Mopar Electronic Pickup Polarity and Rotor Phase

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KitCarlson

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Mopar electronic distributors work on the principle of variable reluctance. The distributor pickup is turns of magnet wire around a magnetic core biased by a magnet. When reluctor tooth approaches pickup nub, magnetic flux is pulled away resulting in a negative voltage. The voltage produced is proportion to the speed of the reluctor. When reluctor tooth is centered with pickup, flux does not vary, voltage drops to zero. As reluctor passes off the flux increases, resulting in a positive voltage at pickup. Between reluctor teeth flux change is small, and voltage goes to zero.
MoparVRCorrect (800x552).jpg

The upper scope trace shows a typical pickup waveform. The lower trace shows a trigger circuit, where the falling edge is ignition point. HEI ignition modules basically look for a negative signal, followed by a positive signal for trigger point. The trigger point is above zero, so it is not triggered by electrical noise. The trigger voltage threshold varies between Mopar boxes, HEI and other ignition boxes, but usually only a few degrees. It is useful to know the trigger point, for use in static timing an engine. Static timing is a great way to start an engine, if an engine does not start, something is wrong! Rotating distributor to get it to fire may get it running like cr.p, not solving any mechanical or wiring issue. Bench testing a distributor with grounded coil HV cable, using timing light to examine reluctor/pickup position gives you the trigger point.

Things that can go wrong:
1. Incorrect reluctor install, there is a small block position, big bloc position, to compensate for rotor clock wise (SB) vs counter clock wise for (BB). Rotor phasing with cap changes when reluctor index moves in relation to rotor, rotor with reluctor as timing mechanical advance increases, and when pickup location varies with distributor body (vacuum advance). I have measured the rotor terminal width, and cap terminal width. The rotor reach is twice the rotor terminal plus cap terminal width, in crank degrees is 40. As above bench testing, and marking rotor positions on distributor body for advance changes, vacuum and mechanical changes. I use vacuum source (mity-vac) and 1/2" variable speed drill, turn shaft end, correct rotation. When turning with drill, reverse for SB, forward for BB. Fast RPM, wrap many turns of fishing line, pull, check with tach.
2. Incorrect pickup polarity results in rotor phasing, and trigger point location issues, the trigger point will also vastly change with RPM and noise. Engine will not run correctly....
MoparVRIncorrrect (800x566).jpg

The above was done flipping top waveform using paint. Notice signal starts with positive followed by negative, trigger point wrong, phasing wrong. Play with distributor timing to get it to run, it may, but never right. Possible back-fire..... when static timed!
 
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That's a nice oscilloscope, makes me wish I had five finger discounted the BK precision digital one at the last place I worked at, LOL! But thank you for taking the time to share this information, it's great to be able to see the physical characteristics of how it's supposed to operate to help to understand the description...
 
The scope is TDS 210 it is about 25 years old. First low cost digital. I took picture with camera. Have ADS 1102C that exports to USB drive, for about $300.
 
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you can pick up a 100Mhz non CRT scope now for about $100+ used. CRT scopes are $20-50 on craigs. I got this model for $100, no box, still had the protector on the screen. Hacked to 200Mhz
upload_2019-8-9_15-16-7.jpeg


motivates me to spin up my Bosch moduled LA "ready to run" distributor for a looksie.
 
I am a poor communicator. Perhaps someone with better writing skills can improve, make it readable.
Kit,
You're doing good. All writing, especially explaining technical subjects, is difficult. Just keep at it! Make as many edits as you feel helps.

It's a good thing to bring up. When looking for something else I came across a post mentioning polarity used by different systems. When I find it I'll put a link here.
Maybe it will help to explain what the scope traces represent and the measurements made. There's guys like Pishta and 67Dart273 who know at a glance, and probably many who have no idea. I'll put myself somewhere in between. Let me try to fill that in and you can correct me.

Here's a picture of showing a reluctor tooth just passing the pickup nub.
upload_2019-8-9_19-48-25.png

Maybe this is too simplistic but if I understand correctly,
when the tooth gets very close to the nub on the windings, the magnetic pull on the tooth causes small shift of electrons in the windings creating a voltage drop and rise.
The top line on your scope shows that voltage drop and rise. Time is left to right
The blue + is where the tooth is moving away and the voltage is rising. (This is voltage across the black and gray insulated pickup wires)

The lower trace, if I understand correctly, is the signal within the amplifier allowing and breaking current flow through the coil.
 
The signal generation is due to change in flux with time, not position. When reluctor is still, flux is constant, signal is zero. Flux value is different with position. The proximity of tooth to nub, reduces flux, the event of passing increases, hence positive. Rotation direction changes trailing edge, not signal appearance.

At cranking signal level is low, only a volt or two with AC meter. At high RPM signal can increase to 60 V or more.

My favorite timing sensors are optical, they provide static signal, zero speed, and have excellent noise Immunity. They also provide a logic ready signal without detection circuit. The lower trigger trace was generated using ncv1124 variable reluctance interface. NCV1124: Comparator, Dual, Variable-Reluctance Sensor Interface
 
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You can likely get a used digital scope cheap on ebay if patient. I recall that people have made software which converts an audio card into a scope, which should be fast enough to capture these waveforms. I have a Fluke Scopemeter 123 which I won by sending in a card from a EE industrial magazine 20 years ago. Problem is the battery would never hold a charge, nor a replacement NiCd pack. Same thing with one we had in my department at work, so I suspect a generic software problem for the charging. I just plug my adapter into an AC outlet when I use it.
 
While a scope shows signal, timing light method of observing trigger point of pickup/reluctor works. Trigger point is at trailing side, leaving center alignment.

Knowing polarity with scope, still leaves uncertainty of connections to HEI module. Timing light at grounded/plug coil output verifies ignition activity, it also verifies rotor positions. Good procedure for bench tests, observing advance operation. Timing lights, and 1/2" portable drills are common.
 
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Just clear as mud I tell you what. lol
 
Perhaps you can help me out. The intent post, was to show how something simple, swaping leads on pickup, or improper assembly of reluctor can be diagnosed using timing light. Timing light used at distributor, observing rotor position, pickup/reluctor. Timing light pickup clamp on coil outputest grounded.

Slapping on a distributor without bench testing is like farting before you know it can be sorted out. Messy pants, is similar to a tracked distributor cap. May not see the tracks, but know pants will stink.
 
Awsome was just trying to explain it to a member having issues. You nailed it!
 
:rofl:
Slapping on a distributor without bench testing is like farting before you know it can be sorted out. Messy pants, is similar to a tracked distributor cap. May not see the tracks, but know pants will stink.
Nailed it again!
:rofl::rofl:
 
When I find it I'll put a link here.
Maybe this was it. Refering to using a 4 pin HEI module with a points distributor:
Yes that works but it works backward, it fires when the points close not open, so you will want to re-clock the rotor, and...
It is customary to instead use approx a 40 ohm resistor. This runs enough current through the points to keep them clean.
MSD boxes use 40 ohms, and so did delta before them.
TFI fires "going high", like points do, opposite of the GM , so using the TFI module is a lot easier.

Speedtalk username: Modok

IIRC modok is Brad Urban.

The lower trigger trace was generated using ncv1124 variable reluctance interface.
So the scope is picking up just one signal. It's clipped on to the magnetic pickup leads.
The lower trace is that signal run as modified by the interface so it looks like (works like) a falling edge trigger ECU .
yes?
 
Yes, that is what was on scope. Think how that follow edge trigger trigger could directly used to drive an ignition IGBT. It is bit more complicated, there needs to be dwell control, to turn transistor on earlier as RPM increases, and current limit, serving to replace ballast resistor.

I have a cheap timing light, added BNC connector to clamp-on leads, use BNC to BNC cable with scope, to see timing light trigger.

Other methods use 50 Ohm 20 W resistor instead of ignition coil, trigger will be when what would have been coil (-) goes high.

Don't try hooking normal scope probe to coil (-). Only use 1kV or more probe for that.
 
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I bought a Hantek scope on eBay or amazon (forgot which) a few years ago. It hooks up to your pc or laptop. Was really cheap and works nice. You get all the digital scope features. You can print, save, whatever you want. There are many different varieties and prices, so you can get as much as you can afford.
 
Hey all, sorry to bring back an old thread but I think my question might be helpful to others... If the leads from the dist pickup are reversed but then the ignition module being used (aftermarket) is set to rising edge trigger instead of falling edge, would the trigger signal end up being correct? @KitCarlson
 
Don't apologize. Good to add to an existing thread about a general issue which interests everyone. Renewing it keeps it alive. After a time, the photos become archive-only.

Re your question, with reversed polarity, most ignition modules will still trigger and you can adjust the distributor so the timing is "about right", but it won't be ideal because the trigger point will jump around so won't get consistent spark timing. That is because when reversed there is not as sharp a leading edge on the rising part of the signal. I just scrolled up to post #1 by Kit. Didn't you see that clearly explained in his O-scope plots?
 
Don't apologize. Good to add to an existing thread about a general issue which interests everyone. Renewing it keeps it alive. After a time, the photos become archive-only.

Re your question, with reversed polarity, most ignition modules will still trigger and you can adjust the distributor so the timing is "about right", but it won't be ideal because the trigger point will jump around so won't get consistent spark timing. That is because when reversed there is not as sharp a leading edge on the rising part of the signal. I just scrolled up to post #1 by Kit. Didn't you see that clearly explained in his O-scope plots?
That is not the full story. Polarity reversal also moves the trigger point in relation to the rotor, which is exactly why, in this day and age, we have the term "rotor phasing." Maybe the term should more correctly be, "rotor to trigger phasing"
 
Don't apologize. Good to add to an existing thread about a general issue which interests everyone. Renewing it keeps it alive. After a time, the photos become archive-only.

Re your question, with reversed polarity, most ignition modules will still trigger and you can adjust the distributor so the timing is "about right", but it won't be ideal because the trigger point will jump around so won't get consistent spark timing. That is because when reversed there is not as sharp a leading edge on the rising part of the signal. I just scrolled up to post #1 by Kit. Didn't you see that clearly explained in his O-scope plots?

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.
 
That is not the full story. Polarity reversal also moves the trigger point in relation to the rotor, which is exactly why, in this day and age, we have the term "rotor phasing." Maybe the term should more correctly be, "rotor to trigger phasing"
Thanks for the correction. I didn't think about that (mentioned by Kit), which can be an even bigger problem since the spark could jump to the wrong cylinder post. What I've done to verify rotor phasing is spin the distributor by hand (out of engine) and flash a timing lamp at the innards (cap off). Insure the reluctor wheel teeth aligns with the magnet center when it sparks. If not, reverse the polarity and try again. When doing this, connect the coil HV output cable to gnd, either directly or thru a spark tester.
 
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.
 
If only this was corrected in dumbed down English.
 
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.

The Summit Billet distributor uses the same trigger setup as factory Mopar electronic distributors; it looks exactly the same under the cap. Where it differs is in the mechanical advance, it uses weights and springs supposedly adapted from the old Mallory units; much easier to adjust than the Chrysler stuff although according to @Mattax doesn't provide as linear/consistent advance with RPM. From what I've seen online it's also the same as the Firecore distributors.

I drove the truck to work today and it seems better but still not as good as it should be. Idk if I'm expecting too much but I feel like at idle with a stock cam it should be smooth enough to 'balance a quarter on the air cleaner'. I also filled it up with gas and this past tank only got 8.5 MPG, the horrible gas mileage is another reason I've been making myself crazy trying to get the tune just right. I've already gone through the carb and know that's good (tuned Edelbrock AVS2). I suppose at some point I'll try switching the pickup wires around and go from there. I do know I need to slow down the mech advance, it's all in by about 2000-2200 RPM which is way too quick, should be more like 3000-3200 RPM.
 
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