Neg coil terminal V & o-scope

If all you're getting is a 45 volt spike when the primary switches open, you have a problem in the primary, normally the voltage spike is in excess of 200 volts, well in excess. (300 volts = real healthy)

Sounds like you found (bought) some secondary pick-ups for your scope. d:-) With the engine idling, if you grab one plug wire with some insulated pliers and carefully pull the plug/wire connection apart, you should see an increase in secondary firing voltage as that gap increases, this is one method of assessing how many Kvs the coil is capable of putting out. Do NOT go full open circuit if you can help it! If this is a round, oil filled coil, you should be able to get close to 24 KV out of it under near open circuit conditions.

If not, either the coil is toasted (kaput) or the spark energy is finding an easier path to ground, like thru the ignition rotor to center shaft, coil tower tracking, etc.

I did get a capacitive and inductive pick-up set, and now have my eye on the low-current probe you recommended :-D

Would this work as an alternative to ck the coil's upper KV capability: ground a 5th plug w/a big gap - say 6mm - 8mm - run one of the 4 plug wires to it and idle (catcon test pipe in place)? Enough resistance thru

Has been/continues to be good a learning experience. Was expecting to see spark line sloped for excessive resistance - but didn't. It's pretty erratic at the end of the firing section/start of intermediate section. Also have decent number of coil oscillations.

WRT the primary pattern (at idle), there's a 2.5 ms section where the trace drops 12V below the 0V line, angles up to 7V below for subsequent

I failed to mention that I do get some arcing to a grounded lead, at the spark plug wire distrib cap boots, when wetted down (occurs with 2 sets of OEM wires). It's almost like the headgasket is acting like an insulator to the block! Trying to tie all this together with the retarded timing.

Thanks (again)

FWIW, I spent a few hours with my DSO (Digital Storage O-scope) and captured some waveforms. I wrote up some software to download the captures from the scope and allow them to be saved on my PC. I'm not sure what kind of comparison it would be, but I saved the capture of the dizzy on my '90 Corolla. If you like, I can zip it up and email it to you; just drop me an email.

-Tim

There are some good links I can point you to once you get the low amps probe.

You might want to consider buying:

The picture doesn't do it justice, you can adjust the size of the gap via a threaded adjuster, there is a KV scale on the body which represents the voltage required to fire across the gap. Handy tool for stress testing the secondary and you can use it to verify/calibrate your new bought secondary capacitive pick ups for accuracy (highly recommended). Fairly cheap tool to purchase, and available at many retail outlets.

Secondary resistance can manifest itself in multiple ways. High initial firing voltage can mean an open in the secondary (wires, rotor air gap too wide, wide plug gap), a fairly even spark line around 2KV with a slight sag is typical. If the spark line hooks up at the end it usually means a lean condition (higher voltage required to sustain arc due to depleted HC molecules). The events thru and including the initial ionization of the spark gap are capacitive events, once the spark has bridged the gap, the circuit pretty much mimics a series circuit, so we have high initial voltage (firing line/spike) which settles down to the spark line (2 KV or so) and then as energy runs out the coil oscillations.

You might want to call Sun Electric/Snap-On at

Sounds like maybe you have the ground (zero volts) misadjusted. This 2.5ms drop is the coil turn on, i.e., 14 volts being pulled(switched) to ground.

Coil saturation and/or current limiting being applied by the control module. The low amps probe when used on the primary will allow you to see the above events in much greater detail, plus since secondary capacitance effects certain events in the primary, you'll be able to use that to determine the relative health/function of the primary circuit.

As I mentioned in my last post, hopefully that initial primary firing line reaches a nice high voltage, although if you're using an analog scope (IIRC) it may not render the verticle traces very well.

point. Off course,

Double check that zero volts actually corresponds to where ever/whatever you're using for ground reference.

This is not a good sign. You've got leakage for some reason. (good test technique though!) OEM wire sets are not always all they're cracked up to be.

Using a scope involves performing various dynamic test procedures, under a good hard snap of the throttle, observe how high the secondary voltage goes, after the throttle snap, when the engine is under decel, observe how low the secondary voltage goes (this measures the rotor air gap KV value). Cranking the engine with the fuel system disabled at wide open throttle puts a quite high load on the secondary, another good method for spotting problems...

My pleasure.

Thanks, Tim - I just sent you an e-mail.

Pre-dawn I changed out the dizzy again, and fiddled more with the scope controls. The lower light level helped, with engine cold, saw 275V w/the primary switching on. But it seemed to fade back down to yesterday's readings of 45V-55V as engine got hot (have driveability probs regardless of engine temp). Expectedly, some of the burn line hash was reduced

Also could see secondary firing 14KV-17KV, w/engine hot (will ck that again w/cold engine). Depending on what sources are referenced, that's either normal or high. Still pretty erratic 3KV variation going into start of coil oscillations

A fair part of this is probably user error. Back to the books & sample waveforms

I was using, what I call the "main line" (points closed on a convential system), as a relative 'zero' or in differential reference (to the events) - as am often vertically shifting the display - but perhaps that's flawed technique. I didn't adjust the ground, per se

Also, my ground lead is longer than I'd like - at first pass necessity - but will try to rework a shorter solution

I don't fully understand the wet-arc test. Does failing it _always_ mean bad wires? That was my initial understanding. But what if _good_ wires are subject to quite excessive 2dary pressure (for example, substantial carbon build-up that has increased cylinder compression; or retarded timing; or funky cylinder turbulence; or ??)?

When tested, my spark will jump approx 1 cm to the ground wire (quite a leap!) - but only when wet.

With plugs wet and no ground wire, no arcing is evident (at night)

Hopefully not rambling too much here. One thing that has always stood out, while both driving and revving from idle, is a transitional pt, that seems to correlate well with the abnormal ignition retarding 2800 RPM. There is a marked return of power > 2800 RPM (+/- 300 RPM, and broadening)

Wish I had a digital display advance light, but w/a standard inductive light run into the cabin, during substantial bucking, there is no break up in the pulse

Also, O2 sensor doesn't show a lean out during the bucking (but I don't have a max/min/avg DMM [on the tool buy list])

Hence my interest in the knock sensor - active waveform! - and the possibility of greater V @ or 2800 RPM (but it is not evident: KS output seems to seamlessly increase w/increased RPM).

And other phenomenon that retard electronic ignition timing

Back to primary ignition this a.m. - I did notice a distinct laterally shift, sometimes quite rapid, occurring circa 2800 RPM. Otherwise, pattern is pretty stable above and below 2800 RPM.

I had some lateral shifting previously when scoping the Hall-Effect sensor, but was able to quell it by switching from auto to normal trigger, along w/some other adjustments. Wasn't able to do it this time w/the primary ignition

Understood that you may need to shift the pattern up or down in order to see certain parts of the waveform due to screen size limitations...

Ground lead (within reason shouldn't effect much.

I wouldn't want my wires leaking to ground just because they're damp or wet.

To a point, the wires shouldn't leak, retarded timing drives the firing (ionization) voltage waaaaay up since cylinder pressure is higher the closer to TDC the spark occurs. You mentioned previously that there was a timing retard problem, care to expound on what's happening there? So yes, all mentioned above can stress the secondary insulation to the point of breakdown.

I would anticipate that that could cause a driveability problem on damp, wet, rainy days, it could also be a clue to problems on dry days.

Because there is no proximity like you have with your grounding probe, but that doesn't mean that the spark energy is not shunting around the sparkplug gap.

I believe the timing retard is at the core of my problem, but I can't identify the root cause

At base idle, timing is fine per inductive light on cyl #1.

As RPM is slowly increased and held steady - below ~2800 RPM - the timing retards! Snap throttle will advance timing, after a slight lag

Once RPM is above ~2800, and held steady, timing appears to be properly advanced.

The above correlates well to the driveability probs

No single bad cylinder stands out either on the scope or when intentionally driving on varied 3 cyls

Hall-Effect sensor and knock sensor waveforms, at the Ignition Control Unit, appear good (no aberrant or transitional changes below, at, or above 2800 RPM). I've yet to test the Fuel Control Unit's "load signal" to the ICU (FCU pin-out info just states "Freq incr with throttle opening"). Both the ICU & FCU were temporarily replaced, w/a 20 minute long driveability improvement with the former

I will keep pluggin away at it

I got this book, along w/two others, from library loan yesterday (had ordered about 10 days ago, but didn't recognize title). Helpful, along w/"How to Read & Interpret Automotive Oscilloscope Patterns" by Gerald Brown. Have also studied suggestions (and several internet sites)

Update fwiw:

Working pre-dawn, at idle, was able to see ~275V on the primary firing trace (engine cold and hot)

Secondary seeing 28KV - 37KV cold (seems a bit much, even cold); 13KV -

Firing section 1.2ms; coil oscillations 0.8ms. Seems same for all cyls - seems possible indication of a partial short

Burn line still has a fair bit of hash in it (marked reduction w/new rotor & dizzy), especially revved a bit

Most consistent stand-out, to my untrained eye, is at the start of the intermediate section/coil oscillation first peak, which has a 30V rapid variation to it. The first coil oscillation trough is stable

The first peak variation/amplitude inevitably pulls the end of the firing section up (and then back down levelish, erratically). It isn't quite the same as a "typical" fuel/combustion/lean sample I have - wrt to hook concavity or convexity - but perhaps the general upward slope is what's important

O2 sensor doesn't show lean on the scope

Still working on Parade pattern (tough to see/interpret)

I'll be running through everything a few more times

Seems above might suggest all injectors dirty? but seems atypical w/symptoms. Also trying to context in w/timing retard problem

In the Sun book above, there's a wf sample for a GM HEI primary (pg 97), showing a "Current Limiting Hump": pretty similar to what I'm seeing