Ignition System - HEI-Est (TBI Models): Operation
EST system consist of distributor module, and connecting wires. See Fig 1. The EST distributor circuits perform the following functions:
- Ground Circuit. This wire is grounded in distributor and makes sure ground circuit has no voltage drop which could affect performance. If it is open, it may cause poor performance.
- By-Pass Circuit. At about 400 RPM, ECM applies 5 volts to this circuit to switch spark timing control from module to ECM. An open or grounded by-pass circuit will set a Code 42 and the engine will run at base timing with a small amount of advance built into module.
- Distributor Reference Circuit. This circuit provides ECM with RPM and crankshaft position information.
- EST Circuit. This circuit triggers module. The ECM does not know what actual timing is, but it does know when it gets reference signal. It then advances or retards spark from that point. Therefore, if base timing is set incorrectly, engine spark curve will be incorrect.