Mopar Electronic Pickup Polarity and Rotor Phase

MopaR&D said
I did read that in the first post but I got to (over)thinking and confused myself with all of the factors involved. I like to think I have a degree of intuition when it comes to mechanical systems but with electrical... not so much lol.

The main reason I ask is I have a Pertronix Digital HP box in my '72 D200 and a Summit Billet distributor which uses this style of trigger system, same as a Mopar electronic distributor. For a long time I've been chasing an erratic idle and odd running issues that I finally narrowed down to being ignition-related. Yesterday I had the cap off the distributor and noticed what looked like a phasing issue (original thread HERE) where it looked like the rotor was way out of phase with the contacts in the cap. I thought about it some more and remembered that the ignition box can be set to either rising edge or falling edge (it had been set to falling). I changed it to rising edge and re-set the distributor (had to advance it a ton of course) and it seemed to run better. Thing is, in the instructions for the ignition box it says to use falling edge for magnetic-pickup distributors. In my mind I was over-simplifying the signal from the pickup though; I thought it was a simple sine wave when in fact there's more to it.

Today I was doing a bunch of research to figure out what the heck was going on and stumbled upon this thread along with one on Moparts where someone was having a similar issue. It led me to the conclusion that I must have wired the pickup reversed to how it's supposed to be. I was hoping that since I switched the ignition box to rising edge it would trigger at the correct point on the voltage curve with the wires being reversed. However @BillGrissom you are saying that's not the case? I guess I was hoping I wouldn't have to go and swap the wires back because it likely means I need to also reposition the reluctor wheel on the distributor rotor. I did find it strange that I had to do that back when I first installed the distributor because it misfired like crazy and I ASSumed the reluctor had been installed wrong from the factory.

I'm sure many here would be interested in photos of you Billet distributor and its pickup, even if we can't answer your question about the details of rising or falling edge (Kit's O-scope photos show "triggers on rising edge"). Another issue is that as spark timing changes, one needs the rotor tip to still align with the correct post. The biggest change comes from rpm advance. In Mopar distributors, the balance weights make that angle adjustments below the pickup plate, so no effect upon the phase between pickup and rotor. Not true in some aftermarket distributors. I've seen some which have a wide-arc rotor tip to allow the spark to still jump to the proper post despite rotor angle changing with rpm. The vacuum advance adjustment does change the pickup to rotor alignment, but it is a fairly small angle change (>15 deg?).

There are youtubes where someone drilled a hole in the rotor cap to view where the rotor is when #1 spark fires, to verify rotor-phasing. One can even buy a transparent cap for some GM distributors. I feel for your concerns since I dealt with my 1964 slant idling very poorly after years of non-use. New gas, and no improvement on shots of starter fluid (usually runs smooth). Since last run, when it idled perfectly, I changed to electronic ignition, but instead of an entire 1972+ distributor, I put the innards plate into the 1964 points housing. Then I wondered if parts were compatible. Bought a new (cheap) Chinese e-distributor, but no better, but no history with it. For next pass, I have a factory e-distributor and also going to have better diagnostics (wideband O2), and a rebuild kit for the BBS carburetor. Frustrating when you don't know if the problem is fuel or spark. At least with later crank-triggered engines, you don't need to be concerned with spark timing, only that it does spark, assuming you have the proper "toner ring" for your year.