Ignition System: Description and Operation

Ignition And EST System:

Electronic Spark Timing (EST) is ECM controlled. The ECM monitors information from various sensors, computes the desirable spark timing and signals the distributor when timing changes are required. To perform this function, the ECM must know crankshaft position, engine rpm, engine load (manifold pressure or vacuum), atmospheric (barometric) pressure and engine temperature.

The distributor does not contain centrifugal advance weights, springs or a vacuum advance unit. Base timing is adjustable. If it is not correctly set, the entire spark advance curve will be off.

The ECM utilizes information from the MAP and coolant temperature sensors and engine rpm to calculate spark advance as follows:

1. Low MAP output voltage (low engine load) = more spark advance.
2. Cold engine = more spark advance.
3. High MAP output voltage (high engine load) = less spark advance.
4. Hot engine = less spark advance.

EST SYSTEM CIRCUITS

1. Distributor Reference Input (circuit 430): This circuit provides rpm and crankshaft position information to the ECM. Pulses on this circuit are used by the ECM to time fuel injection pulses. If the ECM receives no input from circuit 430, the injectors will not be triggered.

2. Distributor Reference Ground (circuit 453): This circuit is grounded through the distributor HEI module. The ECM compares voltage pulses on input circuit 430 to this circuit. An open circuit 453 may cause poor engine performance and a "Check Engine" light with no stored code.

3. HEI Bypass Control (circuit 424): Above 450 rpm, the ECM applies 5V to this circuit as a signal to the HEI module to allow ECM control of spark timing and dwell (EST Mode). When the 5V command is not given, the HEI module controls ignition timing and dwell (Module Mode).

4. EST Output (circuit 423): This circuit triggers the HEI module during EST Mode. When the ECM receives the reference input signal, it advances or retards the EST output signal from that point. Because the ECM does not know what the actual timing is, the entire spark curve will be incorrect if base timing is not properly set.