MSD6a Spark Output Setting Correction

[ashford]

under the msd ignition setup page it states to use the igbt driver, mod th ini for 25ms dwell and set spark output to normal. i understand the reason for the ini mod it is to more or less simulate real points for they work on a fixed degree not a fixed time. the high current driver i really wouldnt have any other way. but i think setting spark output to normal is incorrect. the ini mod increases dwell alot. it moves the the rising edge away from the falling edge while the falling edge doesn't move and a coil fires from the falling edge. add the igbt and now its inverted rising edge is fixed and falling edge moves. and what i read on msd and other ignition setups the system fires when going low or when negative current is lost.

quote from msd
CHECKING FOR SPARK
If triggering the ignition with the White wire:
1. Make sure the ignition switch is in the "Off"
position.
2. Remove the coil wire from the distributor cap
and set the terminal approximately 1/2" from
ground.
3. Disconnect the MSD White wire from the
distributor's points or ignition amplifier.
4. Turn the ignition to the On position. Do not crank the engine.
5. Tap the White wire to ground several times. Each time you pull the wire from ground, a
spark should jump from the coil wire to ground. If spark is present, the ignition is working
properly.

would this setting cause me the irratic idle speed and advance problem ive been having. my timing is set to 15 degrees in all cells below 1500.

[Bruce Bowling]

under the msd ignition setup page it states to use the igbt driver, mod th ini for 25ms dwell and set spark output to normal. i understand the reason for the ini mod it is to more or less simulate real points for they work on a fixed degree not a fixed time. the high current driver i really wouldnt have any other way. but i think setting spark output to normal is incorrect. the ini mod increases dwell alot. it moves the the rising edge away from the falling edge while the falling edge doesn't move and a coil fires from the falling edge. add the igbt and now its inverted rising edge is fixed and falling edge moves. and what i read on msd and other ignition setups the system fires when going low or when negative current is lost.

The MSD (when using the white while originally designed for points operation) will fire when the points open. In the MSD circuit there is a roughly 40 ohm pullup resistor to +12V, so when the points open the white wire will rise to a +12V potential and trigger a discharge. This is equivalent to a lot-to-high transition, or going-high edge.

It all comes from the days of points and ignition coils. When the points open is when the magnetic field in the ignition coil collapses and a spark occurs. If you were to measure the voltage across the points when they were open you would get almost battery voltage (depends on the input impedance of the meter) because the + terminal of the coil is at +12V battery potential, the coil itself is around 1 - 2 ohms resistance, and there is practically no current flow (again depends on multimeter).

As you indicate, with the IGBT the rising edge (from low to high) is the fixed one, and this is the discharge point.

quote from msd
CHECKING FOR SPARK
If triggering the ignition with the White wire:
1. Make sure the ignition switch is in the "Off"
position.
2. Remove the coil wire from the distributor cap
and set the terminal approximately 1/2" from
ground.
3. Disconnect the MSD White wire from the
distributor's points or ignition amplifier.
4. Turn the ignition to the On position. Do not crank the engine.
5. Tap the White wire to ground several times. Each time you pull the wire from ground, a
spark should jump from the coil wire to ground. If spark is present, the ignition is working
properly.

Step 5 is the key. Each time you pull the white wire from ground, the internal pullup resistor pulls the white wire to +12V - and you get a spark.

- Bruce

[TheMonkey]

ashford-

i originally set up my MSD exactly as the manual suggested. my timing was off and i broke the ring gear on my flywheel and had to have my starter rebuilt because of kickback through cranking.

it was firing at beginning of intended dwell instead of end of dwell. by setting max dwell and max spark duration both to 25ms, it basically splits starts dwell exactly half way between each ignition event. not sure how many cyls you have but on a V8, that added 45° advance which was a perfect recipe for kickback.

i bought a scope and sorted through it. my confusion continued a bit because on the scope, dwell period which is a grounded signal appears at about 0.6v, and floating signal appears at 0.0v, and i had these confused. the 0.6v was the ground offset from my ground reference point, and the floating of course should have 0 voltage. but ended up setting everything up just as if it were firing a coil directly and it straightened out my timing.

my main deviation from manual was to set spark output to 'going high' instead of 'going low'. my timing is setup perfect now.

i wanted to eliminate the MSD box for a test, and fire the coil directly. without changing any settings or hardware except for dwell period, it fires the coil directly perfectly and timing is accurate that way too. i might just keep dwell programmed for direct coil fire in case the MSD box craps out on the road or i need to bump-start the car, i can just by-pass the CDI box. not clear to me why it makes sense to always split dwell/discharge in half when using an MSD box. this would require keeping a ballast resistor in the glove box if using an unresisted coil like MSD Blaster2 or 3.

so, i agree. i have stated a couple times that i think it deserves a closer look, because i can't find an explanation of what is different from my setup and the MSD portion of the manual.

read through this post: viewtopic.php?t=30222

[ChevelleFan]

Just a datapoint here...

I'm still trying to get my ignition control fully sorted out, but I had my car running with rock-steady timing tonight using Going High (Inverted). My setup is wired just like the MegaManual describes for a 2.2 board (JP1-5 triggering a 2n2222a, which in turn triggers my MSD)

I did the MSnS-E HEI input mod, using an HEI-4 module to condition the my VR signal, so I think I need to use Falling Edge for the ignition input capture.

-Dave

[krisr]

I have no objection to changing the manual, but want to ask if there's any objections (from people who have working MSD6 systems) before making the change.

I can change 'Going low (normal)' to 'Going high (inverted)' easily enough. The setting that's there now was written by a third party, and we don't have the MSD box to verify it, so we rely on user feedback in this case.

Hey, I think it might be more beneficial to add into the manual something along the lines of "If you are using the VB921 coil driver, please change your output edge setting to the same as Direct Coil Control, i.e use Going High (Inverted)". As essentially an MSD box fires no differently to a normal coil, when going low, but with the IGBT there by default in alot of installs I think it confuses people that the signal has to be inverted at the processor.

FWIW.

Kris

[Mr Gadgets]

Wow!! I just read this post and the one referenced by TheMonkey. I went through this confusion last year when I was trying to set up my ignition. Or was it two yrs ago? Last summer I went from an eratic tach signal using the MSD VR in the dist to an MSD Crank trigger. I built a dist machine from a spare MSD dist and experimented till I was blue in the face. I could see the problem. Over 4krpm, the tach signal would vary as much as 120-150rpm at a 50ms sample rate.
We had a great deal of discussion on how my setup should be configured. In the end, I found that a Ford Dist turns opposite of a BBC. Not sure which Ford engine we were discusing. But trying to follow the sequence of events was very confusing for me.
In the end, before the crank trigger, I changed Spark Output to Going Low (normal). When I switched from the dist VR to the Crank trigger, all I rememeber doing was changing the offset by 3*.
My tach signal became much more stable at idle. I could change my timing in the table by one degree and see it jump one degree on the dampner. After making a WOT pass, my tach signal is also very stable, with a variation on the order of 2rpm to 30rpm, but PW is not affected by it. The reason this was important to me, was because this motor is in my boat. Very similar to running on the dyno, full load all the time.
After that I was able to lean down the motor and I ended up picking up 4.1mph and I regularly see rpm over 6300rpm.
So now I am confused. It works the way I have it, I have not rethought it since then, cause it was suppose to be correct. My setup is as follows:
Ign Input capture... Falling edge.
Cranking trigger..... Trigger Rise.
Coil Charge Scheme... Standard Coil Charge
Spark Output... Going Low (Normal)

I am using the VB921 to drive the MSD 6AL box..
Hope this helps.. and doesnt add confusion..
Thanks!
Dick

[TheMonkey]

Dick-

it can absolutely work when setup inverted, but it just inverts dwell & spark duration. MSD box does not care how long dwell is, it only cares when it ends. but, when inverted, MSD triggers at beginning of dwell.

what's your max dwell set to? i think that if dwell is a small number, the kookiness will be less because the beginning of dwell will remain consistent until RPMs increase enough to start trimming it. if dwell is big (like suggested 25ms), then kookiness will increase because after you adjust offset to compensate for the dwell period, that dwell will change at all RPMs.

there's another thread discussing how long dwell actually needs to be for MSD box. i'm trimming mine back to an ordinary direct fire dwell amount so that if CDI goes bad (in the middle of the lake for you), it can be re-routed to direct fire without reprogramming.

don't forget... when you try switching to 'going high' it will retard your timing, so you'll need to adjust trigger offset. also may need to adjust your whole spark table depending on how big your dwell was set. spark table could have compensated for all the kookiness.

take a look on the stim for what RPM dwell starts to shrink (larger max dwell starts shrinking earlier). with a timing light, you prob have offset wizard lined up just right at IDLE now. as you rev past the RPM where dwell starts to shrink.... if you have it programmed inverted, then you will see the offset wizard begin to go wrong.

Scott.

[Mr Gadgets]

My dwell is set to 25.5ms. So you are saying I should set it up this way, only changing the spark output?
Because the dwell varies and will change my timing. I could rev the motor to 3500rpm and it would stay steady. Not sure what happens above that rpm..

Ign Input capture... Falling edge.
Cranking trigger..... Trigger Rise.
Coil Charge Scheme... Standard Coil Charge
Spark Output... Going High (Inverted)

I have added my msq.. my timing table is set up to be pretty close to a locked out dist.

Thanks!
Dick

[ashford]

i was building a ms for a friend who was going to buy a msd unit later and use direct coil control till then. while bench testing it with a real coil and setting it up, i thought why would the phase change with msd?

the phase changes out of the cpu-igbt-coil is what confused the living heck out of me. So i changed my mindset from voltage to current and that made it more clear. going high out of the processor is current which turns on igbt and charges a coil(points closed) when it goes low current is lost igbt turns off(points open) coil fires.

to be sure on this i kneed to know if the falling edge(from the processor) is the calculated fireing point and not the rising edge.

the roads her finally cleared up and temp is high enough to start my car(north dakota) ill take it to the shop and verify in a few days.


the monkey: make sure your "crank trigger" is set to calculated mine wasnt at first and had kickback problems(bent throttle body butterfly) it was firing at the offset degrees so i changed my distributor orentation to a less advanced spot but then i had cross firing to the wrong cylinders in the cap. then posted this viewtopic.php?p=177480&highlight=#177480

[Anders Karlsson]

I found this thread now, i have some strange problems with my MSII & MSD 6A setup.

I posted this earlier viewtopic.php?f=89&t=34259&start=0
Not the exact same problem, but some similaritys.

Go easy on me with the english, especially "computer language"!