Improvised oscilloscope probe for Automotive ignition analysis.

[Jim-0000]

A stock car racing buddy has a miss firing engine in his race car. We would like to put an oscilloscope on the high tension leads. At present, the ignition system is the standard, pre-EFI, contact points and capacity, Kettering System.
It did have (and will have again) electronic ignition, but this has been replaced with the above in the fault elimination sequence that has been done to try and locate the fault. A lot of stuff on the engine has been swapped and replaced in the search for the miss.
The car miss fires most under load; classic HT breakdown behavior.

In the attached document from "Electronics Today" 1977, an improvised high ratio probe is described using a common peg and some aluminium foil. This then, allegedly, enables a more common low max voltage oscilloscope to be used with safety.

My questions:
Is this a safe way to protect the oscilloscope?
If so, should it be tested somehow before plugging it into a high tension lead?
Or is the probe simply wrapped around the outside of the HT ignition lead?
It is hard to tell how this probe is made. If someone reading knows, please advise?

I do hope this is the best section of the forum to post this. If not, please advise and re-direct me.

Jim.

[soldar]

The sensing is done capacitively by wrapping a bit of metal around the HV cable.

Those ignition systems are fairly straightforward. Things to check are that points and capacitor are good, coil is good (they do fail), coli polarity is connected correctly, HT cables are good and spark plugs are good. All are easy to test and with all things being as they should I can't think of anything else which could go wrong. I guess the distributor but that is less likely or it would be obvious.

All cylinders have the same problem?

I do not understand something. The electronic ignition has been replaced by the older Delco? How are you going to diagnose the electronic with the Delco? And both are misfiring in the same way?

You are not using the same coil are you? Again, try replacing the coil and make sure of the polarity.

[Microcheap]

On Aliexpress you can find this capacitive probe used to test the secondary of the ignition as is described in the article you linked:
https://www.aliexpress.com/item/Hantek-HT25-Automotive-Oscilloscope-Probe-HT25-2-5-meters-Ignition-Capacitive-decay-of-up-to-10000/32822817192.html there is another type of probe but I've never tested one http://www.hantek.com/en/ProductDetail_15_11165.html

[Jim-0000]

The sensing is done capacitively by wrapping a bit of metal around the HV cable. ..

Is the normal probe just connected to the metal film?

Those ignition systems are fairly straightforward. Things to check are that points and capacitor are good, coil is good (they do fail), coli polarity is connected correctly, HT cables are good and spark plugs are good. All are easy to test and with all things being as they should I can't think of anything else which could go wrong. I guess the distributor but that is less likely or it would be obvious.

All cylinders have the same problem?

I do not understand something. The electronic ignition has been replaced by the older Delco? How are you going to diagnose the electronic with the Delco? And both are misfiring in the same way?

You are not using the same coil are you? Again, try replacing the coil and make sure of the polarity.

The two brothers who are working on it are both career mechanics. They have changed all the above parts more than once, and some, like the spark plugs more times than they can count. They have fabricated a new inlet manifold and fitted a different carburettor. They even changed the camshaft. For a while, the cylinders were behaving differently, with #6 cylinder missing more than others. Then they turned the carb around 180 degrees and the cylinders now (appear) to behave similarly. Coils changed, new capacitor and points fitted to the present Kettering ignition system. etc. etc.
This might give you a bit of an idea into the recent history of this Holden Grey motor. Much modified I should say.
The engine missfires with both ignition systems in the same way.
I have suggested the ignition analysis. And, also, running the engine in the dark of night. Observe for sparks etc. However, it is possible to hold all ignition leads while the engine is running, suggesting all insulation is sound.

[Jim-0000]

On Aliexpress you can find this capacitive probe used to test the secondary of the ignition as is described in the article you linked:
https://www.aliexpress.com/item/Hantek-HT25-Automotive-Oscilloscope-Probe-HT25-2-5-meters-Ignition-Capacitive-decay-of-up-to-10000/32822817192.html there is another type of probe but I've never tested one http://www.hantek.com/en/ProductDetail_15_11165.html

Thanks for that link. I will follow it up.

[Johnny10]

The best way to test for problems in firing is with an oscilloscope.
A HT probe on the sparkplug wire and a pressure transducer in the cylinder hooked to oscilloscope.
This will show where in the cycle spark happens and the position of valves in the cycle.
It will show advanced or retarded firing.

This was my first oscilloscope project.
The pressure transducer signal identifies all the engines internal pressures during the combustion cycle. The HV spark signal shows where the spark ignites fuel as a second channel.

https://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1004&context=auto_pres



These should get you started.

[Jim-0000]

The best way to test for problems in firing is with an oscilloscope.
A HT probe on the sparkplug wire and a pressure transducer in the cylinder hooked to oscilloscope.
.........................................

Pressure transducer? That is new to me, can you please explain Johnny.

https://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1004&context=auto_pres

I will download them and watch them.
Thanks,
Jim.

[David Hess]

The car miss fires most under load; classic HT breakdown behavior.

At high compression the breakdown voltage across the spark plug is greater so if the voltage is low, it is more likely to miss.

[joeqsmith]

A home made probe for looking at the ignition waveform.

Hunting a problem that only happens under load is always fun. Keep in mind it may have nothing to do with the electrical or electronics at all. 

[Jim-0000]

A home made probe for looking at the ignition waveform. ....

That is interesting. Can you provide some more details please Joe?
What cable you use, where to get it etc.
Is the attenuation sufficient enough to plug it into a small oscilloscope with a 200 volt input max limit?

............. Keep in mind it may have nothing to do with the electrical or electronics at all.

Of course, that has been the awareness all along. Evidenced by the many changes and mods done, some of which I listed above.

Thanks for the picture and the comment.

[floobydust]

I've had similar weak spark problems on my old musclecar and none were in the textbook.
You can look at the HT waveforms but you've replaced everything there, so the problem is likely elsewhere.

Some things to check:

1. It was a defective manufacturing batch of Champion spark plugs, the internal resistors were pooched/missing and the arc was inside the porcelain making up for that.
2. Problem was the tachometer, its input circuit was loading the coil primary.
3. The ballast resistor - it was nichrome wire in the harness, partially broken so a high ohms value.
4. A set of points with a weak spring, the contacts bounced at anything much above idle RPM's.
5. I would look at dwell and see if the distributor is stable - one had loose bearings and the shaft wobble changed dwell at speed and it was very hard to track down.

I would check the 12V feed to the ignition system is not sagging, and you are (now) using a ballast resistor/condenser with points?... so as to not damage the coil. Transistorized ignition does not need the ballast resistor, and move the condenser to the 12V power input.

Most digital scopes can't handle automotive ignition systems. The EMI will make the scope add false signal spikes and noise to the displayed waveform. So be prepared to go on a goose chase looking for a "blip" on the waveform that actually isn't there.

[Jim-0000]

I've had similar weak spark problems on my old musclecar and none were in the textbook.
You can look at the HT waveforms but you've replaced everything there, so the problem is likely elsewhere.............................................................................

4. A set of points with a weak spring, the contacts bounced at anything much above idle RPM's.
5. I would look at dwell and see if the distributor is stable - one had loose bearings and the shaft wobble changed dwell at speed and it was very hard to track down............

Point #4 is not likely, as they are new points.
But............
Point #5 might be the problem!  They are using an old Holden grey motor distributor. (however, come to think of it, they did have a Falcon distributor in it with the electronic hall sensor type ignition.........same problem exhibited).

I would check the 12V feed to the ignition system is not sagging, ......

That is what one of the mechanic (brothers)  is going to check first opportunity on Sunday. He will power the ignition system only with a separate battery and cables. The car battery is in the back of this Stock car, so there is a long wiring loom. Potential there for voltage drop as you mention and/or one or two poor connections. Also, there is a "kill" safety switch on the dashboard!

and you are (now) using a ballast resistor/condenser with points?... ...............................................

No ballast resistor during the last iteration of the ignition system. In other words, the last time we worked on it. Might have had one previously.

Thanks for the detailed reply.

Jim.

[joeqsmith]

Coax is RG400/U.    I have a trimmer cap up near the clamp to trim the voltage level.   The large braid goes to the block, the clamp goes around the plug wire.  As mentioned, this forms a capacitive divider.    I feed this into a 50 ohm termination (built into the scope).   I test the ignition parts off the engine.   

[tautech]

Things to double check:
Battery to chassis and engine block grounds.
Coil ground.
Points plate ground.
Broken cable conductors, typically old cabling where bending forces have broken the internal strands.
Cracked dizzy cap or rotor.
Dicky carbon and/or spring in the dizzy cap.

[vk6zgo]

Things to double check:
Battery to chassis and engine block grounds.
Coil ground.
Points plate ground.
Broken cable conductors, typically old cabling where bending forces have broken the internal strands.
Cracked dizzy cap or rotor.
Dicky carbon and/or spring in the dizzy cap.

All of the above!
Plus, if you are using suppressor leads, check that they are around 15 to 20 k resistance.
Back in the day, when the carbon core looked pretty much like burnt string, it was not unusual to find one' or more leads had drifted up to 5 megohms or so.

Sparkplugs are not all equal, even when they have the same part number & brand.

I fitted a set of German made Bosch spark plugs to my HQ Holden 6 back in the day----should have been good, eh?
Nope, the insulators cooked up, went brown, & started leaking badly.

The same number but made by Bosch in Australia were noticeably different, in that the insulation was thicker where it went into the metal body of the plug.

It turned out that there were two plugs using that part number, one which was designed for aluminium heads, & the other for cast iron ones, with their worse heat dissipation.
The "aluminium head" ones were never meant to be sold in Oz.

Holdens fitted AC Delco Spark Plugs  as standard, & crappy things they were, too!
My HQ had just run over the time for the last warranty service, so I fitted (Oz made) Bosch plugs, thinking that as I would have to pay for spark plugs anyway, they might as well be the ones I like.

I ran into an acquaintance who worked at the Holden dealer, who said "You are only a "bee's whisker" over, bring it in!"

Well, I thought I had a win, till the next week I was travelling down to a country town about 200km away from home, when suddenly, there was an almighty "bang" followed by "chuffing" noises.

The entire insulator &  "guts" of one spark plug had been blown out, never to be seen again
On investigation, I found the dealer had removed all my beaut new Bosch plugs , fitting  AC "crappo" ones instead!

[Jim-0000]

Things to double check:
Battery to chassis and engine block grounds.
Coil ground.
Points plate ground.
Broken cable conductors, typically old cabling where bending forces have broken the internal strands.
Cracked dizzy cap or rotor.
Dicky carbon and/or spring in the dizzy cap.

All good points and all will be checked again this Sunday. I hope to be assisting.
I suspect point #1 the most. Battery is in the back, as I have explained.
The distributor earthing is another one we will check, (I hope).

[Jim-0000]

Maybe this will help:
https://www.princeton.edu/ssp/tiger_cub/library/ignition_waveforms.pdf

I have already linked it in the first post. Very good paper from 1977.

[Jim-0000]

Coax is RG400/U.    I have a trimmer cap up near the clamp to trim the voltage level.   The large braid goes to the block, the clamp goes around the plug wire.  As mentioned, this forms a capacitive divider.    I feed this into a 50 ohm termination (built into the scope).   I test the ignition parts off the engine.   

Where did you buy the clamp please Joe?

[tautech]

Things to double check:
Battery to chassis and engine block grounds.
Coil ground.
Points plate ground.
Broken cable conductors, typically old cabling where bending forces have broken the internal strands.
Cracked dizzy cap or rotor.
Dicky carbon and/or spring in the dizzy cap.

All good points and all will be checked again this Sunday. I hope to be assisting.
I suspect point #1 the most. Battery is in the back, as I have explained.
The distributor earthing is another one we will check, (I hope).

Some points plates used a slipper contact while others used a very fine and soft braid or wire. If intact they give no problems but a slipper contact can if the wiper hasn’t enough tension.
Good hunting.

[soldar]

If the spark has enough voltage, and it should, and it only fails under load, that means is is not high enough to spark through the compressed mixture or it is sparking somewhere else. Some spark plugs can be defective and the spark jump internally. Or if the plug's cap are not good the spark can be jumping totally outside the plug directly to the block. Or it can be inside the coil. But you say all these items have been replaced....

Yeah, it's no fun troubleshooting something which happens only under heavy load.

[tautech]

If the spark has enough voltage, and it should, and it only fails under load, that means is is not high enough to spark through the compressed mixture or it is sparking somewhere else. Some spark plugs can be defective and the spark jump internally. Or if the plug's cap are not good the spark can be jumping totally outside the plug directly to the block. Or it can be inside the coil. But you say all these items have been replaced....

Yeah, it's no fun troubleshooting something which happens only under heavy load.

Depends on how hardcore your setup is.
Add some logging to a race car and examine a pass and the issues generally stand out like a sore toe ! 

[Jim-0000]

Add some logging to a race car and examine a pass and the issues generally stand out like a sore toe ! 

Can you explain what this means to a "non racecar" experienced, but otherwise generalist mechanical person please?
(I have maintained my own road vehicles for over 60 years. The very few times I went to a mechanic in remote locations etc., the job was not done properly!)
So I sympathise with vk6zgo in reply #13.

[tautech]

Add some logging to a race car and examine a pass and the issues generally stand out like a sore toe ! 

Can you explain what this means to a "non racecar" experienced, but otherwise generalist mechanical person please?
(I have maintained my own road vehicles for over 60 years. The very few times I went to a mechanic in remote locations etc., the job was not done properly!)
So I sympathise with vk6zgo in an earlier post.

Full logging of all systems can get pretty complex with sensors all over the vehicle and they can help fine tune most of a cars performance. All I’ve had experience with are on door slammer drag cars but one of the most telling are EGTs that not only give feedback on tune vs load but when they might drop unexpectedly against another cylinder indicating what’s generally an ignition problem.
They are great when doing dyno runs to optimise tune at all points over the rev range.

So what might you log ?
RPM
Boost
Fuel pressure
TPS
Driveshaft RPM (slider clutch slippage)
Suspension (launch squat and bumps in the pass)
Battery voltage etc etc

For the guys right into it, logging is of paramount importance to squeezing what’s possible out of any setup.

[joeqsmith]

Coax is RG400/U.    I have a trimmer cap up near the clamp to trim the voltage level.   The large braid goes to the block, the clamp goes around the plug wire.  As mentioned, this forms a capacitive divider.    I feed this into a 50 ohm termination (built into the scope).   I test the ignition parts off the engine.   

Where did you buy the clamp please Joe?

A friend of mine had given me a couple of those.  As the paper mentions, a clothespin would work fine. 

So what might you log ?
RPM
Boost
Fuel pressure
TPS
Driveshaft RPM (slider clutch slippage)
Suspension (launch squat and bumps in the pass)
Battery voltage etc etc

 
A few others I look at are: EGT, exhaust pressure, oil pressure, AFR.

I have had similar problems under load where the motor would stumble with nothing to do with the electonics.    Just thinking about pressure for example...

The exhaust pressure is roughly equal to the intake, if things are matched.   Once the pressure was high enough, conditions were just right and the motor would stumble.   The exhaust valves were sticking open.   Bigger springs....     

As pressure goes up, it puts a strain on the ignition parts.  No surprise.  You can see how it arched under the boot and etched the side of the plug.   To combat this, the gap had to be closed down. 

Fuel type and amount of fuel ... is another fun one.   As the pressures go up, this adds to the fun...


Shown is another test jig to evaluate wideband sensors. 

[floobydust]

The coil primary waveform is also very useful, it's how modern ECU's do real-time misfire and ignition fault detection.
Electronic ignition clamps the primary voltage to just under 400V, so max. output voltage is limited compared to points.

There's two grounds in a car. One high-current ground is from engine block to battery, and the second smaller ground is from the engine block or battery to the car body/chassis. This is for the in-car loads like fuel pump, ignition, lights etc. A common mistake is forgetting the car body ground which the ignition module may not have been grounded to the block. The firewall is no good unless you have the ground strap from block to body. You can't rely on current flow through motor mounts and U-joints etc.

Flipping the carb around, it would mean poor mixing in the intake manifold? I don't see a single carb used on the Holden hi-perf mods.
If possible you can fit an O2 sensor and see each cylinder firing lean/rich to check balance. Maybe joe has tried this.
Otherwise you'll blame the ignition system when nothing is wrong with it.