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 hi- (Circuit 430)
  
  This is a 5 volt square wave signal that originates at terminal pin R of the HEI module and is input at terminal D8 of the ECM. This signal provides crank shaft position to the ECM.
• Reference ground - (Circuit 453)
  
  This is the ground circuit for the distributor reference signal. This is the low voltage reference for the distributor reference signal. High resistance or an open in this circuit will prevent the ECM from receiving a clean signal from the HEI module.
• Bypass - (Circuit 424)
  
  This is a 5 volt signal that originates at ECM terminal C7 and is input at the HEI module terminal pin B. This signal switches the HEI module from module control of the ignition coil primary to ECM control to the ignition coil primary.
• EST - (Circuit 423)
  
  This is a 5 volt square wave signal that originates at terminal C8 of the ECM and is input at terminal pin E of the HEI module. The circuit triggers the HEI module when the ECM is controlling spark timing. The ECM does not know what the actual timing is, but it does know when it gets a reference signal. It uses the reference as a base point and advances or retards the spark from that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect by the amount that the timing is misadjusted.
• CAM Position - (Circuit 633)
  
  This is a 5 volt signal that originates at the cam sensor and provides the ECM with information to determine when the intake valve on number one cylinder will open. The cam sensor is a hall effect type sensor. The ECM sequences the injectors in cylinder firing order based on the input from this sensor.

Fig 1: Testing Pick-Up CoilCircuit

Courtesy of GENERAL MOTORS CORP.