MSD users. A survey and a very simple question.

[Jonny H]

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.

[bpu699]

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...

[Jonny H]

Originally Posted by bpu699

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.

[Jonny042]

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.

[Jonny H]

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

[VFR750]

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

Using a "timmy2" plug

[Jonny H]

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

[VFR750]

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.

[VFR750]

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

November 22, 2016 is when I copied this.

[VFR750]

Jonny,

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

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

[Jonny042]

Originally Posted by Jonny H

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

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.

[Jonny H]

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?

[Jonny042]

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.



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?

[Jonny H]

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

[Jonny H]

Originally Posted by Jonny042

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.

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

[Jonny H]

Originally Posted by VFR750

Jonny,

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

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

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.

[VFR750]

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

All others are violet or black is negative. Including their own distributor!!

[Jonny042]

You can say that again....

[Jonny H]

Originally Posted by Jonny042

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?

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?

[Jonny H]

Originally Posted by VFR750

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

All others are violet or black is negative. Including their own distributor!!

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!