|Originally posted by Custom2M4:|
How would this effect the tach? He didn't say the engine jumped, just the tach..
The ICM generates an ignition pulse about 10 deg btdc. The ICM passes that pulse onto the ECM on the Purple/White wire. It is and also out sent out of the ICM on the tach port. Once then engine is rotating over 400 RPM the ECM puts a voltage on the Tan/Black that the ICM should be firing now on the ECM's generated signal. That signal will have the spark timing occurring perhaps 40 deg BTDC. The ECM sends that signal on the White wire and that signal is what is used to actually fire the coil. (Go figure that how does the ECM generate a signal that occurs BEFORE the reference signal from the ICM. It does)
Anyways that signal is ran into the coil and the coil produces that high voltage output. If that high voltage output instead of jumping the spark plug gap instead shorts back to the ICM, there will be a pulse at about 40 BTDC and a second pulse at 10 BTDC.