? about Mopar elect. dist.

Is the reluctor in a Mopar electric distributor a rising edge or falling edge device?

Reason I ask is, I put a Pertronix digital HP box in my bracket car years ago to fix an ailing digital 6 plus. I chose it based on several factors... shift light output, ground triggered launch rev limit, and 12v burnout rev. limit all as delivered. Works great. So I built a street car, and buy another digital hp box. Well there is two coil out put settings one at 145 mj., the other at 187 mj. So I figure I'll call them to find out why they put 2 outputs on them, the racecar is set at 187mj. I figure running the 187 setting on the street might burn something out. The tech guy says 187mj setting under continuous use could overheat the coil. He says I should run it on 145mj, multispark, falling edge setting for the street car. I have been running the bracket car on the rising edge setting. He says no that is wrong, so we "discuss" this for a while and I respond I will change it to falling edge trigger. Well as I suspected the car won't start. I switch it back and to rising edge and low and behold the car fires instantly, as it always did. But now this has me thinking it has something to do with where the dist is rotated to and maybe I am triggering off an inconsistent signal. Does anyone know what all this rising edge/falling edge stuff really means and how it relates to the trigger signal?

I'm not up very well on current terminology, but maybe I can give you some insight.
The original Mopar electronic ignition used a magnetic pickup. With that in mind, as the blade on the reluctor turned and approached the pickup, the magnet should sense the the approaching reluctor blade as it comes around (which I would call the rising edge). The magnetic pickup would also sense the reluctor as it passed (the falling edge). The distance between the magnetic pickup and the reluctor was a critical measurement in the early Mopar electronic ignitions. Were I betting, I would suspect the Mopar unit would fire the spark at the point at which the magnetic signal was probably within the predetermined range (as determined by the tolerances built into the magnetic pickup), which should have been close to straight up.

The thickness of the reluctor edge can't be more then a couple degrees either way (between the rise, the center, and the fall), I don't see how that could cause a no start unless your timing it at the very edge of function. The gap between the reluctor point and the magnetic pickup could probably be effected by the condition of the distributor bushings more then the leading or falling edge of the reluctor. Later versions of Mopar electronic ignition use a crankshaft pickup to eliminate the variables a distributor could cause. In short, your electronic signal could be fluctuating between cylinders or between dist revolutions. You have to keep in mind the Mopar electronic ignition was the 1st ever production electronic ignition system and compared to the old point system everyone else was using, was very far advanced. Automotive electronics have come a long way since 1972 when the 1st Mopar electronic ignition systems showed up on the option sheet. Gene

The mopar reluctor setup is more a spike than a nice square wave like an optical pickup would give you. It has both positive and negative voltage components. As for it being a leading or trailing edge component, that's not relevant to the distributor, it's relevant to what ever the distributor is triggering. I do know if you get the two wires from the pickup swapped going to the stock ECU it will change your timing. As for what your Pertronix wants, well I think you figured it out in spite of their tech support.

Pic of typical magnetic pickup signal

It is the falling negative voltage that turns off the Mopar ECU and fires the coil.

Thanks!. However this makes me wonder. Looks like the falling edge is the sharp one. Wonder why the Pertronix won't start the car set to falling edge trigger. The rising edge in that scope pic is not the one you would not want to trigger off from what I believe.

You can set the box to trigger off a rising or falling edge signal. Is that the same as swapping the wires to the dist? I'm not sure it is

swapping the leads essentially just changes what happens first, the negative portion of the spike, or the positive portion. Which is why the timing will change if you do that.

still can't understand why triggering the box on the falling edge it won't run. Only thing I can come up with is that the spark happens later, and the rotor is past the cap terminal at that point Or they are backwards in the use of their terminology.

the mysteries of circuit design can make you scratch your head. Been working on electronics since 83 and when I run into this situation I just go with what works and not worry about the rest. What maybe happening is that you are feeding it the negative going portion first and it is looking to trigger off a positive signal, either the leading or trailing edge of that positive going signal. It may have a signal clamping circuit designed to protect the input that is triggered by the negative going circuit, it essentially shorts the input to ground to prevent damage and doesn't release in time to use the positive going signal that comes later. Just a guess though. Even though we like to think electronics are instantaneous, they really aren't. Though they do work faster than we can see it happen it has built in lag that can sometimes rear it's ugly head.

Gives me some more to contemplate....however in the mean time I'll just keep running what works but as I get time here and there I am going to dig deeper. First thing I'll do is check the timing over rpm very close.

Like most or all magnetic pick-ups the Mopar dist produces an AC signal. From all I have read as the reluctor comes close to the pick-up it sees it generate a + signal and then as it leaves the reluctor it sees it generate a - signal. Its the changing of the AC signal thats triggers the ECU from what I have read. Ron

Thats true as when I worked for Ford in the mid 70's to 1980 we were told switching the pick-up coil wires changes the timing about 13 degrees I believe. I think that what I remember as it was a long time ago as it may change the timing even more.
I also found out something about the MP orange ECU. Years back I put a 383 in my 318 Dart and I had to lenthen the pick-up wire harness wires. Well the car had an aftermarket ECU and it ran fine. But when I put the MP orange ECU on it the car would not run as it had no spark. So I got another orange ECU and still no spark. So I check it and do the test where you scrape the male terminal in the pick-up wire harness to ground with the key on and look for spark and I got no spark. So for the heck of it I put a nail in the female terminal and scraped it to ground and it had spark !!! So I must have reversed the wires when I made the harness longer and the MP ECU would not work like that at all but the cheaper aftermarket ECU worked fine. Live and learn. Ron

Funny thing is I also worked at a Dodge dealer for 24 years and when they started using the 3 wire Hall Effect unit in the dist one of the Mopar tech guys told me its better then the AC pick-up because it uses a higher voltage (5 volt signal wire) then the AC pick-up thats about 1.6 volts AC signal so it wont be effected by a dirty connection as easy. So I thought we would not see many AC pick-up units much more but thats not true as many still use them so I guess the Hall Effect is not all that much better. Ron

Hall effect is a cleaner signal than the old reluctor setup, computers don't like dirty signals. As for the signal strength of the old setup, it varies with RPM, at idle it's small, get up in rpm and it can approach 100v. It is essentially an unregulated AC generator.

It is all about how the designer approached what they wanted to do. The magnetic pickup is versatile and reliable and only a 2 wire set up. The box can condition the input signal, and in the end whether they use an PNP or NPN transistor determines whether it triggers off a rising voltage or a falling voltage. Those 50 year old systems still work fine from the distributor portion, even when left to the weather. Clean right up and work. Chrysler was first, other had the advantage of seeing what they did, then follow-up along with improvements that where occurring in the electronic field.

I agree the magnetic pick-up is an AC generator basically. I never checked them at high RPM but I know when cranking the eng they give around 1.6 AC volts on my meters. I assumed the Hall Effect unit was a better signal since its a digital 5 volt on-off signal. But I am not and electronic engineer just a lifetime tech who tries to understand any systems I work on. And that can keep my busy most of the time trying to stay on top of the latest technology as I am sure you know. Sometimes I feel like I wish technology would stand still for a few years so I can stop learning for a little while and take a break. Course no company can afford to do that and fall behind. Ron

I agree as it makes sense. I worked for Chrysler from 1986 until 2011 and when they started changing over to fuel injection they started using pretty much all Hall Effect units on them for the crank and cam sensors which made me think its a cleaner signal with the instant switch of the digital on-off 5 volts compared to the magnetic pick-ups AC signal that generates its AC signal to positive and then negative. But the Hall Effect is a 3 wire unit with a signal , power and ground wires. Ron

Once you added a computer and the conditioned 5V power source going a different route was easier. The cars where not built to be performance racers, heck the first version still used points, but with a CD ignition system and the points only fired a simple transistor at low current. Then it was crank triggers.

looking at the posted sine wave (thanks!) where would rising edge/ falling edge occur? The way I see it there is two rising events, and one falling. in one signal. Is that correct?

Generally speaking, the first one to change states is the leading edge, the last the trailing edge, but it really depends on what signal is being looked for. If the circuit expects to see a negative going voltage as the valid trigger then the leading edge will be the first one to drop below zero volts and the trailing edge will be the first one that goes back towards zero. Usually there will be a minimum expected voltage, such as once it passes -.3v it counts as a valid signal, this is to minimize noise's effect on the circuit causing false triggers.

I'm using a lot of caveats here simply because I don't know the actual circuit design and am guessing on a lot of it. If I had the schematic I could, eventually, sort out how it's supposed to work.

OK, so it is only looking at half of the wave, either the positive or negative portion?

most likely, it's just a trigger so the ECU knows when to fire the coil. Back in the days of points the points would both charge the coil and cause it to fire when they opened. With the advent of electronic ignition the ECU would charge the coil and the pickup would trigger the ECU to fire the coil.

You are right as I remember looking at how some of the point operated electronic and CD ign was wired. I believe Mopar , Ford and GM all had point triggered electronic CD ign in the later 60's. And you are also right about the basic ECU not having the 5 volt system that once they went to a Fuel injected PCM they had to have a 5 volt circuit. Ron

To be honest, they didn't have to have a 5v setup. there are ways to have conditioned the old style signal to be compatible, but it was cheaper to go the way they did if they were already adding a bunch of sensors and such that used 5v. I am putting a dual TBI setup on my 51 Plymouth's flathead and am using the old school mopar electronic ignition distributor guts from a /6 in my 51's distributor as the ignition trigger.

[quote=Sniper]To be honest, they didn't have to have a 5v setup. there are ways to have conditioned the old style signal to be compatible, but it was cheaper to go the way they did if they were already adding a bunch of sensors and such that used 5v. I am putting a dual TBI setup on my 51 Plymouth's flathead and am using the old school mopar electronic ignition distributor guts from a /6 in my 51's distributor as the ignition trigger. [/quote

I just meant once they went to 5 volts to work all the sensors as you said then it was easy to go with Hall Effect. Good luck with your 51 as it sounds like a cool ride. Ron

I said that originally. As I also stated points where used to trigger CD type box. Points are opening against a higher current flow for a normal point system, hence the blue spark and wear to contact surface. In the petronix race set up the points no longer directly fired the coil. The coil is charged and discharged through the large power transistor that is heat sunk on the large hunk of aluminum. The points just handled a much smaller milliamp lever of current to fire a circuit to turn the power transistor off. That is why they moved to the shorter and smaller contact surface points. They could handle much higher rpm without bounce. If you used them as normal points though, they would wear out much faster because of the smaller contact area.

Through resistors, capacitor you can create electronic filters (high or low freq pass). So that AC input signal that looks like DC pulses but is considered AC because it reverse polarity can be cleaned up to simple negative pulses never even seeing the positive voltage spike. Early transistors needed a higher Vbase typically to fire. .5 to .7V range. New stuff much lower voltages, plus the move to FETs and who know what now. Let alone what can now be done by a microchip controller.

Your right as I understood right away that the points in the electronic CD ign only carried a low current compared to normal point ign systems. That was one of the good features about them way less point wear and much more point life with more spark. Ron

Just a update I did get it to run on falling edge.....The dist. had to be advanced significantly from where it was at rising edge.

was there any increase in performance doing that ?

Not sure yet from a run through the gears, full throttle stab...but It did change the idle, sounds better, and if I hold it steady at 5000 It sounds different now too. The timing, after adjustment, seems to be the same but the idle is completely different. Don't know why. If I had to guess it would be that the trigger point is more consistent, event though I couldn't see it in the light. It could have been a rotor phasing issue before also with a different trigger point. They will run both ways though.

What do you mean by rotor phasing? At low RPM your giving a cleaner signal in to the box as discussed earlier; off that falling voltage. So I imagine it did clean up your idle. That is what the box wanted. You had to reposition the distributor to get the correct timing for the back side of the reluctor tip.

Same box here - Pertronix digital HP, and I set mode 9 to 4000 for "magnetic trigger falling edge". Still using stock dizzy but I replaced the Mopar ecu with this box. Timing was nearly same as it was before. Using this box because it has start retard which is mode 7- and I set that to 4500. My initial is 24 degrees.

Position of rotor to term. in cap at time of spark. Was thinking changing trigger point would move this.

These replaced MSD's. I wonder if MSD is a rising edge set up? Because just changing the box leaving all else the same it would only run on rising edge.

I remember having a RB motor on a engine dyno that had aftermarket magnetic pick up distributor female plug that wouldn't plug into the Mopar ECU boxs so I adapted a Mopar type male mag pickup plug onto it and I didn't know the color code of the Mopar mag pick up versus the after market and of course I got them reversed. the motor wouldn't start on he dyno until I retarded the distributor a bunch.
Luckily the dyno operator recognize the symptoms and we fixed it, it was 40 crankshaft degrees difference in having it wired the correct way and having it reversed