Fox Please help: Electrical load creates missfire ?!

CassieL · Dec 8, 2024

Ok first you can have electrical noise in the power circuit. And the alternator can throw noise and put a load on the engine.
Don't be concerned about the start signal
It's only used on push to start models.
If yours is running then that circuit isn't relevant. If your hunting a misfire then use an induction pickup on the coil secondary lead. Then you can see each cylinder firing.
Sometimes a bad pip can cause a random misfire. It's seen as a ragged edge on the pip waveform. I have seen an alternator cause a rough idle due to electrical noise from a bad rectifier.
What scope are you using and what is your scale and time base?

CassieL · Dec 8, 2024

On this, the green trace is pip , this is what a normal pip should look like at idle.
The blue trace is number one injector pulse. You can see the flat line is at 12 volts and is smooth.
All your 12 volt signals can have an AC ripple but less than a half volt won't affect anything. Your scope should have a filter for that.
So again if you're experiencing a misfire, is it on the primary or secondary side?

martroy · Dec 8, 2024

CassieL said:
Ok first you can have electrical noise in the power circuit. And the alternator can throw noise and put a load on the engine.
Don't be concerned about the start signal
It's only used on push to start models.
If yours is running then that circuit isn't relevant. If your hunting a misfire then use an induction pickup on the coil secondary lead. Then you can see each cylinder firing.
Sometimes a bad pip can cause a random misfire. It's seen as a ragged edge on the pip waveform. I have seen an alternator cause a rough idle due to electrical noise from a bad rectifier.
What scope are you using and what is your scale and time base?

I'm hunting for a misfire, yes and no. Here's the catch. Because PIP signal gets "corrupted" and Megasquirt see a RPM higher than rev limiter (6k RPM), than I get spark cut for the split second.

If it's a bad PIP, changing distributor would have fix it and it's not (Cause I've tried another motorcraft distributor). What confuse me the most is when alternator is unhook (terminal B+ or from regulator), I cant reproduce the "misfire". Again, I've replace the alternator and issue still there. I do have another (third one) brand new alternator I could test to see?

Scope I use is this one (https://www.fnirsi.com/products/2c53t). Scale I've tried several from 1ms to 100 ns and scale is 2V/div.

martroy · Dec 8, 2024

martroy said:
I have changed the alternator and issue is still the same. I do have a third one brand new on the shelf.

It doesnt make sense to me 2 on 3 are defect but I will do the swap and report back because this make me sick not finding the cause....

Thank you guys

I've swap for my third one today, a brand new 3G and issue remains the same.

Here's some shot while idling. Yellow is 12V RAW and blue is PIP (Idle_09). We clearly see it's not always a perfect square wave?

And I've capture some shot while creating a load (to get the misfire) and we clearly see noise on the line (3kRPM+blower_01 to 3kRPM+blower_03). Now, what is the right one to find it?

Thanks

CassieL · Dec 9, 2024

The last picture at idle is a good pip. That's the sample rate i would use.
PIP should never look like it soes in the other pictures. On the yellow, is that more than .5 volt oscillation?
Is this taken with the alternator plugged in?
Is this taken with the spout in?
Have you captured the misfire?

CassieL · Dec 9, 2024

This is an example of a pip dropping out and causing multiple misfires and stalling.
This was a bad tfi module.
The yellow trace is coil control.
If you probe the coil control you'll see if you're losing it. A problem with the pickup or module will always result with an issue with coil control.

martroy · Dec 9, 2024

CassieL said:
The last picture at idle is a good pip. That's the sample rate i would use.

When I look the 4th pattern, there is a edge missing, witch I tought is not right?

martroy · Dec 9, 2024

CassieL said:
PIP should never look like it soes in the other pictures. On the yellow, is that more than .5 volt oscillation?
Is this taken with the alternator plugged in?
Is this taken with the spout in?
Have you captured the misfire?

As for volt oscillation, grid is 0 to 16V (2v/div) so almost 2V oscillation and pretty sure if I had took that shot at a faster rate, that would be higher...
Yes with alternator and spout in. Misfire is capture on all 3 others pictures.

CassieL · Dec 9, 2024

I have reread this thread and i really i have no experience with mega squirt only factory efi. I didn't know mega squirt could run the tfi ignition.
I would first want to identify the mis.
A secondary ignition prope so you can see the spark line. Also looking at coil control and pip together. A bad pickup can sometimes be caught when it runs better with the spout unplugged.
Also, is the condenser on the coil?
That's what filters out radio noise.

Have you done a voltage drop test on the battery, engine, alternator, distributor and computer grounds?

martroy · Dec 9, 2024

CassieL said:
This is an example of a pip dropping out and causing multiple misfires and stalling.
This was a bad tfi module.
The yellow trace is coil control.
If you probe the coil control you'll see if you're losing it. A problem with the pickup or module will always result with an issue with coil control.

Make completely sense but I get same RPM spike (higher than spark limiter) when there is electrical load and that makes the spark cut.

I'm still looking option/idea how to scope the right thing to fix the thing.

things to remember is Alternator has been replaced by 2 brand new units and distributor and TFI replaced by brand new motorcraft unit. Pretty sure I do have a bad connection somewhere but can't figure it out.

martroy · Dec 9, 2024

CassieL said:
I have reread this thread and i really i have no experience with mega squirt only factory efi. I didn't know mega squirt could run the tfi ignition.
I would first want to identify the mis.
A secondary ignition prope so you can see the spark line. Also looking at coil control and pip together. A bad pickup can sometimes be caught when it runs better with the spout unplugged.
Also, is the condenser on the coil?
That's what filters out radio noise.

Have you done a voltage drop test on the battery, engine, alternator, distributor and computer grounds?

In that situation, I think wiring issue is located before going into the megasquirt and probably unrelated. The sparkcut is coming from the revlimier set in the megasquirt cause the RPM signal (PIP) jumps to the moon.

No I dont have the condenser on the coil, I never had one the past 20 years but may help?
Yes I have done voltage drop test. Alternator case to battery ground in trunk was 73mV while Alternator positive to battery positive was 130mV.

Not quite sure why higher on positive side?

CassieL · Dec 9, 2024

Okay, so spark is cut by rev limiter in ms.
Because pip signal goes nuts.
Ecu uses pip for a crank/cam reference.
This is used to tell rpm and fire the injectors. If you're seeing a rpm jump in ms, are you also seeing it on the tach?
You do need a condenser on the coil. It filters noise.

I'd want to see the actual misfire taking place. A secondary ignition probe is needed. Coil control can be taken from the module or control side of the coil.
Try to capture the coil control or secondary and pip together.

Make sure the voltage drop test is done with the circuit loaded. Max is 500 mv or half a volt.

CassieL · Dec 9, 2024

A ragged pip can indicate a bad pickup and just because it's new doesn't matter.

CassieL · Dec 9, 2024

This is two parts, watch both.

View: https://youtu.be/JUv3D2Q21mQ?si=OAzU_sLlEuz_lECc

View: https://youtu.be/9OR2yaV-3VM?si=HUOPXEiZoO8oI_Nc

martroy · Dec 9, 2024

CassieL said:
Okay, so spark is cut by rev limiter in ms.
Because pip signal goes nuts.
Ecu uses pip for a crank/cam reference.
This is used to tell rpm and fire the injectors. If you're seeing a rpm jump in ms, are you also seeing it on the tach?
You do need a condenser on the coil. It filters noise.

I'd want to see the actual misfire taking place. A secondary ignition probe is needed. Coil control can be taken from the module or control side of the coil.
Try to capture the coil control or secondary and pip together.

Make sure the voltage drop test is done with the circuit loaded. Max is 500 mv or half a volt.

You understand clearly how MS is working in that situation. Yes I see it on the tach too.

My scope is having 2 channel so you want 1 channel on PIP and on SPOUT?

Yes voltage drop was done with circuit loaded and based on results, I checked in.

martroy · Dec 9, 2024

CassieL said:
A ragged pip can indicate a bad pickup and just because it's new doesn't matter.

It a ragged pip split second only when creating load.

martroy · Dec 9, 2024

CassieL said:
This is two parts, watch both.

View: https://youtu.be/JUv3D2Q21mQ?si=OAzU_sLlEuz_lECc

View: https://youtu.be/9OR2yaV-3VM?si=HUOPXEiZoO8oI_Nc

That is really interesting, I've watched both. Thanks for sharing.

martroy · Dec 11, 2024

Update:

I've put scope on Ignition ground (Yellow) and Case ground (Blue) from the harness and seems OK to me, (First picture)

When I rev to 3K and turn on load like blower fan, pretty noisy. (Second picture). As @CassieL asked me, I've probe several things but everything seems to me irrelevant.

PIP (yellow) vs Secondary coil (Blue) (Third picture).

I've put a third alternator brand new 3G and I still can reproduce behavior while creating a load at 2k or higher so now, I think we should remove it from the equation.

I've unhook green wire from Alternator just to get better capture, PIP (yellow) vs TFI power (Blue) (Forth picture). Wondering what means blue spike like this?

I've created a video while idling, alternator disconnected and creating a load by turning on and off blower fan. We clearly see RPM dropped or at least, the needle. View: https://youtu.be/6E8SjcZLfyY

Any help is appreciated cause now, I'm losing my minds...

Martin

CassieL · Dec 12, 2024

So after looking at these and reading through your symptoms i might could solve this but there's details i don't know.
Such as your setup.
I know you're running mega squirt and ford tfi ignition.
Stock harness? SD or MASS? Stock distributor? Any CDI box?
Where does the ECU get power and ground? TIF power and ground source?

I see that without an electric load the symptom isn't there. This looks like a short to power, But i need to know the details. If this car was stock i would suspect a short to power. Meaning that positive power is feeding back though the ignition ground.

I can offer you some tests to try.
I assume you have factory schematics
Or schematics to your system.

martroy · Dec 12, 2024

CassieL said:
So after looking at these and reading through your symptoms i might could solve this but there's details i don't know.
Such as your setup.
I know you're running mega squirt and ford tfi ignition.
Stock harness? SD or MASS? Stock distributor? Any CDI box?
Where does the ECU get power and ground? TIF power and ground source?

I see that without an electric load the symptom isn't there. This looks like a short to power, But i need to know the details. If this car was stock i would suspect a short to power. Meaning that positive power is feeding back though the ignition ground.

I can offer you some tests to try.
I assume you have factory schematics
Or schematics to your system.

Here's my setup

1991 5,0 Mustang
Megasquirt 3 with MS3X for sequential injection with EECIV adapter (it connect straight to stock harness)
I do have stock harness but since I run MS (the past 10 years), I'm not using MAF but MAP (like SD).
Stock distributor with stock coil (No CDI box)
ECU get power and ground from factory harness. Positive is still on the relay in the kick panel. Ground from kick panel and the one that is connected to battery (when located in front) is in the driver front fender. (Yesterday, blue wire from solenoid (PCM power), I've moved it straight the battery and issue still there so I've put it back on the solenoid)
TFI power and ground is stock. If I recall correctly, ground is from PIN 16 and distributor body while power, one from ignition switch and the other from clutch/neutral switch (stock schematic)

I do have factory schematic and pinout for MS EECIV adapter I have.

I'm pretty sure its something really easy to fix (since was working fine the past 10 years) but as now, I'm stuck and would like to have new eyes to probably see something I dont see or even try.

Again, thank you

Fox Please help: Electrical load creates missfire ?!

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