DIYPNP 91 Miata only firing one coil output

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DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Mon Sep 25, 2017 7:06 am

Hello,
I have a DIYPNP that I built that has been running for about 4 years on the car. After about 10 minutes of driving after about a month of sitting, the car began misfiring. Looking at the spark plugs, the car is only firing one coil (this is a wasted spark system with 2 coils driving 2 of the 4 cylinders each). I have since ruled out almost every part of the ignition system - plugs, leads, coilpack, crank angle sensor. One of the terminals on the coilpack plug just has no signal.

This seems to point to the ecu. One thing I have found is that the stock ecu puts out 5v on both the 2E and 2G pins (these are the two signals from the CAS on my car - on the megasquirt they are instead called 4E and 4G). However, the megasquirt is putting out nearly the battery voltage (about 0.5v less ~11.5v) to one of these pins of the CAS, the 4E pin on the ecu. It is giving a normal 5v to the 4G. When rotating the CAS by hand with the stock ecu, the CAS will click audibly, and the voltage will drop between 5v to 0v signalling the crank position. With the megasquirt there is no noise, and the voltage drops oddly between 11.5v to 0.5v for example. I believe this is the issue causing no signal on that one coil terminal.

I have a log here of a short drive with the car, the forum would not let me upload as it is over 1mb (https://drive.google.com/file/d/0BwOgVA ... sp=sharing). When driving the car seemed to function normally for perhaps the first 1/2 of the log, and then misfire heavily during the second half. I am not sure how to interpret this, or if it shows anything valuable.

Does anyone have any idea what part of the circuit would cause this, or how to diagnose it? Is this definitely not a standard thing that the megasquirt would run this 4E signal pin as a much different voltage than a stock ecu would? The ecu looks fine visually and I am stumped now what to do to fix the issue.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Mon Sep 25, 2017 9:08 am

If you are using the stock ignition, the ECU isn't physically capable of firing one plug without the other. What are the odds this is an injector that isn't firing instead? If so, that is likely to be a dead injector.
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Re: DIYPNP 91 Miata only firing one coil output

Postby billr » Mon Sep 25, 2017 11:14 am

I don't think the OP is saying that only one cylinder is dead; says it is one coil.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Mon Sep 25, 2017 10:44 pm

Yes, thats correct - one whole coil is not firing (and thus 2 plugs are not firing) the other coil and other 2 plugs do fire.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Tue Sep 26, 2017 4:29 am

Can anyone see anything in the log out of place? Any help on where I should inspect on the circuit board to find why the ecu is not firing one whole coil? I'm utterly stumped on this. Been through every single ignition part except for the ecu and everything is working as it should..
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Tue Sep 26, 2017 5:54 am

Sorry, I misread this one. If you set the spark output to Going Low (disconnect the coils while doing this test), do you have 5 volts on both spark outputs?
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Wed Sep 27, 2017 4:42 am

Matt Cramer wrote:Sorry, I misread this one. If you set the spark output to Going Low (disconnect the coils while doing this test), do you have 5 volts on both spark outputs?


Hi Matt, Thats ok.

Just measured the spark output on IGN1 and IGN2 (1G and 1H) while on 'going low' and it gave me 1.7v for each. I had the coilpacks unplugged, with the key on but not cranking. Battery voltage was a little low, at 11.7v?

I was thinking, is there a way I could swap part of the circuit inside the ecu to diagnose that one of the ignition outputs is definitely not firing? E.g. if I swapped the outputs then the car would fire only on the opposite coil (because the other coil would thus get the working input and the current working coil get the failing input)? I thought about just swapping around where IGN1 and IGN2 went into the ecu plug, but this would mean that I would have to swap my plug leads too (or ignition would be out of phase?) and not end up showing me anything useful?

The difficulty with this issue is I guess that it seems a little irregular. I tried driving the car again yesterday, and it drove and presumably fired perfectly for the first minute before suddenly misfiring. This leads me to think swapping parts of the circuitry and checking the spark plugs after a drive might be the best way to check it? I worry that anything I measure while the car is off, or just idling may look ok, but not actually be.

Is there a component on the ignition circuits that I could swap, e.g. a transistor from one ignition output to the other in an attempt to rule out it dying? Or is this something more driven by the cpu itself, and thus likely to be a fault in that?

Thanks, Hugo.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Wed Sep 27, 2017 7:32 am

If the spark outputs are only giving 1.7 volts, first check the VREF voltage. If that's OK, I'd clean the board to make sure you don't have stray flux shorting something out.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Thu Sep 28, 2017 4:54 am

Matt Cramer wrote:If the spark outputs are only giving 1.7 volts, first check the VREF voltage. If that's OK, I'd clean the board to make sure you don't have stray flux shorting something out.


Hi Matt,
Just tested VREF voltage, it is at 4.9v. I have also found that the spark output causing issues is IGN1 (connected to 1H terminal on ecu) by following it back from the coil to the ecu.

I had a thought - would it be possible to use one of the other spark outputs (c or d - ALED/WLED?) instead of diagnosing the broken output? Otherwise, is it possible to build another spark output in the proto area? I saw the 'spark A output pin' setting in tunerstudio with WLED as an option? How does the car select the second spark output then (I only need 2)?

If this isn't a possibility, do you know of a component on the board that may have died and could be tested or replaced - e.g. a transistor?
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Thu Sep 28, 2017 7:58 am

IG1 comes directly from the processor.

Switching the spark mode to wasted-COP would make ALED and WLED both operate as spark outputs that duplicate the first two outputs.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Thu Sep 28, 2017 9:10 am

Matt Cramer wrote:IG1 comes directly from the processor.

Switching the spark mode to wasted-COP would make ALED and WLED both operate as spark outputs that duplicate the first two outputs.


I see, does that mean that the processor is probably dying then if it isn't outputting?

So WLED copies IG1? Could I then try setting it up as wasted cop (add pullup resistor, wire to adapter board) and just using two of the outputs (say, WLED and IG2) to fire my stock coils, ignoring the other two unused outputs?

Is there any other diagnostic work I could do that would help figure this out - logging, testing parts of the circuit?
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Fri Sep 29, 2017 6:28 am

That would work - often a processor will continue working indefinitely with one failed output and not have the problem spread.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Fri Sep 29, 2017 7:51 am

Matt Cramer wrote:That would work - often a processor will continue working indefinitely with one failed output and not have the problem spread.


Great, I will have a go at that tomorrow! Are you able to confirm which of ALED/WLED would copy IG1? From what I gathered it goes by firing order, so 1-3-4-2, A-B-C-D, meaning that spark C (WLED) would be 1+4 for a wasted spark setup and replace IG1?

EDIT: Have wired in the new WLED output and removed the IG1. Car ran perfectly on wasted COP mode until I got it out of the driveway. Revved it a bit harder to get it moving up the hill and it instantly started misfiring like before. At a loss what to do now.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Mon Oct 02, 2017 6:16 am

Yes, they copy the first two spark outputs. Do you have a data log of what's going on with the new settings?
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Re: DIYPNP 91 Miata only firing one coil output

Postby Hugo999 » Fri Oct 06, 2017 12:15 am

Matt Cramer wrote:Yes, they copy the first two spark outputs. Do you have a data log of what's going on with the new settings?


Hi Matt,
Sorry for the late reply. The car is in a shop that specialises in my model locally. They have tested the car for fuel and spark and are saying that it always has spark. They are suggesting that the problem is the ecu, and that it is causing the car to overfuel? Don't have a log of the new setup sorry, and won't be able to get one till I can tow the car back.
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Re: DIYPNP 91 Miata only firing one coil output

Postby Matt Cramer » Tue Oct 10, 2017 6:29 am

Well, it's definitely possible for the wrong settings to make a DIYPNP overfuel this car. A log and MSQ will give us more to go on.
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