secondary voltage question

[Jim Lusk]

Been far too long since I took automotive classes in college and everything was different back then anyway.

Having an issue with my 440. I put my scope on it (unfortunately I need to fix or replace the #1 trigger lead). At one point I had peaks of 20-21kv on all cylinders (from the coil wire). After some time that peak dropped to about 8-12kv on most cylinders, but 14-18kv on #7.

What I cannot remember is if plug condition affects secondary voltage (my guess is that it does, but I'm venturing into new territory here). I'm going to put plugs in it tomorrow just because.

The symptoms are rough idle (perhaps one cylinder not firing - at least that's how it sounded today - no the scope wasn't on it at the time), difficulty revving (no load), but when I got the RPM up it seemed to smooth out, but still had the miss.

Carb is suspect, but float level is good, bowls weren't overfull (which is what I thought I might find). It's an Edelbrock 1407 that I've had since 2001. I'll probably put a kit in it mostly because it has sat for a few years now, but the engine ran well in May with no changes.

 

[Murray]

Boy, this has been a long time since I scoped! The initial spike has more to do with the spark plugs, their condition and their gap. If all the spikes are not the same we would suspect something uneven in the cylinders like plug gap, carbon build up, valve condition, plug wires. Also we would want to do a compression check on all cylinders. After checking these things you should be getting a good idea of the condition of the engine and the tune-up parts. Make sure the electrical parts are correct before you diagnose the fuel part.

 

[Joey4speed]

Yes,
I'm betting its ignition wire(s) or plug(s).
If I remember correctly, the tall spike voltage on the scope would indicat resistance, like what you may have seen on #7.
Before you start chasing your tail,
the used plugs tell the story, it should indicate which cylinder may be missing.
Before you go all out swapping things, I would systematically rule out the secondary ignition system:
- Check inside distributor for signs of carbon tracking.
- Remove and compare all plugs, especially #7 to the others.
- Replace that single #7 plug and that plug wire first, with new ones.
- Run engine to test it again.
If that doesn't get positive results.
-replace all 8 plugs and all wires, including coil wire.
Then, if it still misses, you can move onto compression checks, fuel, etc.

 

[67Dart273]

The initial spike in part indicates the voltage it takes to get the plug to fire. This is affected by many things including wire condition, especially back in the resistance wire days, the gap in the rotor contact, whether the plugs are resistor/ booster gap and what their gap is, and cylinder mixture.

The horizontal line is while the plug is firing, and this too is affected by the sum total of resistance and gaps in the system, so resistance in coil and plug wire, gap in distributor/ rotor, any booster gap in plug and plug resistor, and plug gap and of course any faults along the way like burned spots in wires, etc

I don't pay too much attention to indicated voltage as the scopes are generally not that accurate and are affected by WIRE INSULATION type and thickness

Biggest thing is uniformity between cylinders and plenty of "overhead" AKA coil reserve voltage

Before Google stopped working, you could search and find example traces online

 

[cudamark]

If you suspect low secondary voltage, and you have a points ignition, bypass the ballast resistor for a quick test to see if the idle smooths out. Otherwise, I'd go through a full tune up. Check compression, check the resistance in the plug wires (I don't like anything over 1000 ohms per wire), points, condenser, plugs, etc. Once you determine the engine and ignition system are sound, you can then play with the carb and fuel system. If the bypass test improves your idle, you may have a weak coil, bad plugs, or plug wires with too much resistance......or a combination of the three.

 

[Jim Lusk]

Custom ignition. Mopar factory electronic distributor, HEI module, MSD SS e-coil, no ballast. Swapped modules. I do have an MSD Blaster II coil that I could also use. Plugs are old, most else is relatively new. That's why I'm starting with the plugs, which I should have tomorrow.

 

[Mattax]

I may have something pdf format Jim. If so I'll email it to you.*
I agree gap and conditions affecting the difficulty th espark to jump will increase or decrease that voltage to initiate.
Here's expected secondary shapes for Chrysler, and HEI. This was found on g**gle awhile back. Now *uck*uckGo is better but doesnt have older web stuff.

*Looked but didn't find what I was thinking of .
Did find this page bookmarked which has Tektronixs Concept Books scanned and available. Tektronix "Concepts" Series Books

 

[67Dart273]

^^How in the world did you find that!!! I just downloaded the whole mess!!^^

 

[Jim Lusk]

May or may not be able to see the waveform until I get the #1 trigger fixed as it puts a big note on the screen telling me it's not working. I'll get back on it in a day or two.

 

[Mattax]

^^How in the world did you find that!!! I just downloaded the whole mess!!^^

Like you said. Back when g**gle actually showed a real search I found that and bookmarked it.
I probably didn't have enough hard drive space to save a lot of big files. Nor did I think good internet info like that would ever disappear or get lost. Boy was I wrong, or guess niaeve (sp?) !