[tekuhn]

I know this is an old thread, but I thought I'd add some information to it. I have been rebuilding a couple of Delco-Remy T.I./CD Ignition distributors and was searching for information related to the electrical polarity of the pickup coil that generates the magnetic pulse signal to trigger the module. The mag-pulse signal generated by the pickup assembly (coil, magnet, rotating pole piece (reluctor)) is an A/C signal. It is not a pure sine wave because the induced voltage increases as the pole pieces near alignment and immediately reverse as they begin to move away from each other. The time the signal is positive is shorter than the time it is negative. Therefore, the peak on the positive side is sharper and shorter in duration than the peak on the negative side which occurs in the dead space between poles.

I have HEI, TI, and MSD distributors available for testing, which provided some valuable information.

Since the magnetic pulse signal is an a/c voltage, referring a wire as positive or negative must be qualified as to WHEN it is positive or negative. I intend to refer to a wire as positive if it has a positive voltage on it as the pole pieces are approaching each other and swings negative as the pole pieces move apart. With this in mind - On an HEI pickup assembly, the GREEN wire is positive, and the white wire is negative. Following the same logic, on a TI distributor, the white w/green stripe is positive, and the white wire is negative.

Here's where it gets interesting. MSD says on their pickups, the black/orange (purple on extension) is positive and black/purple (green on extension) is negative. However, testing reveals that the wire MSD calls positive is going positive as the pole piece is moving AWAY from the coil. This is important if you try to use an MSD pickup to trigger an HEI module. You might be aware that MSD says for their products to use the pickup polarity that retards the timing the most with an older analog box and to use the polarity that gives the most advanced timing with the newer digital boxes. Clearly, the digital boxes trigger a spark as the "positive" wire swings negative (approaching) and the analog boxes trigger a spark as the "positive" wire swings positive (away).

Back to our GM products. Why does polarity matter? Two reasons come to mind. First is dwell time for coil saturation. Unlike capacitive discharge, HEI is still an inductive (Kettering) system just like a points setup. The length of time the ignition coil primary winding is energized is important to building a strong spark. Unlike a points system where the ignition coil is building a magnetic field charge anytime the points are closed, HEI always defaults to being off (points open) when there is no signal from the pickup coil. When you crank the starter and the distributor begins to spin, the pickup coil starts to generate a signal. As the signal pulse swings positive, the module connects the ignition coil primary to ground (equivalent to points closed) and the coil builds a magnetic field. As the positive pulse begins to drop (but it's still positive), the module opens the coil primary (equivalent points open) and the coils fires. If the polarity of the pickup signal is backwards, the positive peak is not the proper duration and the coil saturation timing is off. The other reason it matters is because of the rotor tip position. If the spark is occurring as the reluctor points are moving away from each other, the rotor tip is moving toward the zone in between plug wire contacts and misfires will happen.

I can't comment about different HEI pickup coils and 4-pin versus 7-pin modules. The HEI distributor I have is a 1977 unit and appears to have the original D-R 4-pin module.

Here is a link to a discussion on a different forum complete with oscilloscope wave patterns to illustrate what I was attempting to describe above....

https://www.slantsix.org/forum/viewtopic.php?t=56383