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Old 02-20-2010, 05:57 PM   #1
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Default Dwell observations on the Toyota COP.

For some time, I'd been experiencing an occasional misfire. Review of my datalogs showed that this was caused by a dropout in the crank trigger signal, as indicated by the fact that when it occurred, my RPM reading would spike to an impossibly high number.

Simple enough. Adjusted the position of the sensor relative to the crankwheel, tweaked the decoding circuit a tad.

The problem went away... almost. Every now and then I'd still get an occasional hiccup. I was just about ready to tear back into the crankwheel system, but decided to do a bit more logging first. Good thing, that.

The new logs showed that I was no longer getting sensor dropouts, just the tiniest little burble in RPM that you'd expect from a bona-fide ignition misfire. I'd have realized this earlier if I put two and two together and noticed that the tach on the dashboard was no longer dropping out when it happened. So this is now definitely a separate issue- one relating to the ignition system itself.

First order of business: new plugs. And this time, I gapped them down to about .035", as opposed to the .045" that I'd previously been running.

Didn't help. Well, maybe it did, maybe it didn't. The misfires were so infrequent at this point that it was hard to tell. But they were still there.

This morning, I decided to put my fancy new inductive DC current probe to work. Dragged out the scope, hooked channel 1 up to the brown / yellow ignition trigger, and put the current probe on channel 2, measuring the +12 supply into the #1 coil.

Up till now, I've had my "Running Dwell" set at 2.5ms, as commonly espoused by those who came before me in documenting the behavior of the Toyota COP. But wait- the Megasquirt automatically adjusts your specified dwell up or down depending upon battery voltage! And while you can adjust the ratio by which this is done for injector duration, you cannot adjust it for dwell, at least, not on the MS1. What's worse, the ECU's only knowledge of system voltage comes from its own power supply, it can't know what's going on at other points in the harness, such as at the coils.

So, here's what I saw with a warm, idling engine, with 2.5ms specified dwell:



For those not accustomed to reading scope charts, the green line shows the voltage on the line between the ECU and the coil. The yellow line is the current flowing into the coil primary. For the green line, each major division on the vertical is 1 volt, and for the yellow line, each major division on vertical is 2 amps. (My current probe was set to 100mv / 1A, and the scope is set to 200mv / div). Each major division on the horizontal is one millisecond.

So you can see that although I'd specified a 2.5ms dwell time on the MS, it was derating this to approximately 2.1ms based on what it believed to be the system voltage: 14.24 to 14.35 volts as indicated by Megatune. At this dwell, the coil primary current made it to about 8.5A, and was still rising sharply when the coil fired, meaning that the coil was nowhere near saturation.

I then entered 4ms dwell into the MS, and got this:



It's now giving me 2.9ms actual dwell (suggesting that the correction is proportional, rather than fixed) and perhaps more interesting, we can see that after about 2.5ms, the coil itself has activated an internal current-limiter, clamping the primary at just over 11A. I didn't realize that they did this- probably goes a long way toward explaining why folks have been able to run them on the stock ECU without the coils self-destructing.


I continued playing. Next, I set the MS to a dwell of 3.2 ms:



Actual dwell is now about 2.7ms, and we're just barely hitting that current limiter at ~11.1A.

Hmm. Let's try giving the MS a 3.0ms dwell value:



That did it. Actual dwell is now 2.5ms, and the coil primary is peaking right at 11A without hitting the limiter. I could probably run just a tad more dwell without putting the coils at any risk, and likely will.

So, this makes me feel a lot better. It will be interesting to see if this fixes the problem, but if I had to bet Hustler's mom's life on it, I'd lean towards "yes." An extra 30% coil current should translate to an extra 30% potential ignition energy, and that's gotta help.

It also confirms the measurements which were taken long ago by others, with the obvious caveat that you have to make sure that your ECU is actually doing what you tell it to.
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Old 02-20-2010, 06:10 PM   #2
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Dear Joe,

Please help me fill in my dwell voltage correction curve while I digest your information.

My current curve looks like this:

Code:
Voltage    Correction
6               500%
8               248%
10              168%
12              128%
14              102%
16              88%
EDIT: rereading, it seems like my curves should be okay as is. at 12v I'm adding 28% vs. your 17% number. Maybe I need to back it out.
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Old 02-20-2010, 06:15 PM   #3
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Note to self:

If car misfires, take it to Joe P's house.

--Alex
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Old 02-20-2010, 06:22 PM   #4
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Quote:
Originally Posted by Joe Perez View Post
But wait- the Megasquirt automatically adjusts your specified dwell up or down depending upon battery voltage! And while you can adjust the ratio by which this is done for injector duration, you cannot adjust it for dwell, at least, not on the MS1.
On 029v, open "msns-extra.asm", go to line 7068. There's your dwell Vs system voltage table. Correct to your heart's content, run "compile.bat" to generate the new binary (I run "wine ./casm08z.exe msns-extra.asm S L I M" on Linux), correct your therm values with easytherm, upload the new firmware. Easy as 1,2,3,5,8,11...

Jim
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Old 02-20-2010, 06:40 PM   #5
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... 21, 34, 55, 89, 144

Thanks, Rev. Never occurred to me to look at the source.

Dwelltv is obviously the batt correction factor. Very curious as to what dwelltf is. I see it getting called twice, once is a straight STA, the second is an STA after a DEC. Curious to see where those wind up getting called...
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Old 02-20-2010, 09:03 PM   #6
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You're seeing 8.5A @ 2.1 ms @ 14.3 V which is pretty much spot on with what I've measured here:

https://www.miataturbo.net/forum/t11744/

When the internal high current driver current limits at 11A, it will dissipate a lot of power, so it should be avoided. In the above tests I did, the current limit varied with temperature so a bit less dwell will keep things safe.

The firing capability of the ignition system is pretty much a function of the voltage at the plugs, which is proportional to the current. I will guess that the spark voltage at 8.5A is plenty for firing plugs with a 0.035" gap, so something else is causing your misfire, or if the extra dwell fixes it, perhaps the extra ignition strength is hiding another problem.
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Old 02-20-2010, 11:25 PM   #7
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Joe, when were your misfires occurring? 11A is none too shabby, but 8.5A through .035" should be strong enough to light off the charge at any 'moderate' manifold pressure. We scoped a fairly hot OEM system on Friday that was hitting only 7A, and I think the factory gap on that car (SRT8 Challenger) is something ridiculous like .049".

MS2 and later has a dwell correction to battery voltage table.
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Old 02-21-2010, 04:41 AM   #8
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Quote:
Originally Posted by Joe Perez View Post
... 21, 34, 55, 89, 144

Thanks, Rev. Never occurred to me to look at the source.

Dwelltv is obviously the batt correction factor. Very curious as to what dwelltf is. I see it getting called twice, once is a straight STA, the second is an STA after a DEC. Curious to see where those wind up getting called...
dwelltv is the voltage. dwelltf is the factor. So, with the following table:

051T,068T,085T,102T,119T,136T (which is 51%, 68% 85%, 102%, 119% and 136% of 12 volts)
250T,124T,084T,064T,051T,044T (which is x3.9, x1.9, x1.3, x1, x0.8, x0.7 of the running dwell)

if your voltage is 6V (51%), your final dwell will be running_dwell*3.9, etc.

Jim
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Old 02-22-2010, 02:04 AM   #9
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based on Jason's curve. I went ahead and changed my curve to this:

Code:
Voltage    Correction
6               180%
8               160%
10              120%
12              100%
14              86%
16              76%
any thoughts?
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Old 02-22-2010, 05:57 AM   #10
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It would need to be scoped. I would imagine, if using the oem power supply to the coils, that you are under dwelling them.
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Old 02-22-2010, 07:48 AM   #11
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Quote:
Originally Posted by Braineack View Post
based on Jason's curve. I went ahead and changed my curve to this:

[code]Voltage Correction
6 180%
8 160%
10 120%
12 100%
14 86%
16 76%
[code]

any thoughts?
Maybe I understood something wrong but my curve looks like this:

Code:
9                156
10               140
11               128
12               116
14               100
16               88
I did this because I thought - that if you were running at ~14.0V most of the time - you want that 2.6ms aka 100% at that spot - wrong?

Greets
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Old 02-22-2010, 07:59 AM   #12
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I was going off this data:



It runs fine on the original curve that was in place.
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Old 02-22-2010, 08:09 AM   #13
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But as Joe scoped above - even at 3.0ms running dwell set in the MS (i.e. at ~14.0V] it gives him a total of 2.5ms actual dwell...

What do your ignition settings look like? (There the need for you msq again... ]
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Old 02-22-2010, 09:32 AM   #14
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Well if we look at the code:

Like Reverant says, at 14v you'd take 51/250*4 = .816

so 2.5 x .816 = 2.04ms, pretty close to what Joe got.

same with his 3.0ms values x .816 = 2.448, again pretty close to Joe's actual findings.

am i wrong? seriously I'm dumb correct me if I'm wrong.
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Old 02-22-2010, 10:12 AM   #15
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Don't get me wrong - I have as much knowledge about this as a 3month old...

I think the main question is - what is the 100% point - 14v or 12v - because otherwise we did the same thing - we took the graph and looked up the values und made them to percent.
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Old 02-22-2010, 10:27 AM   #16
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the code straight up says:

Code:
dwelltv: db     51T,68T,85T,102T,119T,136T    ; 6v,8v,10v,12v,14v,16v
dwelltf: db     250T,124T,84T,64T,51T,44T
;Values in table are /4 (i.e. 250 = 250/256*4 = x 3.9)
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Old 02-22-2010, 10:38 AM   #17
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O.K. - what have you put in your ignition settings in MS2 now. (Pleeeeeease post your msq....)
I understand that 12v thing now, or better I think I understand.

Would that mean, Maximum Dwell Duration should be set to 3.0ms?

Greets
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Old 02-22-2010, 10:57 AM   #18
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the way I read it, here: http://msextra.com/doc/ms2extra/MS2-...on.htm#moreign , you'd set the max dwell time to 2.5ms (like I have) and then just use the battery compensation to compensate for voltages. I just can't tell if it'll never spark above the max dwell time or not, but i see no other way to set it other than using "time after spark."

MY spark settings like like the 9494 DIYPNP values but with 2.5 set as max dwell duration. So far it's been running without a hitch.
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Old 02-22-2010, 11:03 AM   #19
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Quote:
Originally Posted by Braineack View Post
am i wrong? seriously I'm dumb correct me if I'm wrong.
You are absolutely right, but the problem lies in the fact that you're referencing voltage at the ECU for the correction factor, which is a problem because there's less voltage at the coils than at the ECU. The only way to properly set this up is with a scope, otherwise we're just guessing!

And my 'best guess' is that your original settings in post #2 were probably the most correct.
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Old 02-22-2010, 11:04 AM   #20
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I guess if it ain't broke... it's running hella strong, and I'll be dynoing my MS-II unit vs. my MS-I unit on the same spark map on April 10th.
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