System Operation

While the engine is cranking, the ignition module sends a reference (RPM) signal to the ECM on the reference wire of the 4-wire EST connector. When engine speed exceeds 400 RPM, the ECM applies 5 volts on the by-pass wire to switch ignition timing control to the ECM.

NOTE: The reference signal also triggers the fuel injection system. If the ECM does not receive a reference signal during cranking, the vehicle will not start.

The ECM expects to see no voltage on the EST line when timing is controlled by the ignition module. If the ECM senses voltage on this line, it will set a Code 42, and it will not go into EST mode.

While the system is running on the ignition module, the signal on the "EST" wire will be grounded through the ignition module. When engine RPM is about 400 RPM, voltage will be applied to the by-pass wire. As the EST should no longer be grounded in the ignition module, the EST voltage should be varying.

If the by-pass wire is open or grounded, the ignition module will NOT go to EST mode. In turn, EST voltage will be low and a Code 42 will be set. If the EST line is grounded, the ignition module will switch to EST but, there will be no EST signal. Code 42 will be set.

If the by-pass signal is missing because of ECM failure or open circuits, the engine will run at base timing plus a small amount of advance built into the ignition module.

CAUTION: If base timing is incorrect, the entire advance curve will be incorrect.

Fig 1: I-Mark (Turbo) Ignition System

Courtesy of ISUZU MOTOR CO.