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file:///C:/Users/pweick/Downloads/ETI_Feb77_ScopeTheIgnition_1b.pdf

Would these help?

Peter Wieck Melrose Park, PA

I am not a member of any organized political party. I am a Democrat.

Will Rogers

Thank you PF. The link does tell me what is possible. I suppose by 'high pressure pdobe' they mean 'high voltage. Hookup diagrams would be nice to have. Of course the only electronics in a 1948 auto is the ignition system. Would love to look at 4 cylinders (8 total) at a time with the Rigol. I do have inductive pickups that I can salvage from timing lights.

Exactly what do you expect to see? What are you gonna do with the information when you get it?

Is the benefit derived from that worth the risk of blasting the front-end out of your scope?

What you see on the scope is likely to be more dependent on your probing technique than the signals present. When I experimented with monitoring fuel injection, I had trouble getting rid of the ignition noise so I could look at the injectors.

I have a Snap-On automotive scope with an input voltage rating of 500V with 30KV surge rating, whatever that means. I wouldn't go poking around aimlessly with that either. I'm sure it's completely safe...except when it isn't.

Keep your digital scope out of your car unless you really, really need to measure something that you can control to FIX something that's broke. Even the 12V...I guess it was 6V in 1948...electrical system is risky.

Thanks Mike, I thought if I could see all the ignition activity at the same time (or at least 4 at a time) I could look for anomalies and identify a poorly firing spark plug or semi-faulty wire.

Thanks for the cautionary note. Will heed.

Ivan Vegvary

Yep, the low-voltage terminal of a spark coil does give that kind of info. You can identify (or so I'm told) variations in fuel mixture from cylinder to cylinder that way (higher breakdown voltage for rich fuel mixture). It takes a 500V scale for the o'scope, though ('low voltage' means +12 to minus 300 or so...). A transformer clip-on probe can trigger your 'scope from cylinder

1, which simplifies the analysis if your auto has a single coil for all the cylinders.

(or at least 4 at a time) I could look for anomalies and identify a

poorly firing spark plug or semi-faulty wire.

You have to be careful NOT to assume normal conditions when detecting faults, which by definition, are NOT normal conditions. Stuff happens. It's best if that stuff doesn't happen when you're probing around with an expensive digital scope.

300V might be a fine number under normal circumstances, but, if a wire is bad and the plug fires at a much higher coil secondary voltage, the primary may also show a much bigger spike. And there's always the temptation to crank up the sensitivity to see some smaller wiggle on the trace. That may be a bad idea in this case.

A plug either fires or it doesn't. If it fires, the thing you are interested in is the current. After it arcs, the voltage will be determined by the plug and wire/coil resistance/inductance. High current >> fat spark >> good. Of course, that assumes that the current is all going thru the plug and not arcing to ground thru a crack in wire insulation.

If you have a clip-on current probe of known transient response, you can clip it on the coil output and look at the relative currents for all the cylinders. Properly insulated current probe designed for ignition service should be safe to use on your scope. If you try to move the probe to different plug wires, you'll probably see more variation in the coupling than in the actual signal.

It's a vintage car. If it runs, drive it proudly. If it runs like it has ignition problems, change the plugs. If that doesn't fix it, work your way back toward the battery. Run it in the dark and poke around the wires with a grounded probe to look for insulation faults. You can learn some interesting things by pulsing the coil with the engine off and listening to the coil. But, you can smoke the coil if you're not careful.

Rule number one... If it ain't broke, don't try to fix it using your expensive scope. Rule number two... If it IS broke, don't try to fix it using your expensive scope, except as a last resort.

Ok but it still begs the question, why?

I can't see a set of new plugs, wires, cap, rotor, points and condenser running more than $50 dollars or so. I'd just replace everything and if you think something is wrong, look elsewhere.

You didn't say where and what condition this car is in, but time and money spent on a compression test and carb rebuild kit wouldn't be money ill spent. That and a timing light.

It's a novel idea but the scope isn't going to give you any more ideas than a good set of ears and eyes will.

-bruce snipped-for-privacy@ripco.com

Thank you all. Scope will not be used on ignition. Car runs fine. Lesson(s) learned. Great group! Ivan Vegvary

I agree that unless there is some odd problem, just replace everything for a few dollars insted of maybe messing up a scope that is a thousand or so dollars.

If there is an odd problem, maybe you could find someone that has one of the old Sun scopes.

For the old cars just see if you can find a timming light and dwell meter.

(s) learned. Great group!

This is one of those times when an old CRO would come in handy. A 20 MHz jo b from the 1970s would be fine.

You would connect the vertical input to where the wires goes from the point s/condenser to the coil. The other channel you use to trigger by loose coup ling to cylinder #1.

Set the time base so you see eight pulses if it has a eight cylinder. They should all be the same amplitude. If some are higher the gap of the plug mi ght be wider or the wire starting to fail. If some are lower that would ind icate a spark leak, like loss of insulation or a fouled/semi fouled plug.

Once you eliminate all ignition faults having the plug gaps all the same an d the wires all good, then if there are variations in the pulses that indic ates the relative compression of that cylinder. A cylinder with low compres sion will yield a lower amplitude pulse if all other things are equal.

I don't remember what kind of voltage you get to the coil on one of those b ut I think it was around 300 volts. Many newer scopes only go up to 5V/div which with a 10X probe (always use a 10X probe to protect the front end, un less you REALLY need all that gain) gets you to 50V/div. You can go into "U NCAL" mode if it goes off the screen but you are at the upper limit of the front end of the scope now.

You can experiment with it by slowly pulling off one of the wires from the distributor. Don't pull it from the plug itself - I learned that the hard w ay. ZZZAPP !

You should see the pulse for that cylinder increase in amplitude. Take a wi re and short the output of the distributor for that cylinder to ground and the amplitude will decrease. Don't let the thing run alot with o plug wire or anything on the distributor because those coils are current operated. Wh en there is no arc the voltage tries to climb until there is a load and it might break down the insulative properties of the distributor cap or rotor. (this is even more important on newer cars)

Actually the easiest way to find a spark leak is a AM radio. Tue to the low er end of the dial and listen to the ZZZZZZZ sound which will increase in p itch as you rev the engine. But if you get a POP POP POP sound that means y ou probably need wires or a distributor cap. The does not tell you which cy linder but it doesn't matter, the wires should be changed as a set and the distributor cap is common to all so that is that.

The one time I ever used a scope on a car was a 1990 Olds with a VIN C 3800 . It stalled sometimes when hot and would not restart. The way it acted sme lled like a semiconductor fault and sure enough the crank sensor was failin g. We read up on how the system works and I stuck the scope probe where it needed to be. A garage would have taken a month to fix this but we were fli pping cars at the time. I used to have a couple thousand worth of books on it but later sold them because I don't do that kind of work anymore for a c ouple of reasons.

Anyway, does that old Pontiac have a radio in it ? Now that can be a blast. You're looking at tubes ad a vibrator based B+ supply. I assume it is 6 vo lts, right ?

What can blow some people's mind is that those cars and even the radio will work if you put the battery in backward. It has been known to happen actua lly, that is why they used to polarize the generator. But they didn't reall y have to, nothing cared much about polarity. Even the radio, because the v ibrator changed it to AC for a transformer the rectifier(s) after that took care of polarity, the filaments of the tubes don't care.

I think it did make a little bit of difference in the spark, but minor. The problem comes when you need or give a jumpstart. Sometimes you get that bi g spark and then you know that one or the other car is mis-polarized.

Later came the hybrid radios with the solid state output, then polarity bec ame important, also then the alternator came out and it was not an issue an ymore. It simply wouldn't run on reverse polarity because the diodes would short out the battery. In cars with a generator the charging current went t hrough the ignition switch, or else the coils would discharge the batter fo rthwith. Therefore it was not a good idea to leave it on, to play the radio for example. then they came out with the "ACC" position on the ignition sw itch. It also wouldn't heat up the points if they happened to be closed.

While I greatly prefer 1960s and 1970s cars it is still a good thing to kee p that old beast running.

I drove a 1950 Chevy once, it was almost like work. People don't know how g ood they got it now. They have no idea what it means to double clutch, and they have no idea how undeveloped the steering and suspension was back then . Driving those old cars is almost like driving a semi truck, but the new o nes aren't even that hard.

Well OK, it isn't really all that difficult but I bet you don't take that t hing to work every day...

han

One thing a hillbilly taught me was if you have a miss and want to know whi ch cylinder it is to put your hand on or near the exhaust manifold (dependi ng on how long it ran) and one part of it will be cooler than the rest.

Another backyard mechanic taught me about timing chains. Take off a valve c over ad see how far backwards you have to turn the crank (with a wrench) to get the valves to move, more than a certain amount the needs timing irons.

The olman taught me that when an engine jumps time the cam is almost always late in phase which will give you higher measured compression but low vacu um. However that has changed because almost all cars use a tensioner. Some of them jump at shutdown and could be advanced instead. Sometimes it is the tensioner that goes bad.

The buddy who taught me about the timing chain trick actually brought a car to me after it had been to Mr Goodwrench and they said "Replace the engine ". It was down to cam timing or the crank sensor which was terribly hard to replace in that model. It was running like shit and pretty much couldn't g et out of its own way.

I looked at it and drove it and did not know much, just that it ran like sh it. Eventually I removed the cover from the throttle body to see if it was getting a good stream of fuel. Well it was but when I tried to rev it up it spouted gas right out. I said "Tear down for timing chain". He said Mr Goo dwrench said there is no timing chain, this old 151-4 has gears. The next d ay he shows up at my house with a broken timing chain tensioner.

Think Mr Goodwrench would give him back the fifty bucks they charged to mis diagnose the thing ? Hell no.

In my business you had hell to pay for a misdiagnosis. apparently the ASE c ertification males you like a doctor, you get paid even if the patient dies . Buncha shit IMO.

While I greatly prefer 1960s and 1970s cars it is still a good thing to keep that old beast running.

My dad had a 1950 Chevy when I had my learners permit. Not too bad to drive. Even my mother drove that car. It even had the modern push button on the dash to start it. You did not need the key if you left the switch in just the off position.

That is a link to a file on your C: drive.

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

{about displaying ignition on an oscilloscope]

No '500V' scale on an oscilloscope is gonna burn up with a brief transient, nor can such transients get over 1 kV (the distributor or coil would arc). Probes, on the other hand, are at risk. You'd be best served by a suitable divider probe, they exist. Or, a few cheap parts and you've made one.

Not true; probing the current is no less dangerous, stick with the V-versus-time and there's plenty of information present. More important, a dirty or worn spark plug might eventually pass the right amount of current, it matters what peak voltage it fires at, You'd be remiss in disregarding the voltage rise before conduction.

Gotta disagree strongly there. If you never 'scope out a working car, the diagnosis of a nonworking one is forever beyond your experience.

Oh, wow, have you bought any car parts lately? A set of wires ONLY for a 4- cylinder car can run $50 at the NAPA parts store. Parts for a 1948 car are likely to be VERY hard to find specialty items. IF J. C. Whitney doesn't have them, you could be out of luck.

The guys at the local Auto Zone would not even know what the term "points and condenser" means.

Jon

For working around car ignition systems, I would look for one of the older tube type Tektronix models. Try 535, 545 for the large ones or a 453 with the nuvistor tubes in the vertical front end.

These would survive better than a usual fet input amp.

Just my 2 cents.

Yeah but if you expand your horizons a bit beyond NAPA and AutoZones, there are places like Rockauto.com, which is probably 1 stop shopping for it.

He never mentioned models and engine types but I just looked up a 1948 Pontiac Streamliner with a 3.9L (239 cid) L6...

condenser is $1.31 to $8.52 dist cap $8.72 rotor $3.12 to $4.78 points $9.64 to $12.29 ignition coil $10.11 to $14.56 spark plugs $1.10 to $1.90 each

I don't see wires listed for those in 1948 but, those are just wires. Looking up a 1964 Buick Wildcat, 425cu 8cyl (at random), they have closeout sets from $6.74 to high performance ones with ceramic boots, $73.89.

So maybe $50 was on the light side, so if I say under $100 make you feel better?

Point is, it would be money more well spent just replacing all that compared to doing whatever he wanted to do with the scope, which probably isn't going to tell him anything.

Like someone else pointed out, a dwell meter (forgot about those), timing light and volt-ohm-meter will be far more useful.

-bruce snipped-for-privacy@ripco.com

I have not had a car with points in it in many years so may be way off on the following.

Wasn't the usual thing to change out the points, condenser, plugs and a few other things about every 10,000 miles or so back then ?

Seems to me that the 3 or 4 cars I owned with the old nonelectronics in them needed to be worked on all most all the time. I remember replacing a lot of plugs and some points, distributer caps and wires. None of those had over 100,000 miles on them. Now lots of cars go 100,000 without anything but oil changes and tires.