Ignition System - 4-CYL: Operation
During cranking or in event of EST ECM failure, a by-pass signal from ECM terminal 11 to the HEI-EST module terminal "B" is either absent or low. See Figure. This notifies the HEI-EST module to take over control of spark advance and to ignore any EST information coming from the ECM. During this period, poor engine performance may result under some conditions, but the diagnostic "CHECK ENGINE" light will not come on.
The HEI-EST module will then convert pick-up coil RPM signals and transmit them through the by-pass switch and terminal "C" directly to the negative "TACH" terminal of the ignition coil. These signals turn the coil primary circuit on and off, causing a surge in the secondary that fires the spark plugs.
When engine speed reaches 600 RPM or more (about 5-15 seconds after starting), the ECM transmits a constant 5-volt signal to the distributor HEI-EST module, changing the position of the by-pass switch. This has the following effect:
- On models equipped with a regular distributor (without a Hall Effect switch), the pick-up coil's RPM signals can no longer flow directly to the ignition coil as this circuit is now open. Instead the signals are converted in the distributor module and routed through terminal "R" to ECM terminal 10.
- On models equipped with a distributor having a Hall Effect switch, the change in position of the by-pass switch opens the circuit between the pick-up coil and ignition coil. The pick-up coil has no RPM reference connection to the ECM, as HEI-EST module terminal "R" is not used. The function of supplying RPM signals to the ECM is then accomplished by the Hall Effect switch. The wires from the Hall Effect switch connect directly to the ECM and do not connect to the distributor module.
The PROM (Programmed Read Only Memory) portion of the ECM carries the basic spark advance curve based on engine RPM. Spark Timing is calculated by the ECM whenever an ignition pulse is present. However, spark advance information is only sent to the distributor when the engine is running (not during cranking). Engine sensor values are used by the ECM to modify the PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions.
The coolant temperature sensor advances spark on a cold engine and reduces advance as the engine reaches normal operating temperature. If the engine is too hot, spark is retarded to prevent detonation.
During light throttle operation, the throttle switch allows for additional advance. Additional adjustment results from input from coolant temperature, engine RPM and manifold absolute pressure (MAP) sensors. When MAP is low, spark is at maximum advance.
As load increases and pressure increases, spark timing is retarded to allow the engine to maintain its proper performance and emission level.
After computation of all information from the various sensors, a compensated ignition spark timing signal is sent through the HEI-EST module's terminal "E", the by-pass switch, and terminal "C", to the ignition coil negative terminal. Each time the signal is flashed on and off, the coil's primary circuit is turned on and off. As this occurs, a voltage surge occurs in the secondary that fires the spark plugs.
The ECM is continually computing sensor information to maintain efficient engine performance with low emission levels, doing so under varying engine conditions.