Ignition System Components: Module

The EST system consists of distributor module, ECM, and connecting wires. The distributor has four wires that connect HEI module to ECM. The connector terminals are lettered as shown. See Fig 1 . These circuits perform following functions:

• Distributor reference - (Circuit B)
  
  This provides ECM with RPM information. If wire becomes open or grounded engine will not run, because ECM will not operate the injectors. If circuit B is grounded or open to ECM, a code EO12 will be set.
• Reference ground - (Circuit D)
  
  This wire is grounded in distributor and ECM 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 C)
  
  At about 400 RPM, ECM applies 5 volts to this circuit to switch spark timing control from HEI module to ECM.

  An open or grounded bypass circuit will set a code EO23 and engine will run in bypass timing.

• EST - (Circuit A)
  
  This circuit triggers HEI module. The ECM does not know what actual timing is, but it does know when it gets a reference signal (circuit B). It advances or retards spark based on information received from other sensors (Coolant, MAP, RPM, etc.). Since ECM modifies base timing, it must be set correctly for ECM to control spark correctly.

  An open or ground in EST circuit will set a code EO23 and cause engine to run on HEI module timing. This will cause poor performance, possible detonation, and poor fuel economy.

• CAM Position - (Circuit E)
  
  The CAM position sensor provides ECM with information to determine when intake valve on number one cylinder will open. The CAM sensor is a Hall Effect type sensor which signals ECM that cylinder number one is next to be fueled. The ECM will sequence fuel delivery once number one cylinder intake stroke is determined. The ECM sequences injectors in cylinder firing order and assures that fuel injection is completed as respective intake valve opens. If Hall Effect sensor goes open, ECM will randomly select from reference pulses a starting point to sequence injection and Code EO41 will set. Driveability will not be noticeably affected on steady cruise. Tip-in hesitation may be noticed depending on how much ECM fueling is out of synchronization.

For testing, see PFI CHART C-4: EST SYSTEM CHECK in SELF-DIAGNOSTICS article.

Fig 1: Identifying Electronic Spark Timing Circuits

Courtesy of GENERAL MOTORS CORP.