Signals, Interfaces and Shields

Wiring Diagram For EST:

CIRCUIT DESCRIPTION:

The ignition module sends a reference signal, circuit 430, to the Electronic Control Module (ECM) when the engine is cranking. While the engine speed is under 400 rpm, the module controls the ignition timing. When the engine exceeds 400 rpm, the ECM applies five volts to the bypass line (circuit 424) to switch the timing to the ECM control (EST circuit 423). When the system is running on the ignition module, that is, no voltage on the by-pass line, the ignition module grounds the Electronic Spark Timing (EST) signal. The ECM expects to see no voltage on the EST Line during this condition. If it senses a voltage, it sets a Code 42 and will not go into the EST mode. When the rpm for EST is reached (about 400 rpm), the ECM applies 5 volts to the by-pass line, and the EST should on longer be grounded in the ignition module so the EST voltage should be varying. If the by-pass line is open or grounded, the ignition module will not switch to EST mode so the EST voltage will be low and Code 42 will be set. If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set.