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Old 07-30-2010, 02:22 PM   #1
N3v
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Default F**king pullup resistors, how do they work?

Why do lower resistance pullup resistors across the LEDs on the board create a more powerful spark? I'm talking about the revision made to braineack's writeup where 1k ohm resistors are replaced with 330 ohm.

I didn't think this would affect the ignition's strength because it is only modifying the trigger signal, which should only matter if it is on or off.

This question stems from my troubles using ls1 coilpacks on my 13b swapped miata. The leading plugs are on wasted spark, so I just tied two ls1 coil triggers together, but the other two ignition outputs (i'm using all 3 leds for igniton) fire only a single trailing coil. I was wondering if this fact should be considered in regard to the pullup resistors. Do I need a different value resistor on the one output which has to trigger two coils?

My problem, btw, is that it creates a visible spark but it is very puny and won't start the car or even trigger a timing light. I have the correct dwell settings and 0.5ms minimum opening time. The wiring is good, I ran the power and ground and two of the triggers from the existing miata coilpack harness.

I've deduced that it can only be the pullup resistors (which are all 1kohm) or insufficient grounds. If anyone has ever messed with ls1 coilpacks and megasquirts and has any insights, I would greatly appreciate it.
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Old 07-30-2010, 02:35 PM   #2
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Quote:
Originally Posted by N3v View Post
Why do lower resistance pullup resistors across the LEDs on the board create a more powerful spark? I'm talking about the revision made to braineack's writeup where 1k ohm resistors are replaced with 330 ohm.
The resistor is between the source of power and the consumer of power. Higher resistance values are... well... more resistive. The more you resist the flow of current, the less current flows. Lower resistance, more current flow.



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I didn't think this would affect the ignition's strength because it is only modifying the trigger signal, which should only matter if it is on or off.
Even digital signals are analog.

The thing here is that the inputs on the igniter require a certain amount of current to operate. If you throttle the trigger too much, the igniter acts a little funky and either doesn't dwell the coils for as long as it should or doesn't allow full primary current to flow in the coils.



Quote:
I've deduced that it can only be the pullup resistors (which are all 1kohm) or insufficient grounds. If anyone has ever messed with ls1 coilpacks and megasquirts and has any insights, I would greatly appreciate it.
Are you sure the LS1 coilpacks are designed to operate on a 5v trigger? I honestly don't know, but there are some COPs (Subaru, for instance) that use a 12v trigger.
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Old 07-30-2010, 02:39 PM   #3
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Try using 100ohm pull ups.

yeah the are 0-5V.


A dwell figure of 5.5-6.0ms is advised for LS1 coils.
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Old 07-31-2010, 04:26 AM   #4
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you both are gentlemen's gentlemen. For answering all of my stupid questions over the years, you both are entitled to drive my rotorcar if/when we meet in person.

Now, an update on where I'm at with this:

-I switched the resistors out for 220ohm (matt from diyautotune suggested "270, or even 100" so 220 seemed like it'd be in the ballpark.)
- I tried to crank again, very bright and clear trigger sparks, more visible sparks on crank, but still no timing light reading

I decided that it must be my grounding and I went about measuring it. my battery dropped from 11.9v to almost 8 volts as measured at the coilpack harness during cranking. yikes. I ran a very thick 8 gauge wire (one of those fancy amp install kit wires, says theres as many strands as a 4 gauge) from the negative terminal to the starter, and this improved my cranking voltage drop from 11.9 to around 10v. still no start though!

So I did one more test: I removed the cas from the engine and turned the key to 'on' and spun the cas wheel with my hand. the plugs lit up like a christmas tree, and got a beautiful signal on the timing light. This indicates that push starting it would probably work, which I plan on trying tommorrow, but I need to get this cranking thing figured out!

What could cause that? I'm using the stock miata fuse box, I extended the 12v starter wire from the stock harness to make it all the way to the rx7 starter. I think the rx7 starter might be a beefier starter unit than a miatas, but i'm not sure.

I also tried using a much larger battery from a minivan. cranked a little faster, but still no start and no signal on the timing light. I didn't measure the voltage drop on that test.
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Old 07-31-2010, 06:27 AM   #5
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Slight tangent but some Toyota COPS are 12v triggered, took me a bit to sus that out...
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Old 07-31-2010, 10:49 AM   #6
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LSx coils are 5V triggered. I would use a 100 ohm pullup resistor.

What does your entire ignition system consist of? What CAS? How many coils of what type? How are they wired to the MegaSquirt? LSx coils have a pretty high dwell requirement. I don't think I'd want to run the leadings wasted spark.

Does the MegaSquirt have 12V when cranking?
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Old 07-31-2010, 12:34 PM   #7
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Quote:
Originally Posted by Ben View Post
LSx coils are 5V triggered. I would use a 100 ohm pullup resistor.

What does your entire ignition system consist of? What CAS? How many coils of what type?
ls1 coils, stock 91 cas, megasquirt has the dual vr sensor daughterboard.
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How are they wired to the MegaSquirt? LSx coils have a pretty high dwell requirement. I don't think I'd want to run the leadings wasted spark.


lsx coils have a maximum dwell internally regulated to 8ms, they normally are set to 5-6ms.

I have four in total with three ignition outputs from the LEDs on the megasquirt. it's setup like this:

I tied the two middle coils' trigger wires together then crimped them, that's how it goes into my weather plug
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Does the MegaSquirt have 12V when cranking?
no, the megasquirt drops in voltage the same way that the coilpack harness does. i measured at the battery terminals, and it drops there too.

Edit: newest theory, my 12v to the starter isn't a big enough wire, causing resistance on my wiring harness? it's a 4 gauge though. :/

Last edited by N3v; 07-31-2010 at 12:46 PM.
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Old 07-31-2010, 01:01 PM   #8
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Quote:
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I don't think I'd want to run the leadings wasted spark.
Is this to say the LSx coils will not operate properly in a wasted spark setup?

I've been considering using some of the LS2 coils Leatherface24 mentioned in another post regarding misfire.

My plan(given my coils are the source of my problem) was to swap out the stock coils and for the LS2 setup and run wasted spark until I can get around to setting up sequential ignition...

Is this plan fail?

- sorry to bust into the thread -
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Old 07-31-2010, 04:02 PM   #9
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I just tried to push start it. it brap brap brapped for a second then died. so I checked everything, and apparently a few push start runs drained it of it's 1.5 gallons in the tank, and the battery voltage was 7.5 volts. yeesh. that battery is toast huh
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Old 08-02-2010, 11:17 AM   #10
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Quote:
Originally Posted by N3v View Post
ls1 coils, stock 91 cas, megasquirt has the dual vr sensor daughterboard.




lsx coils have a maximum dwell internally regulated to 8ms, they normally are set to 5-6ms.

I have four in total with three ignition outputs from the LEDs on the megasquirt. it's setup like this:

I tied the two middle coils' trigger wires together then crimped them, that's how it goes into my weather plug

no, the megasquirt drops in voltage the same way that the coilpack harness does. i measured at the battery terminals, and it drops there too.

Edit: newest theory, my 12v to the starter isn't a big enough wire, causing resistance on my wiring harness? it's a 4 gauge though. :/
OK, I was going to suggest running true coil on plug (RX8 mode) but you'd need an MS2 for that. You will need a scope to properly set dwell, but I would guess that you'd need to be closer to the 3.5 ms range to have any longevity out of them at high rpm.
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Old 08-02-2010, 11:21 AM   #11
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Quote:
Originally Posted by Turbobarber View Post
Is this to say the LSx coils will not operate properly in a wasted spark setup?

I've been considering using some of the LS2 coils Leatherface24 mentioned in another post regarding misfire.

My plan(given my coils are the source of my problem) was to swap out the stock coils and for the LS2 setup and run wasted spark until I can get around to setting up sequential ignition...

Is this plan fail?

- sorry to bust into the thread -
I was more referring to running these coils in wasted spark on a rotary since they require long charge periods to deliver hot spark, and a 13b will spin at higher RPMs than your B6 or BP will.
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Old 08-02-2010, 03:25 PM   #12
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If your battery voltage is dropping that low while cranking, it's probably a bad battery.

That's why shops "load test" batteries - make sure your voltage stays solid with a heavy current drawn. The voltage should sag a little, but not excessively.

Where'bouts in Nashville are you? I'd like to check the 13B miata out sometime.
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Old 08-02-2010, 03:27 PM   #13
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fwiw, mine drops to about 10v when cranking.
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Old 08-02-2010, 03:31 PM   #14
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From what I can tell, the minimum voltage you want to see while cranking is right at 10 volts. Anything less means bad battery... too much internal resistance and all.
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Old 08-03-2010, 12:37 PM   #15
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I got it figured out guys. I laid two more ground wires and got my battery replaced (bahaha, autozone warranty) and I got the thing started. All my settings were actually fine, and it might have even worked with the 1k resistors. I just had to pull start the car to get it running for the first time. silly rotary. it also has an exhaust leak and it sounds like a pissed off dirtbike.

I really appreciate everyone's help, and I'll post vids soon. I took it back to my fab guy to get the exhaust fixed, then it should be drivable.
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Old 08-03-2010, 01:48 PM   #16
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Quote:
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it also has an exhaust leak and it kinda sounds like a plane crashing two feet from your face.
I preferred this description, LMAO!
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