I am installing my MSD 6A box but do not want to buy the harness to replace the ICM. Does any one know which wires i connect together. On the MSD side they are Green and Purple and on the Distributor they are Green and White... See pic below.

 

You need the green and white wires from the pick up coil. I see them in the sketch, at the upper right corner of the module. The MSD Probillet distributor has a much better pick up design that doesn't use the troublesome coil that the GM has.

 

But which wire goes to the white and which one goes to the green?

 

I would go white to white just for the best color match. It's an AC signal so polarity shouldn't really matter.

 

There isn't white on both sides....MSD side is Green and Purple... distrib side is blue and white.

 

Sorry. I thought you said green and white in your first post. I should have said green to green, but that wouldn't fit either would it? These are the wires from the pick up coil, which the MSD will use as a magnetic trigger. GM usually used green and white for this. If they are blue and white it makes no difference. Just be sure you have the right wires. You can trace them back to the pick up coil to be sure. Polarity should not be an issue.

The PU coil produces an analog signal It may in fact be 0-2.5 volts and not AC as I said before. I don't remember the last time I looked at this signal on a scope. If you dont get spark, you can try switching the wires. The module is just looking for a pulse to trigger generation of a spark event. The magnetic trigger pick up doesn't seem to be real picky. I have used the MSD and ACCEL triggers and polarity didn't seem to matter.

 

Polarity doesn't matter to get it to fire, but the timing will be affected if it's backwards.

 

I don't understand how. The pick up coil produces a pulse as the teeth of the timing core and pole peice pass each other. What am I missing?

 

It's not a pulse like you're thinking. A magnetic reluctor produces a voltage that gradually ramps up to a largeish positive value as the poles approach, then abruptly changes polarity as they pass to become a largeish negative value that gradually decays. The box triggers on the falling edge of the polarity reversal when the voltage reaches about +0.5v going negative. The signal timing at this point is extremely stable, and the box will always fire within a tiny fraction of a degree of the correct location.

Reverse the polarity of the reluctor, and you've got a signal that ramps up to a large negative value, reverses polarity to a large positive value, and then decays. The box still triggers on a falling edge at +0.5v, but this doesn't happen close to the zero crossing anymore, it happens long into the decaying tail of the signal. The voltage is only changing gradually here, and the rate of change varies with speed, so the exact point where the signal crosses +0.5v to trigger the box varies by several tens of degrees. Connect it this way and you'll have an unstable trigger point that also gets a bunch of extra unstable advance or retard with engine speed.

 

So how do I make sure I am hooking the correct wires up?

 

Easiest is to pay the $6 for an MSD 8861.

 

google image search, 100s of wiring disgrams

 

Okay, so my old school memory was correct. I guess I was told 20 years ago that it was an AC signal but I talked myself into thinking it was a pulse. Thank you for clarifying the relationship between polarity and timing. See? Even old guys can still learn. I'm going to do some studying and see if I can refresh my memory on HEI function.

If the harness is $6, I would buy it and have a clean install.

 

I just went ahead and ordered the harness. I didn't mind the 6 bucks but i have to pay shipping and i hate doing that.

 

I know the info is easy to find, can't member as it been a while and not gonna take mine apart to see but I know it wasn't hard to find. I still got the little connector I cut off in my pocket

 

Its cool. Once i get the harness ill figure out what color goes to what and post it on here for future reference.

 

pretty sure all you need to know is which wire is + and which is - on the pick up coil and the MSD box and hook up accordingly.

since the pick up plugs into the HEI modual, it should give it away, think they are marked + and - on some.

 

There are exceptions, though. For a while I was fighting the timing on my car until I figured out that the polarity of the reluctor signal was wrong, even using the MSD harness. I never tried it with another pickup coil to see if it was just a factory freak, though.

 

A note / tip on top of that.

The wires run out of different ends of the MSD on purpose, don't bundle them all together, cross talk can happen. [been there] I run my coil wires the long way around, so the pick up wire n rest can run direct to Dizzy.

course note its my V8 car with a MSD, not my Bird in sig. I run the MSD HEI cap that lets me use and external oil filled can coil. [better by design, oil cools the coil, in cap epoxy filled ones hold in heat, and being stuff in the HEI just makes it worse. Heat kills.]

 

That's right, especially don't run the pickup wire closely parallel to the coil primary or secondary wires.