GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Electronic Spark Timing (EST) System


Most of the vehicles covered by this manual utilize an electronic control system for all spark timing changes. Many of the HEI distributor equipped engines use the EST system instead of mechanical or vacuum advance mechanisms. The HEI (EST) spark control system makes the ignition timing changes, necessary for emissions control, economy and performance. For more information on this system, please refer to Chapter 2 of this manual.