.256mV Alternator A/C ripple

Good afternoon all. New Premium member, also new to the lab scope. I took some readings on a 93 Wrangler 2.5L M/T (amp clamp on primary negative) 2 wire coil and am trying to identify what I am seeing here. Looks like nothing I have seen while doing my research. Any input would be greatly appreciated. If I did this correct there should be 3 screen shots attached here. Looking at these wave forms I am leaning twords the ignition driver in the SBEC is not switching correctly. The vehicle has a hard constant miss, feels like it is only firing on 2 cylinders. Good Cam and crank signal happy to post those if needed.       

Your screenshots are wild, DGB! Is your amp scale correct? 'Cause it looks like the coil is drawing 80-ish amps at times. I don't have a known good dwell time for a coil on a 4.0L, but 30+ ms is an eternity. I'm surprised the coil hasn't overheated, or popped the fuse. Or smoked the coil driver.

I don't blame you at all for suspecting the PCM, but I'd want to have another look at the cam/crank before pulling the trigger on a replacement. Would you mind posting the waveforms?

Glad I'm not the only one that thought this signal looked nuts. Not what I expected at all. Clamp 20a scale Scope at 10:1 ignition preset. The jeep is a 2.5L the coil was recently replaced, went bad. Another thing I should mention, I had .256mv AC ripple coming from the alternator. .256mv is excesive but I do not know if it's enough to damage a computer.  Alternator has been replaced. Need to download cam and crank. Back shortly

Here are cam and crank   

Another afterthought. Scan tool is reading a crazy RPM signal. While the engine tach show bouncing between 500 and 2000 rpm scan tool shows it jumping to 6000, 7000 and 8000RPM. Scan tool issue or computer issue, I do not know. Tool is a snap on Red Brick

Scope at 10:1 ignition preset

This is why your reading such high amperage. The scope is multiplying the signal by 10x.  You do not need this setting with an amp clamp. 
This setting is for when you are using an attenuator to reduce voltage input to the scope, like you would when capturing a Coil Primary Voltage waveform.

Ah, gotchya. What do you make out of that crazy wave form I posted? There is somethin definitely wrong. It looks like the ignition coil driver is not firing correctly. I am trying to get a second opinion before I condemn a computer. It looks like the coil is charging up and saturating but failing to be pulled down consistently, or am I reading it wrong?

It looks like the coil is charging up and saturating but failing to be pulled down consistently, or am I reading it wrong?

To see if it is "pulling down", consistently, capture a Primary Voltage waveform. Swap out your amp probe for test leads. Negative lead to BAT (-). Positive lead to the Coil control wire.
When the coil is not energized, you should see battery voltage on the control wire. When the PCM energizes the coil, the voltage on the control wire will be pulled down to zero. When the PCM De-energizes the coil, there will be a voltage spike on the control wire that could reach 400 volts.  This is where you need 10:1 attenuation because the maximum voltage input of the uScope is 40 volts. An attenuator is used to reduce the 400 volts down to 40 volts before it enters the scope. To account for this, the 10:1 setting on the scope will multiply the input voltage by 10 and display the proper, original, voltage output on the screen. 

Are your Coil Current waveforms captured with the engine running, engine cranking, or energizing the coil manually? Because I agree with Tyler. 30+ ms dwell time is crazy. 

 

Got it, I will redo the test. Tests were done with engine at idle.

Here are the screen shots from the new test. Primary coil control back probed. Engine at idle. The shots showing 2 wave forms back to back, is that something I am doing wrong with the scope? none the less these don't look good

             

Is .256mV of A/C ripple leak enough to damage a computer?

I assume you mean 256 mV, or 0.256 V.

I would not consider anything less than 300 mV, 0.3 V,  to be a problem.
300-500 mV would start to get my attention, only, if I was chasing electrical anomalies, But I would not be real concerned until 500 mV, or more.

Oops, yes you are correct. I typed that wrong. And yes I do believe I have some sort of electronic anomalie. That reinforces the info I have learned so far. I have a 93 wrangler that is kicking my @$$. I believe I have narrowed it down to the ignition side. But ignition components are checking good. I am also getting a crazy RPM signal on my scan tool that does not reflect what the actual tach is reading. Why I am leaning towards the computer and if it is the SBEC did something else fail and damage it. Thank you for the input greatly appreciated!

I do believe I have some sort of electronic anomalie... I am also getting a crazy RPM signal on my scan tool that does not reflect what the actual tach is reading.

Excessive AC Ripple will, absolutely, affect a 2-wire VR type sensor. However, this crank sensor is a 3-wire hall effect sensor, and I am not very confident that a ripple of less than 300 mV could be your problem. I suppose, you could fully charge the battery and disconnect the alternator for comparison. 

Here are the screen shots from the new test. Primary coil control back probed. Engine at idle. The shots showing 2 wave forms back to back, is that something I am doing wrong with the scope? none the less these don't look good

 
Base on those Primary Voltage waveforms, I would pull the spark plugs to make sure they are not fouled. 

But, the thing that concerns me the most is the back-to-back firing events. It will be impossible to do with a single channel scope, but I would like to see a waveform of Cam, Crank, Coil control, and #1 injector control, all at once. 
 

I unfortunately do not have access to a 4 channel scope. What caught my eye is on some of these snap shots the voltage is not coming back up to 14V after coil discharge like the very last screen shot (S015.bmp) which looks like it should. 

What caught my eye is on some of these snap shots the voltage is not coming back up to 14V after coil discharge like the very last screen shot (S015.bmp) which looks like it should.

Good eye! If you had a 2-channel scope, I bet the current waveform would show these areas as some crazy-long dwell times. I think that I would disconnect the coil control wire from the Coil and PCM. A continuity to ground check with an Ohm meter should display OL. Give the wiring harness a good wiggle and watch the Ohm meter for any continuity to ground. If no faults are found with the control wire, I would begin to suspect a faulty PCM. However, I would still want to see Cam/Crank and coil and injector controls, all together. 

I opened up the harness previously. Control wire was not in good shape, it has been replaced and re-crimped with new female pins. I will have to look around and see if I can borrow a 4 channel from someone. Not sure how that will work out for me. lol we'll see

BTW you are correct about the crazy dwell times. Lol Don't need to put them side by side to notice. Scroll up to the beginning of the thread.

I didn't think of it before. Voltage drop on that coil control wire, no telling how long it has been that way. Could have stressed the driver?