronic IC module connector terminals are identified as shown in the Electronic Ignition System graphic. These circuits perform the following functions:
^ 3X reference high-The 7X crankshaft position sensor sends a signal to the electronic ignition control module which generates a reference pulse that is sent to the PCM. The PCM uses this signal to calculate crankshaft position and engine speed (also used to trigger the fuel injectors).
^ 3X reference low-This wire is grounded through the ICM and assures the ground circuit has no voltage drop between the ICM and the PCM.
^ Ignition control bypass-During initial cranking, the PCM will look for synchronizing pulses from the camshaft position sensor and the 7X crankshaft position sensor. The pulses indicate the position of the #1 piston and the #1 intake valve. Five volts is applied to the bypass circuit at precisely the same time these signals are received by the PCM. This generally occurs within one or two revolutions of the crankshaft. An open or grounded bypass circuit will set a DTC P1351 and the engine will run at base ignition timing. A small amount of spark advance is built into the ignition control module to enhance performance.
^ Ignition control (IC)-The PCM uses this circuit to trigger the electronic ignition control module. The PCM uses the crankshaft reference signal to calculate the amount of spark advance needed.
^ 24X reference signal-The 24X crankshaft position sensor increases idle quality and low speed driveability by providing better resolution at a calibrated RPM.