How to determine which misfire generator system my engine requires
Despite the connotation of its name, the connection box contains much more than mere connections to the ignition system; rather, it also houses the electronics responsible for generating misfire. Because the two MisGen3 connection boxes interact differently with the two types of ignition, it is imperative that the appropriate box be selected.
If the technical information on the ignition system is hard to obtain, or to confirm the information that is available, it is highly recommended that the ignition signals in the harness wires be observed on an oscilloscope before hooking up a misfire generator for the first time. Attaching a 10x pobe to just one ignition signal will quickly reveal whether it contains a logic-level signal (0-5 volts) or a primary coil signal having much higher voltage peaks (several hundred volts at the moment of ignition) with significant ringing and ignition noise.
To illustrate this difference, Figures 1 and 2 depict two types of ignitions with typical scope traces carried in each harness. (Be aware that the high voltage spike on the coil primary signal in Figure 1 is very brief – about 20 microsecs – and may not be readily visible at scope scan speeds comparable to engine firing intervals.) Note also that the polarity of the dwell pulse (the interval when the coil is charging) is generally opposite in the two systems – though the possibility exists that a manufacturer of ICD ignitions may utilize a negative-going logic pulse (instead of the positive pulse shown), even though such a design has not yet come to our attention* .
Figure 1 Diagram showing one coil of ignition system with driver housed separately from coil. Typically, the original MisGen3 system requires only a single connection per coil wire, which is not interrupted between the driver and coil. (Driver is not necessarily bipolar transistor as shown; it may be MOSFET, IGBT, etc.)
Figure 2 Diagram showing one coil of ignition system with integrated coil driver. MisGen3 for ICD ignition requires two connections per coil wire, which must be interrupted between the ECM and the coil driver.
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* A trivial change within the Connection Box for ICD is necessary, and mandatory for safe operation, to accommodate such negative-going logic pulses. Contact the manufacturer if your ignition system uses that polarity. This statement does not apply to the original Coil Connection Box (for non-ICD ignitions), which presumes that signals at the coil primary always go low at the onset of dwell.