MSD users. A survey and a very simple question.
 

This survey question goes out to those who have any MSD unit replacing a 6 Pin Bosch CDI. Let me state that this is a survey that (I hope) will resolve an ongoing problem and benefit the community.

The MSD unit has a purple and a green wire for connection to the distributor.

Q: Does the green wire connect to:

A. The signal wire of the distributor (pin 7 on the original 6 pin connector)

Or..

B. The signal shield wire of the distributor (pin 31d on the original 6 pin connector)

Please vote but feel free to comment too!

Thanks.

Whichever one works...

I did this a couple years ago, don’t remember this being a big issue... there were threads on here that walked you through it...

Trouble is they can both appear to work so many threads are incorrect. This isn’t about helping me out, I know the answer.

It’s more about spreading knowledge and understanding.

I voted signal from Porsche to Green on MSD.

Therefore Shield on Porsche goes to Purple on MSD?

My only knowledge on the subject comes from 30 second with google and 2 minutes with an MSD wiring diagram.

^. Can you explain which part of the MSD wiring diagram led you to make that choice?

http://forums.pelicanparts.com/uploa...1556747059.jpg


This is what I did a couple of years ago. Still working

Using a "timmy2" plug

^. You are both correct of course but can you tell me how you made that choice based on info published from MSD?

Jonny.

Good question. I can’t quite remember, but I know I spent a lot of time on the diagram and concluded green goes to “7”.

The comment on shielding is important. The violet went to “31d” for grounding the signal.

I’ll take a closer look on why.

http://forums.pelicanparts.com/uploa...1556748667.PNG

Well, this doesn't help, except I searched the forums and followed this guidance.

November 22, 2016 is when I copied this.

Jonny,

Since it is a magnetic pickup, does it really matter?

It is a pulse. (Semi-square wave) right?

The MSD breakdown on the function of the wires listed the polarity of the green/signal as (+) and the violet shield as (-).

The shield will always be (-) or grounded.

Ok, let me explain why I’m playing this game. We are getting increasing numbers of call outs to resolve ignition problems and mostly this is on twin plug or systems with custom distributors.

The one thing they have in common is they have MSD pickups inside. In EVERY single diagram ever made by MSD and in most vendors of distributors using their pickups, the polarity labelling is wrong by conventional electrical standards.

The two posters above both confirmed correctly that the green wire should connect to the signal, yet the green wire is labelled - in all MSD info. The purple wire is labelled + by MSD yet it is connected to the shield which is ground (-).

Furthermore all the MSD mag sensor wires are also labelled +/- incorrectly which leads to repeated wiring mistakes. If you give an auto electrician a wire labelled - he will connect it to ground every time.

I just don’t understand why I seem to be the first person to notice this?

EDIT!!!!!!!! I transposed my Green and Purple in my original response by mistake. In other words, got it completely backwards!!

Which in this case, according to JonnyH, is completely right. Sometimes you get lucky, not smart.

Edit 2 - I also apparently imagined reading about "signal" and "sheild" in the MSD info.

http://forums.pelicanparts.com/uploa...1556749451.jpg

So Jonny, if you go and put a good old fashioned needle type voltmeter (I keep one around and it's often very handy) on the MSD distributor pick up wires with the red test lead on the green/(-), and the black test lead on the purple/(+) you get a voltage spike in the positive direction when you turn the distributor?

^. Yep, you see it is wrong. GREEN IS POSITIVE, connected to signal as everybody knows!!!!!

I agree with you about the shield but according to MSD, green is negative. Check the MSD info for yourself.

You bet it matters! If the sensor is wired backwards, the spark is fired when the rotor is half way between the posts. Often the engine will idle but not pick up. Also, the shallow slope of the ‘wrong’ edge cause dramatic wandering of timing as the revs rise.

According to MSD, only their crank trigger is green = negative

All others are violet or black is negative. Including their own distributor!!http://forums.pelicanparts.com/uploa...1556750699.jpg

You can say that again....

http://forums.pelicanparts.com/uploa...1556750860.jpg

It’s an AC signal so that test doesn’t prove anything. On an MSD pickup the colours are orange and purple. Purple by conventional electrical standards is positive but MSD label this - . Orange is connected to shield but of course MSD call this +.

Then, on top of that, The wire colours on their sensor don’t match the MSD unit, so purple on the sensor goes to......yep green! And orange on the sensor goes to...purple. Hooray!

Confused yet?

That’s showing the sensor side but it is also incorrect. I had a distributor here today with that sensor listed on the top line. Orange/Black 100% needed to go to shield (-). I’ve corrected half a dozen of these so far. All wired by good auto electricians misled by this abuse of polarity!

I am wondering if this labelling is a throwback to when cars were positive earth.

Interesting problem of manuals giving information which, if followed would be wrong.

Has anyone pointed this out to MSD?

I’m going to fire them an email to get an explanation. I needed to come on here first and get ammo. I really though I was going insane at one point!

Jonny,

I had to reverse the green and violet wire pin outs when I made my MSD harness. I didn’t want to cut into the factory harness — I just wanted to plug in into the MSD directly so I could go back if I ever wanted to.

There is an old thread where this was debated. I think it was timmy2 that brought it to my attention. I called MSD and they confirmed that their units trigger the opposite of the way the Bosch CDI triggers. One triggers on down slope and the other is the opposite. I don’t remember which is which. But the guy I spoke to sounded like he had answered this question many times before.

I’ve been running with the green/violet wires swapped for years.

See post #22 in this thread...

http://forums.pelicanparts.com/porsche-911-technical-forum/687837-msd-street-fire-78sc-2.html#post7655686

green to signal
violet to shield

I understand it as the following.

The 911SC uses a 6 pin variable reluctor. Typically, the voltage signal goes from zero to high, then crosses zero to negative and back to zero as a tooth passes its pickup point. So, this is a falling edge, zero point cross over signal.

The MSD is a rising edge trigger device. If you invert the reluctor signal above by reversing the wires then you have a proper, rising edge trigger that the MSD can handle. IN other words, the signal goes to ground and the shield wire goes to signal.

^^^ Good explanation!

And in other words: MSD has known this for probably decades, and yet still can’t find the time to update their multi page document to make this clear.

I see so many Porsche’s with msd boxes, and all had to figure out how to properly set them up, independently of the multipage, and on box, documentation.

Well, it looks like most of their market is American cars and they do seem to have that stuff covered. Would it kill them to update the docs to include Porsche applications? No. But like I said, when I called their support line the guy I spoke with new immediately what I was talking about and told me that rolling the green/violet pin out was necessary.

And most of us tend to read other people’s experience on the forum and that’s how we learn what to watch for — somebody else banged their head on the problem first. So it becomes “tribal knowledge” in that sense. So, yes, it would be nice to authoritatively confirm this stuff and document it.

I can answer without a doubt as for the 930.
the green (-) goes to the + center conductor of the dist.
the signal out of the dist is not really a plus and minus like a battery. its more like AC power. yes, AC has a hot and neutral.

when I first connected the MSD up the engine RPM was much higher and the engine sounded like it was "thin" or "tinty" sounding. the reason was is because the timing was waaaaayy advanced. I did not check to see how much but it was a lot.
so I flipped the wires and timing was almost dead on for what it was with the bosch unit.

so I investigated as to why.
here is a oscope connected to the output of the dist.
the bosch uses a positive to negative part of the signal to trigger.
the MSD triggers on the negative to positive portion.
looking at the scope, the sharp rise from negative to positive is how the signal needs to be for the MSD to trigger properly. I only have the one pic so you will to imagine this next part.
flip that signal upside down. now the sharp edge is going from positive to negative and the long trailing edge portion is going from negative to positive. that is how it looks for the bosch unit

OR. look at the signal as it is. see the long slope after the sharp rise. see where it crosses the 0v or center of the scope. that portion is not stable. it is also waaaaay out of time with where TDC is really happening. THIS is the part that my MSD was firing on when I first put it in.,
here is the real kicker!!! that part is not stable in time. as RPM's increase, that part that crosses the 0v line MOVES!!!! so your timing will actually advance MORE than it should.

this is hard for me to explain typing so I hope I did not confuse anyone,
this is for a CCW dist, I don't know what it would do for a CW dist.
my suggestion for a CW dist is to just connect it up and see. if the timing is way off, flip the wires.
one other thing. I bought a connector that fits the factory connector that goes to the engine. I flipped the wires in THAT connector and left the factory ones alone,. I still have my bosch unit so now I can just plug my bosch back in and reconnect the wires to the dist if the MSD fails.

http://forums.pelicanparts.com/uploa...1556884439.jpg

^. This is absolutely correct as is jpnovak and tirwin.

I started this thread to illustrate that the use of + and - MSD documentation is misleading.

It is doubly confusing for electronics guys and auto electricians as pretty much all VR /Mag sensors and their chipsets trigger on the falling edge. So do all HEI systems. It appears only MSD have a system that fires on the rising edge.

The simplest thing for MSD to do would be to state in their documentation that their systems are rising edge triggered.

With their mag pickups they could say: ‘Use orange for positive edge systems, use violet for negative edge.’ They could argue that this is what it + and - mean to them but it is not explained anywhere!!!

MSD might even show a diagram of a scope trace like the one posted above would also help (it shows a rising edge signal).

I wonder how many hours are wasted due to this oversight?

When i built my "functional" 3.2 distributor to use with PMO carbs i hooked up my analog MSD box per a description i found online.

It ran horribly, rotor phasing was waaaay off from where i designed it and had me second guessing my distributor build. I switched the wires on the MSD and everything acted perfectly as designed.


I wasted about 2 hours and a distributor cap to find the issue.

problem is people don't know what they have in their car or how it works as far as rising trigger or falling trig. I didn't and I didn't even consider it when I put my MSD in. not until it ran bad.
I don't think the installation documentation says its a rising edge trigger. I have some other documentation that I acquired after I put mine in that goes through how the system works. it even has schematics for the unit.

for those that pull their dist and retime their engine could be looking at disaster since the backside of the trigger does move in time and will advance timing as RPM's go up.