Mopar Electronic Pickup Polarity and Rotor Phase

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.

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....

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!

This should Be Sticky! Thanks

I am a poor communicator. Perhaps someone with better writing skills can improve, make it readable.

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.

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


motivates me to spin up my Bosch moduled LA "ready to run" distributor for a looksie.

KitCarlson said:

I am a poor communicator. Perhaps someone with better writing skills can improve, make it readable.

Click to expand...

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.

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

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.

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!

KitCarlson said:

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.

Click to expand...

Nailed it again!

Mattax said:

When I find it I'll put a link here.

Click to expand...

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.

KitCarlson said:

The lower trigger trace was generated using ncv1124 variable reluctance interface.

Click to expand...

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.

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.