GM Blazer/Jimmy/Typhoon/Bravada 1983-1993 Repair Guide

Description & Operation

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The HEI (EST) distributor uses a modified ignition module. The module has seven or eight terminals instead of the four used without EST. Different terminal arrangements are used, depending upon engine application.

To properly control ignition/combustion timing, the ECM (or PCM) needs to know the following information:



Crankshaft position
 
Engine speed (rpm)
 
Engine load (manifold pressure or vacuum)
 
Atmospheric (barometric) pressure
 
Engine temperature
 
Transmission gear position (certain models)
 

The ECM (or PCM as applicable) uses information from the MAP and coolant sensors in addition to rpm to calculate spark advance as follows:



Low MAP output voltage would require MORE spark advance.
 
Cold engine would require MORE spark advance.
 
High MAP output voltage would require LESS spark advance.
 
Hot engine would require LESS spark advance.
 

Incorrect operation of the EST system can cause the following:



Detonation - low MAP output or high resistance in the coolant sensor circuit.
 
Poor performance - high MAP output or low resistance in the coolant sensor circuit.
 

The EST system consists of the distributor module, ECM or PCM (and its connecting wires).

 
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