What's the normal polarity of the zero-crossing of an HEI reluctor signal at firing time, negative to positive or positive to negative?

Having a little trouble deciding which way to plug in the wires to the HEI switching module?

I honestly don't know which direction the coil is wound, so I couldn't say with any certainty. I've monitors the output (single trace) and can only see the AC. Without a second trace/trigger from the HEI output, determining the waveform timing is impossible. Maybe if I get bored someday I'll drag the scope out to a running engien and check it out.

What I do know is that the threshold level for the HEI seems to be about 0.7 V. If the output of the pickup is wired correctly, the increasing amplitude of the output wave creates a signal that reaches the threshold sooner, and the resultant ignition timing actually gets a bit of advance from the earlier signal. Even without the EST active, the ignition timing will advance a couple degrees at increased RPM due to this effect. If the pickup output is wired backward to the HEI switch/amplifier, the timing will retard at increased engine RPM.

I'm only presuming the reasons you asked, so that may not be of any help to you.

It depends on what you are connecting it to.

If it's wired to a GM HEI module, it wants a positive-going wave first. Then a 7-pin GM module generates a one-shot DRP output, similar to the diagram below.

If you are wiring to an MSD box, the input is usually reversed, IIRC, to match the MSD input circuitry.

HTH

The falling edge signal (pos to neg) is the coil firing time. The pickup coil signal has a lazy rise waveform with a sharp fall time.

The ignition module sends this as a distributor reference pulse (DRP) to the ECM. Same as above the falling edge tells the ECM to fire the coil.

RBob.

Doc,

Your diagram only shows a 600mV P-P signal. From what I've measured, the peak EMF is quite a bit higher at normal engine RPM of 600 or above - unless my old Tektronics needs some calibration.

Merlin

You are absolutely correct, and I'm sure your scope is too .

I only used a graphic that was handy on the computer to illustrate signal orientation. The diagram is not specific to any particular GM VR pick-up. Note the original qualifier "similar to the diagram" & pardon any confusion this may have caused.


DrJ
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GM keyed them for polarity. I don't know what's + or - but Green is the small blade connector, Yellow is the large blade connector. At least on an old non-computer controlled HEI.

Doc,

Thanks for the graphic. That solves the confusion over the zero-crossing polarity for signal trigger. I wish I had a reliable schematic for an HEI module with EST input. Not that I'd want to make any of them, but it would be good reference. I'm almost certain the output is a power Darlington, but the rest is only complete conjecture.

- - - - - - - - - - - - - - - - - - - - - - - -

Damon,

Well, THAT oughtta solve any mystery over polarity, as long as the wiring harness isn't altered, it's a no-brainer (just my speed!).

You'd think it would be nice and simple, but the problem I'm having is with triggering an MSD. Using the MSD harness to replace the 4 pin module it looks like the trigger signal is still the wrong polarity.

Well, the reluctor/pickup coil isn't grounded, so you should be able to safely wire it either way. If you check the base timing with a light as indicated above, you should find the better way to connect it. Personally, I'd prefer the method that adds advance. If the MSD uses the negative-crossing signal, that's going to be tough to accomplish, since the threshold voltage and correct polarity will get later with increased RPM. Almost makes a guy want to go with an optical array, or install points, doesn't it?

The leading edge isn't stable enough, it adds advance, but the amount of advance you'll get out of it is non-deterministic. I think this explains why my advance curve is stable when I put the distributor on the sun distributor machine, but I get too much advance and scatter on the car.

All of my spare distributors have 466 modules in them. I thought I remembered there was something odd about the 466 module. In the instruction manual for an old Carter Engine Knock Eliminator (anyone remember those?) it mentions using a diode with the 466 module, but I couldn't find anything else.

Vader-

1. It's not that complicated. The GM module arms its trigger when the pick up signal goes below ground (negative); then fires at the next zero-crossing point AFTER the wave has gone positive.

The MSD trigger circuit (for a 6 box) behaves a little differently. As I read it, it (sort of) arms when the pick up signal is negative, then fires AS SOON AS the wave reaches +0.7 v, i.e. just past the zero crossing. Hence the reversed wiring.

The GM circuit is designed to prevent noise from triggering a spark. The MSD circuit is designed to be compatible with either points or a mag pick-up. The circuit input for an MSD 6 Box is shown here (if the site is working today - if not try google): ftp://ftp.diy-efi.org/pub/diy_efi/do...s/msd6a_02.pdf

2. I've never seen a full diagram of a 7-pin module either (there is one available for the 4-pin however).

I think if you break the 7-pin module function into two pieces you can see the approximate equivalents by looking at two other standard circuits. See the illustrations below:

The front end of the GM module (up to the power transistor) is AFAIK functionally similar to the diagram of the National Semiconductor LM 1815 chip:

DrJ
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The back end of the module AFAIK is some sort of power transistor, with zener protection and current-limiting built in. An example of such a Darlington is functionally similar to the diagram of the Signetics Thomson ST BU941 unit:

DrJ
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If you ever want to see a complete single-chip ignition control circuit with adjustable dwell, adjustable current limiting, and some other neat features, take a look at the ST L482 application notes. I believe MSD uses a circuit something like this in their Inductive Ignition spark box:

DrJ
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Found this link from another thread and thought this may have been what you were looking for back then?