Electronic Spark Timing (EST)

Electronic Spark Timing Operation:

The Electronic Spark Timing (EST) system consists of a distributor module, Electronic Control Module (ECM), and wiring harness. The distributor has four wires that connect the HEI module to the ECM. These circuits perform the following functions:

1. The distributor reference provides the ECM with engine speed information. If the wire becomes open or grounded, the engine will not run since the ECM will be unable to operate the fuel injectors. If reference circuit 430 is grounded or open to the ECM, a code E12 will set.

2. Reference ground circuit 453 is grounded in the distributor and provides the ECM with a distributor ground. With the engine running, the voltage from pin A8, at the ECM, to the battery ground should not exceed .8 volts. An open in the distributor 4-way pin D circuit 453 may cause a no start condition, codes E12, E23, or other performance problems.

3. Bypass Circuit 424. The ignition module commands spark in two different modes, bypass and ECM-controlled. During cranking, or when the ECM is in backup fuel mode, the ECM grounds the bypass line (circuit 424) and the HEI module controls spark timing. After the engine has started, the ECM provides 5 volts on the bypass line and the HEI module switches to ECM-controlled EST. An open or grounded bypass circuit will set a code E23, and the engine will run on HEI-controlled timing.

4. The EST Circuit 423 triggers the HEI module when the ECM is controlling spark timing. The ECM does not recognize what the actual timing is, but it does recognize timing when it receives a reference signal. It uses the reference as a base point, and advances or retards the spark from that reference. If the base timing is set incorrectly, the entire spark curve will be incorrect. An open or ground in the EST circuit will set a code E23 and cause the engine to run on the HEI module timing. This will cause poor performance and fuel economy. The ECM uses information from the MAP and coolant sensors, in addition to engine speed, to calculate spark advance as follows:

a. Low MAP = more spark advance.
b. Cold Engine = more spark advance.
c. High MAP = less spark advance
d. Hot Engine = less spark advance.
e. High RPM = more spark advance.
f. Low RPM = less spark advance.

Detonation could be caused by low MAP output, or high resistance in the coolant sensor circuit. Poor performance could be caused by high MAP output, or low resistance in the coolant sensor circuit.