ignition pattern problem

[cth350]

OK, I've stumped myself. Someplace around here I have one or more books that show various ignition patterns and what faults they represent.

I just dug out my old scope from the back of the garage, rolled it up the driveway and around the rasberry patch to get to the front of my 250 (M114) sedan, which is parked the wrong way in the driveway to make this easy (why should it be easy on this hot, humid, miserable day?).

Now that I have a pretty picture, I can't find the blasted reference material to check against.

Any opinions out there?

-CTH

[cth350]

Well, the "attachment" option, below, doesn't seem to work, so here it is explicitly...



-CTH

PS. And I neglected to mention that the car is wired up for a conventional black coil, single balast ignition system and I think it's missing the condenser.

[bachlauter]

CTH..

Is THAT a picture of a UFO sighting?????? (hee hee)

Btw....I DO own a 114....could I be of assistance??

~Christy

[cth350]

Christy,

That picture is of the voltage pattern of my ignition system. Check out those books I sent you (since some of the info I'm looking for is in there), and read about the primary and seconardy sides of the "conventional, breaker point ignition system".

You "read" the picture left to right over time, while up and down is voltage, lots and lots of voltage.

The basic idea (as I recall from ignition 101), is that the points close for a while, and during that time, a charge builds up in the windings of the coil. When the points open, WHAM, you get a spark. The picture, above, is what that spark looks like to a very fast volt meter (the oscilloscope).

The "correct" picture of a single spark shows a big spike (like this one) and several little ones, called ringing, right after it (like this one), BUT, I think there should be more little ones than this, AND I am not sure what to make of the extra bump in the middle. Lastly, the little tail before the big wham means something, but I forget what.

So, part of tuning your car is to have the "best" spark you can. And part of the job is making sure that the spark gets where it has to go at exactly the right time.

The quality of the spark is dependent on the health of your battery, the various primary wires (the stuff that carries 12 volts), coil, cap, balast(s), connectors, ignition module (if present), rotor, condensor (if present), points, plugs, engine ground and distributor.

The timing of the spark is dependent on the gap of the points (measured as dwell), position of the distributor relative to the motor, and the timing of the valves relative to the piston (ensured by your timing chain & cam shaft & valve clearance).

We went over your motor's spark timing and with the exception of that 4-5 degrees of timing chain stretch, your car's ignition timing is rather OK.

One sure-fire way to ensure a quality spark is to replace EVERYTHING with new components. That's what you did by getting new points, rotor, cap, wires, plugs (which you've cleaned a few times). Also, the tests to observe the voltage in the primary circuit confirmed that all was rather healthy. You replaced everything but the coil itself. Which MIGHT need to be replaced, but do you want to blow 100$ on a coil just because of a might?

The engine oscilloscope check I just did is how you can confirm a coil problem, and in fact most any problem with the ignition. All you need is this fancy scope. In my case, I just happen to have two of them. One seems to work (thus the picture), the other is very suspect. Tomorrow, I dig that other one out of the basement and try it on for size.

A while ago, there was an article in the star about building your own engine scope for under 100$ (thanks to a used component often available on ebay). If I bothered to pick up a small brass rod like I was supposed to this week, I'd have THREE scopes and would be trying that one out tomorrow too, but I never did get to that.

-CTH

[Jim H]

My take on your 'scope picture.

You're correct about the term 'ringing' and what you have shown there is a classic inductor/capacitor current discharge waveform. While there is no actual capacitor on the secondary windings, there is 'stray' capacitance so that's normal.

The number of up/down excursions will give you an idea of how long it takes to dissipate all of the stored energy from the coil; a few 'rings' means lower resistance load while a lot of rings means higher resistance.

The second set of waves could be from "point bounce" if the points manage to drop back closed briefly because of a worn rubbing block on the points, distortion in the distributor cam lobe, or possible both combined with a too-small points gap. It could also be caused by arcing across the points due to a weak condenser or a small flashover in the secondary either internal or exteral.


Now I'd like to go over what I remember about 'Kettering' spark Ignition. Points, coil, distributor, spark plug(s). Origin of the the vintage engine's 'tune-up' of "Points, Plugs, Condenser (another name for a capacitor) and set the timing" which is now a thing of the past thanks to electronic ignitions.

The coil is actually a complete 'COIL' assembly and consists of primary and secondary insulated wire windings around an iron core. A basic electromagnet. The primary windings are the low-voltage side, and the secondary windings are the high-voltage side.

Apply 12v across the primary windings (sometimes including a 'ballast' resistor in series) and current flows through them. Energy from the voltage and current is stored in a magnetic field in the iron 'core.'

The higher the current and the more 'turns' of wire in the primary coil the higher the magnetic field strength. The proper current and number of turns has been worked out already by the engineers.

The points stop the this primary current flow when the contacts open. Energy cannot be stored in a magnetic field, except at superconducting temperatures, and it doesn't get that cold even in Canada.

The field collapses and the stored energy, looking for a way out builds voltage rapidly; that energy wants to go somewhere and will try anything to do so!

The secondary windings have a lot more turns than the primary, so the voltage builds to very high levels, 25,000 volts or better!

A spark gap in a spark plug looks like a dandy place for this energy to go. The high voltage causes a spark to leap the gap, and the energy heat things up (like a 14:1 air/fuel mixture) quite nicely!

The collapsing field's energy also builds up voltage in the primary windings. While we want the energy to jump the spark plug's gap, we DON'T want sparks to jump the gap in the points, because sparks cause metal erosion and produce carbon deposits. So, we put a capacitor (condenser) across the contacts. The energy from the field collapse in the primary winding is absorbed by and stored in the capacitor rather than arcing across the points.

Now all we have to to is steer this voltage with a 'distributor' to select which cyclinder to 'fire.' Non-electronic multi-cylinder distributors combine the points, capacitor and high-voltage steering functions, with one 'coil' to make the high voltage spark.

The resistance of both the primary and secondary windings can be measured with an ohmeter, and it's an easy test to identify a coil as a cause of "no spark." If there is no resistance there is no continuity in the wires, so current can't flow and the coil won't make spark. The proper resistance values are published in the technical specifications for each engine's ignition system.

Hope this helps anybody, but it was a fun exercise for me!

BCingU, Jim

Not real sure what we're looking at. I am assumeing that it's a single cylinder on the "display" function of the scope? I'm trying to send an attachment with a basic image of what goes where.
Regards,
Randy D.

[cth350]

There was yet another reply by me to this thread last night. Not sure where it went.

The picture there now is one single cylinder snap'd from an overlay of all of them (the effect of the camera on the CRT).

I figured out which book I was looking for, namely the benz technical training manual a friend gave me way back when. It has dozens of sample traces.

The first thing that stands out about that first trace, is that it's upside down. The coil was installed by the PO backwards. I should have noticed that already. I clearly was more interested in playing with the carbs than I was with getting the car in good working order.

The other thing to note in the picture, once that's corrected (please see shot that was in previous & missing post ). Is that lead in section of the pulse has too much of a down-slope. I think that means it's time to clean the plugs and replace the old wires. A more concrete answer than that would be what I'm fishing for.

My goal is to do this though "logical and thoughout" processes, rather than knee-jerk replace the parts 'cause g-d knows when it was done last.

Thanks all.

-CTH

[cth350]

Here's the current image from cylinder 1.

[Jim Villers]

Very interesting discussion on basic electricity but I must have missed the objective. I also suspect that your measurement is voltage rather than current.

Did I read the thread correctly that the distributor does not have a capacitor? The purpose of the capacitor is to prevent arching when the points are opening and therefore creating a "cleaner" spark. If you do not have a capacitor, that could cause the secondary spikes as the arcing occurs. The capacitor also "slows" the initial field build up to prevent sparking when the points are closing that could be your secondary spike.

Since you have spark, does the car run?

[cth350]

The cars much better now that I put the coil in correctly, as opposed to before when I first got it. I believe the front carb needs rebuilding, but I'm not going to do that until I have fully and totally diagnosed the hell out this thing and written it all down. There is a story in the making here and I am documenting the trail as best I can.

As for the condensor, I should be more explicit. The car was converted from the correct, transistorized system to a black coil, "conventional" setup. The PO switched the coil (black for blue) and the balast (single gold for 1 blue, 1 silver). He left the original "green wire" with its integral noise supression consensor, not the correct one for a black coil.

-CTH