Electronic Spark Timing, introduced on the X-Body cars in 1981, monitors engine operating conditions such as engine load, coolant temperature, manifold vacuum, etc. and then constantly adjusts the spark timing so as to maintain maximum efficient engine performance. The EST distributor uses no mechanical or vacuum advance and can be easily identified by the absence of a vacuum advance unit and the presence of a four terminal connector in addition to the normal distributor wiring.
The EST may ignore certain sensor inputs under certain operating conditions and revert to a programmed HEI spark control called the Bypass Mode. The ECM will switch to the Bypass Mode in order to assist in starting, in the event of most EST operational problems, the distributor will almost always switch to the Bypass Mode with its programmed spark advance. This allows the car to be driven in a near normal manner until the problem can be solved.