Ignition System: Description and Operation

Distributor Module:

PURPOSE
The Distributor Ignition (DI) system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the Powertrain Control Module (PCM) controls distributor spark advance (timing) with the Ignition Control (IC) system.

OPERATION
The standard Distributor Ignition (DI) system has a modified distributor module, which is used in connection with Ignition Control (IC). The module has seven terminals.

To properly control ignition/combustion timing, the PCM needs to know:
^ Crankshaft position (via pick-up coil).
^ Engine speed (RPM).
^ Engine load (manifold pressure or vacuum).
^ Atmospheric (barometric) pressure.
^ Engine temperature.
^ EGR.

The DI system consists of the distributor ignition control module, PCM, and connecting wires. There are four inputs to the PCM from the Distributor Ignition (DI) control module.

These circuits perform the following functions:
^ Distributor reference - Terminal "B" refer to C Charts/Chart C4-B Ignition System Check. Chart C-4B Ignition System Check
This provides the PCM with information for RPM and crankshaft position information.

^ Reference ground - Terminal "D" (Refer to C Charts/Chart C4-B Ignition System Check). This wire is grounded in the distributor and makes sure the ground circuit has no voltage drop which could affect performance. If it is open, it may cause poor performance. Chart C-4B Ignition System Check

^ Bypass - Terminal "C" (Refer to C Charts/Chart C4-B Ignition System Check). At about 500 RPM, the PCM applies 5 volts to this circuit to switch spark timing control from the DI module to the PCM. An open or grounded bypass circuit will set a DTC 42 and the engine will run at base timing, plus a small amount of advance built into the DI module. Chart C-4B Ignition System Check

^ IC - Terminal "A" (Refer to C Charts/Chart C4-B Ignition System Check). After bypass voltage is applied, the PCM uses this circuit to trigger the DI module. The PCM uses the distributor reference signal to base its calculation of the amount of spark advance needed, under present engine conditions. If the base timing of the engine is incorrect, the entire spark curve will be incorrect. Chart C-4B Ignition System Check