Using an oscilloscope in place of DVA (peak voltage adapter)?

[badtomatoes]

Hi everyone, I'm trying to chase down an issue with my 1990 evinrude 70. I have an intermittent "miss" issue. Sometimes the engine dies but usually restarts right back up. I've done so many resistance tests that my head is spinning. None of the stator resistance measurements I took were within spec, though they were close enough, I want a little more reassurance. I measured the resistance of the various powerpack leads (SCR's I believe, though I don't know what that stands for), and found one that was way off. I've got a new powerpack ordered, though I plan to further test the stator for peak voltage output. Has anyone used an oscilloscope for this? I am an automotive technician and well-versed in the use of a scope- just curious if anyone around here has used one for this. If I can slip away from the family for a little while, I'm going to give it a shot. I am going to take screen shots of the results and can post them here if there is any interest. I am kind of shocked at how little information is available about this. A little while ago I even read (not this site) a post where a guy was calling BS on the use of a scope. Anyway, sorry for the long post. Just wanted to ask... Cheers.

 

[daselbee]

I have used my scope on the various circuits of the ignition system. I have tried it on Mercs, Evinrudes, Honda four stroke pull start, Suzuki 4 stroke....

It really has not been helpful. First off, it is very hard to get a good sync. The outboard speed is always varying. The resulting waveform picture is very "unstable" and difficult to interpret.
Secondly, there is no published spec for what you should be seeing. Is the waveform right, or is it wrong? Who knows. Sure, you can analyze it yourself and using your best electronic guesswork try to come up with a good or bad assessment.
Thirdly, it is a very dirty signal. The horizontal sweep speed is so slow that the trace is very thick and bright for most of the sweep, only to have that high speed pulse pop in there, which turns out to be almost invisible. Hard to pick out anything to pinpoint as "good".

I have an old Tektronics analog scope.....you refer to screenshots....maybe a storage scope would work better to try to keep that signal still long enough to analyze it.

 

[badtomatoes]

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[badtomatoes]

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daselbee, thanks for your reply. I did my testing this morning and it was pretty simple. Granted, I've got a pretty high-end 4 channel automotive DSO (Bosch MTS 5200). I hope that doesn't come off as gloating or as bragging. I was just curious if anyone else was using this testing method. The resistance measurements I took were out of spec enough to make me seriously question the stator, but not enough for me to drop $200 on one. I don't have a DVA tester and was thinking about buying one, then came to my senses and realized the scope would be perfect for this. I can be a slow learner at times I guess. I don't have a way to upload the screenshots directly from my scope to my iPad, so I had to take a picture of the frozen screen with my phone. The signal shown below is from one of the stator leads to ground. Only took a few seconds to capture the signal. According to the manual, any measurable voltage means the stator is grounded and requires replacement. The scope makes it pretty easy to see how much voltage is being produced. The scope I've got is fairly pricy, but I'm pretty sure you can do the same with a Pico scope at probably 10% of what my scope cost. Anyway, sorry to ramble. I'm just a constant tinkerer and my engine issues were driving me absolutely nuts, mainly because I didn't know what the problem was. Hopefully, I'll be reporting back that all is well in a few days. Cheers

 

[daselbee]

Ok....good pic there.....but what are you looking at? What is the analysis in your mind of that waveform?
What does it tell you?

Enough of the rhetorical questions....for real now...which stator line were you on when that pic was taken?
If I knew that, I could try to figure out what the pic was telling me.
So, am I looking at a 50v P-P AC signal...is that correct? Not sure how to read your vertical settings....
Lines from the stator generally speaking do only two things.....provide alternator style voltage and current to the reg/rectifier for battery charging, and provide an AC voltage to the powerpack to power up the ignition system. That is one nasty looking signal for either of those purposes, if you ask me.....but it works...so....

You see how nasty and choppy that is...the DVA meter/adapter takes that "AC" signal and stores it in a capacitor which charges up to a steady fixed DC voltage, easily readable with simple voltmeter tools. It gives you a steady DC representation of that messy "AC" signal.

Also, if it were me, I would speed up the horizontal trace a bit to spread out that waveform in order to try to figure out what I was looking at.

I would also be very interested in seeing the pic from one of the trigger lines from the timerbase. That is the signal, for me anyway, that is hardest to debug and verify with a DVA since it is such a low voltage.. 0.6v I believe on a 200 looper, 1996 timeframe, CDAL6 ignition family.

I think the DVA method was developed simply because the average shop/mechanics need something much simpler to use for diagnosis. Scopes are kinda overkill in a way.

 

[daselbee]

According to the manual, any measurable voltage means the stator is grounded and requires replacement.

I would not be replacing a stator based on that statement. Something doesn't make sense to me on that one.
Also note that the above statement from your manual was not based on measurements taken with a SCOPE. It was probably based on measurements taken with a DVA....

Again, which line we measuring here? Which engine?
Also, these voltages are an AC power source to the load (PP or RR) so is it logical to put the meter leads across the two wires in question instead of from one of the wires to ground? Waveform pattern will be very different.

PM me with your phone number....would be good to talk this over....

 

[daselbee]

According to the manual, any measurable voltage means the stator is grounded and requires replacement.

I would not be replacing a stator based on that statement. Something doesn't make sense to me on that one.
Also note that the above statement from your manual was not based on measurements taken with a SCOPE. It was probably based on measurements taken with a DVA....

Again, which line we measuring here? Which engine?
Also, these voltages are an AC power source to the load (PP or RR) so is it logical to put the meter leads across the two wires in question instead of from one of the wires to ground? Waveform pattern will be very different.

PM me with your phone number....would be good to talk this over....

 

[badtomatoes]

That was the only screenshot image available to me at the time. The sample was from terminal A (if I remember correctly) on the 5 pin amphenol connector to ground. According to the manual, there should be no measurable voltage present on either terminal (A or B). Both terminals produced a voltage. This test is supposed to determine whether the stator is grounded. The signal is just over 10V peak to peak. The stator isn't being replaced on this signal alone. Both resistance tests available were below specification, though not enough that I felt comfortable justifying the purchase of a new stator- thus the desire to do some further testing. The voltage output of the charge coils were below spec, as were all other outputs with published specs. The engine in question is a 1990 Evinrude 70hp.

 

[woody66912]

I use the snap-on vantage (1) with a D V A . I can always catch the drop outs or low output voltage. I bought mine some time ago when snap- on came out with better obd 2 some dealers have them just laying around, I asked my dealer just last year if i could sill get service on it, no you cant so he just gave me two of them.