Diagnosising old school ignition coil

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hexibot43
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Diagnosising old school ignition coil

Post by hexibot43 » Sun Jul 28, 2019 8:44 pm

Had a car come in a couple months ago that gave me a good slap in the face. 1970 Chevrolet Corvette with stock electronic ignition. I did a re-curve of the distributor, and overhaul / rework of the carburetor. This was done to help deal with the changes in gasoline since the vehicle was produced. I also installed a new set of igntion wires. Customer came back saying vehicle was hesitating. Took it for a drive and didn't notice anything wrong. Sent another mechanic out, and it took him driving 45 minutes to get the problem to show up. Once it got to this state it would almost die on acceleration. If you slowly accelerate, the problem was not showing itself.
I changed the coil based on experience only. Not on empirical data. Yes I fixed it. But it took me to long to get there. Trail and error is a slow process.

I had looked at the ignition with my scope, and I didn't see anything wrong with the pattern. I unfortunately didn't take a snap shot. I could kick myself. I've been looking through old books on Ignition waveform diagnosis. I was able to find one mention of a failing coil pattern. The coil oscillations were more of a line. Did I miss it?

Now my question is - could I have seen something of this before the coil heated up and started showing the problem. Would there have been something obvious? I want to be ready the next time I run into one of these. I really wish I could get my hand on the defective coil. I, we, spent hours on this car. I had thought perhaps there was an issue with the carburetor. Swapped in another carburetor. Thought perhaps we were loosing fuel pressure. We hooked up a gauge to watch while driving. If I had only been given a clue in the ignition waveform. We wasted a lot of time figuring out what it wasn't. Any suggestions? Could there be something more obvious in the current draw, rather than the voltage?

I'm reminded of a friend that had set up a machine to put ignition coils under load to see how it responded. I've only ever seen one of these in my life. It was a machine to simulate the firing of the coil. You could open the gap up to see when it could no longer jump the gap. And from this gauge the coil. I'm thinking the car itself should easily do this. Although his machine tested just the coil so you could easily rule it out.
Last edited by hexibot43 on Wed Jul 31, 2019 3:19 am, edited 1 time in total.

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PicoKev
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Re: Diagnosising old school ignition coil

Post by PicoKev » Mon Jul 29, 2019 11:50 am

Hi Hexi. (Sorry I do not know your name)

There are various clues in the coil primary trace in both the turn on and the turn off zones.
Coil ringing as the energy dissipates at the end of the duty cycle can also be quite revealing.

Coil amperage is also a very good indicator of a failing coil.

Always do at least a WOT test preferably a hard acceleration run on the road under load (up a hill is good!).
Also remember the Old Skool oil filled coils were much more likely to fail under thermal stress so a warm up with the hot air gun is a valid addition to the test procedure, don't forget to factor in ballast resistors if fitted as well.

Back in the Flintstone era we had both coil and distributor test benches, mind you we also sand blasted spark plugs and greased all manner of joints, even the hand brake cables had grease nipples!

Regard,
Kev.

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Re: Diagnosising old school ignition coil

Post by Autonerdz » Tue Jul 30, 2019 4:55 pm

The concepts of coil diagnosis are covered extensively in Primary Failure Analysis on Nerd III.

https://www.autonerdz.com/n1.htm

Followed with a quiz on the concepts:

https://www.autonerdz.com/downloads/apiquiz.pdf

In a nutshell, you validate the primary drive with primary current making sure the current level is sufficient and the current cut off time of fast enough. These two things are all an inductive discharge ignition coil needs to work. A weak secondary output in the presence of a valid primary drive is a bad coil... always.

You cannot validate the primary drive looking at voltage but it's good to have that along with the current should you find a problem with the current to give direction.

If the coil can jump 3/4 inch in the air, that's 30 KV and all the ignition system would require. If it cannot, then secondary output is weak.

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hexibot43
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Re: Diagnosising old school ignition coil

Post by hexibot43 » Wed Jul 31, 2019 5:01 am

Thank you both Kev and Autonerdz,

I will be doing more research along with some further scoping of known good ignition systems. Make myself a map to follow. So far I've been very surprised at what I did find. Mostly a whole bunch of nothing. The graphics in old books were not very definitive at all. Amp probes were not a big thing back when those manuals were written. But I will be pulling my Amp probe out and will be doing some tests. to hopefully make up for the short comings of old school thinking.

I did have something rather odd happen today. I spoke of a coil tester earlier, the only one like it that I had ever seen. My friend that had it passed away a couple years ago now. His son told me this morning that he would be bringing it over and it can be kept at the shop I'm working for now. I don't remember ever asking for it, but perhaps I was thinking loudly !?! That is going to really help me learn the good and bad of coils.

I heard everything both of you said. A current probe takes nothing to hook up on a single coil system. Been thinking of setting my PicoScope up on a cart. I've always kept it in its case. To be pulled out only when necessary. Ever since I dropped it, and had to send it in for repairs. Thanks you Picoauto for getting it fixed fast, and amazing under warranty. Time for a change. Perhaps mounted and hung like my Big scope it will be safe enough.

Thanks,

Marcello

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Re: Diagnosising old school ignition coil

Post by Iver » Fri Aug 09, 2019 9:15 am

Been thinking of setting my PicoScope up on a cart
Forget the cart. it should be in a rucksack on your back. Always ready for action !! :D :D :D :D

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Re: Diagnosising old school ignition coil

Post by HiramTheGreat » Sun Aug 11, 2019 12:56 am

Also think about ''vapor lock''. Here in the U.S. we have a lot of vintage cars and some will call them ''Hot Rods''. The problem with ''Old School'' carbureted systems is the new fuel we have today. With the addition in ethanol and the lack of lead the fuel boils in a matter of minutes, less than an hour. That causes the fuel to boil and cause cavitation...(bubbles)… Air in the system. The real solution to that is to add a return line to the fuel tank to keep the fuel cool enough to not cause ''vapor lock". Also on older cars the main battery cable was routed straight to the starter motor and the rest of the vehicle wiring system was connected to it, causing excess heat from the exhaust manifold...and mainly headers...creating a voltage drop condition. After the vehicle cools all the systems worked well again. Do a voltage drop test from the battery to the coil and see what you have first cold, then when it fails. I would remove the vehicle power distribution away from the starter and add an aftermarket junction block straight to battery positive. Add a return style fuel filter and plumb the return line to the tank (there's aftermarket kit for this) and do some testing with the amp probe as suggested. Do a before and after if possible.
Hope this helps.
Hiram

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Re: Diagnosising old school ignition coil

Post by HiramTheGreat » Sun Aug 11, 2019 1:52 am

Marcello, I misread your post. I see you fixed it with a coil, witch is great. The question is why it failed? Kev has a good explanation on the failure and I agree. Heat is the main source of failure in an oil filled coil. The position of the coil is also important, some would be installed on its side and some straight up. Depending of the vehicle they were in. But heat always used to kill them. Some would leak the oil from heat. And most of the heat, I believe, is from capillary action. I see this on some Mercedes transmissions, not the heat but capillary action, leaking transmission fluid through the pass through connector all the way to the TCM. The same thing can happen in these older systems where the main vehicle power supply is physically bolted to the starter. Heat travels throughout the system causing ghost problems. And causing failure to critical components. Like the coil you just replaced. By the way, I'm not new to oscilloscopes, I'm a newbie here because I just bought the NVH kit trying to solve some transmission issues I have at the shop.
Hope this helps a little and not cause any confusion.
Hiram.

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Re: Diagnosising old school ignition coil

Post by hexibot43 » Sun Sep 01, 2019 8:02 pm

Directed to "HiramTheGreat",

Thanks for your insights. I agree with everything you've said. Our shop has gone from a European Mostly repair shop to a "Vintage Americana" shop. It looks really strange when I open the shop in the morning and there is basically nothing but Americana with a sign outside that reads, "European Motorsports." I do most of the electrically work. And usually most of my work is towards making the vehicles more reliable. I am starting with changing the cables between between battery, alternator, and starter. We usually use 1/0 cabling. Going to 1 wire Alternators. If possible flipping the starter upside down to get away from exhaust systems. Bypassing Amp Meters. I will leave enough current flow to see the gauge move positive or negative but the majority bypassed. These old cars wiring system are not made to handle the power output of the newer alternators. I could go on, and on. Fuel temperature is a real problem as stated. And every car is different. So we've had to create as many fixes as there are cars.

And to Everyone,

I've been watching lots of videos of coil on plug coil failures. The interesting part of it is that they are blaming it on an over worked coil because of very lean fuel mixtures. Nissan having coils melt in place, etc,.. And I thought having a coil for each plug was making the coil have to work much less. And less problematic. LOL! I was wrong. I'm glad all these muscle cars I'm working on are not going to leaner mixtures. Well they are trying with the changes to the fuel. We've had to feed more fuel to make up for it. Along with a different attack on timing. Every Winter I hold my breath waiting for fuel changes to see what they've done this time. Good for business though.

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Re: Diagnosising old school ignition coil

Post by Iver » Sun Sep 01, 2019 10:27 pm

And I thought having a coil for each plug was making the coil have to work much less.
That could be one reason !! Indeed yes, the coil will have a longer rest period but also more time to recharge.

With Turbochargers added to nearly every engine Compression in the Chamber is higher and more Voltage is required to enable the spark to be created and maintained.

Other reasons exist.

One coil per plug also has the advantage of reliability, If one coil fails it creates a misfire not a call for a recovery truck.

Cutting costs is another, with the introduction of "Pencil" coils, assuming all engines have 4 cylinders, the production ans sales of Ignition coils has multiplied by 4. That mass production will lower the cost manufacture and sale.

More so, Emissions are the biggest thing driving & spoiling this industry, BMW & others use "Double Strike" at lower Rpm's, Alfa Romeo were one of the first to use 2 Spark Plugs per cylinder, Mercedes do that on the SL55 AMG & Others.

Where the Coil Manufacturers win, the manufacturers of Distributors, Rotor Arms, HT Leads, Contact Points, Condensers, Strobe Timing Lights Lose out.

We could now start a discussion on KNOCK CONTROL and TRACTION CONTROL, ESP. Both have an input, into the equation that determines Spark Timing. Hence an effect to the output.

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