MSD users. A survey and a very simple question.

[Jonny H]

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

[VFR750]

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

Has anyone pointed this out to MSD?

[Jonny H]

Originally Posted by VFR750

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!

[tirwin]

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.

[tirwin]

See post #22 in this thread...

MSD street fire in 78sc

green to signal
violet to shield

[jpnovak]

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.

[tirwin]

^^^ Good explanation!

[VFR750]

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.

[tirwin]

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.

[T77911S]

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.

[Jonny H]

^. 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?

[LJ851]

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.

[T77911S]

Originally Posted by Jonny H

^. 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?

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.