Ignition System Components: Module
WARNING: This page is about a different car, the 1989 Cadillac Allante. However, it is still accessible from the selected car via links, so may be relevant.
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 430)
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 430 is grounded or open to ECM, a code EO12 will be set. - Reference ground - (Circuit 453)
This wire is grounded in the distributor and provides the ECM with distributor ground. With the engine running, the voltage from pin A8 at the ECM to battery negative should not exceed .8 volts. If the voltage is greater than .8 volts, repair poor engine to battery negative ground. An open at the distributor 4-way pin D (circuit 453) may cause "No Start", code E012, E023, or other performance problems. - By-Pass - (Circuit 424)
The ignition module commands spark in two different modes, bypass and ECM-controlled. During crank, or when the ECM is in backup fuel mode, the ECM grounds the bypass line (Circuit 424) and the HEI module controls spark timing. After the engine has started, the ECM provides 5 volts on the bypass line and the HEI module switches to ECM-controlled (EST) spark. An open or grounded bypass circuit will set a code E023 and the engine will run on HEI-controlled timing. - EST - (Circuit 423)
This 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. An open or ground in the EST circuit will set a code E023 and cause the engine to run on the HEI module timing. This will cause poor performance and poor fuel economy. - CAM Position - (Circuit 633)
The Cam Position Sensor 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 which signals the ECM that cylinder Number one is next to be fueled. The ECM will sequence the fuel delivery once number one cylinder intake stroke is determined. The ECM sequences the injectors in cylinder firing order and assures that the fuel injection is competed as the respective intake valve opens. If the Hall Effect sensor goes open the ECM will record a Cam Sensor Fault (E041) and fuel will be sequenced without reference to cylinder number one. Under such conditions driveability will not be noticeably affected on steady cruise. Tip-in hesitation may be noticed depending on how far out of synchronization the fueling is delivered.