Ignition System: Description and Operation

HEI Distributor With External Coil:

Ignition Circuit:

DESCRIPTION
The Distributor Ignition (DI) system consists of the ignition coil, ignition module, distributor assembly (cap, rotor, magnetic pick-up, distributor shaft etc.), secondary wires, spark plugs, and the Ignition Control (IC) portion of the Engine Control Module (ECM). This 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 ECM controls distributor spark advance (timing) with the ignition control system.

OPERATION
The ignition control system consists of an Distributor Ignition (DI) module, ECM and connecting wires. The distributor has four wires that connect the DI module to the ECM. These circuits perform the following functions:

Reference
Circuit 430 is the distributor reference, which provides the ECM with rpm and crankshaft position information.

Distributor Reference Low
Circuit 453 is the reference ground. 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.

Bypass
Circuit 424 is the bypass circuit. At about 400 rpm the ECM applies 5.0 volts to this circuit to switch spark timing control from the DI module to the ECM. An open or grounded bypass circuit will set a Diagnostic Trouble Code (DTC) and the engine will run at base timing, plus a small amount of advance controlled by the DI module.

Distributor Ignition Trigger
The IC terminal, Circuit 423, triggers the DI module. The ECM does not know what the actual timing is, but it does know when it gets the reference signal. It then advances or retards the spark form that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect. An open or ground, in the IC circuit, will set a Code and cause the engine to run on the DI module timing. This will cause poor performance and poor fuel economy. An open in the circuit will set a DTC 41, and a ground in the IC circuit will set a DTC 42, and the engine will run on the DI module timing and cause poor performance and milage.

Spark Advance Calculation
The ECM uses information from the Manifold Absolute Pressure (MAP) sensor and coolant sensors in addition to rpm to calculate spark advance as follows.

- Low pressure sensor (MAP) output voltage (high vacuum indication) = more spark advance.

- Cold engine = more spark advance.

- High pressure sensor (MAP) output voltage, (low vacuum indication) = less spark advance.

- Hot engine = less spark advance.