Circuit Description

When system is running on ignition module (no voltage on by-pass line), the ignition module grounds the EST signal line. The ECM expects to see no voltage on the EST signal line during this condition. If the ECM sees voltage on this line, Code 42 will be set and the system will not go into "EST" mode.

When engine RPM for EST is reached (about 400 RPM), and by-pass voltage is applied, EST should no longer be grounded in ignition module, so EST voltage should be varying.

If the by-pass line is open or grounded, the ignition module will not switch to "EST" mode. 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, no EST signal will be present. Code 42 will be set.

Code 42 sets if there is an open, or a short to ground, in the EST or by-pass circuit.