nruff wrote:
>
> Hey-
>
> I seem to have very little time to work on my project anymore but plan on
> traveling to my folks this weekend, spending Saturday afternoon and evening
> tackling a miss problem on my 69 2L. I'm 99.9% certain it's in the
> ignition.
>
> This is what I have:
> New Cap and Rotor
> New Wires (Old Wires Did the Same Thing)
> New Plugs
> Pertronics (no points)
> Old Coil
> Old Resistor
>
> The miss is not consistent but is almost always there. I used the inductive
> p/up from my timing light and determined that the miss is present as far
> back as the coil wire. Tell me if I'm correct in my thinking, but this
> leaves only the distributor or coil to be at fault, right?
>
Sorry, Nathan, not right. The points (or Pertronics, in your case) provide the
12v pulse that fires the coil, so any problem there would be present at the
coil output. The distributor houses the points (or sometimes an electronic
replacement thereof) AND distribute the high voltage to the plugs.
So it really houses TWO sections of the firing circuit.
Think of the firing circuit like a chain. It has a beginning and an end. The
END is the spark gap of the spark plug. The beginning is a little harder to
pin down. THe battery is the ultimate beginning electrically. Practically,
the points (or Pertronics or equivalent) is the electrical
beginning of the firing circuit. There is a mechanical component, too. We
have to somehow get the spark in the right place at the right time, so we need
a way to tie in to the engine's cycle. We do that via the distributor which is
driven by the jackshaft. A miss like yours is not likely to be
mechanical in nature, so let's skip that part for now.
So what's the chain? Well, the distributor provides the mechanical timing to
the points (or electronic replacement) to pulse 12v to the coil, which steps up
the voltage and sends it via the coil wire to the distributor to be sent to the
appropriate spark plug.
You've confirmed that the miss is present at the coil wire, which is about in
the middle of the chain. Anything farther back (towards the beginning) is a
possible culprit. What's farther back?
Your coil, your resistor, your Pertronics unit. Or the wiring between any of
these.
You need to verify the integrity of the pulse TO the coil. Your timing light
probably won't do this very well. Checking for a miss before the coil is tough
to do unless you have an oscilloscope (and can fly it) or the kind of
electronics that a garage uses. A regular voltmeter just doesn't respond
fast enough for this.
A good electronics troubleshooting technique is called circuit splitting. Good
technicians do this. (Most technicians throw fixes at a problem and hope
something works.) You've made a good start at circuit splitting - you know the
problem is present at a certain point. You need to go BACK in
the circuit now and find a point where the problem is NOT present. Ah! Now
you've got him trapped! You know where IS and where he isn't - he starts
somewhere in between. Split the circuit again and test for the problem. He's
not here? Split it again. He's there? Take a point in the circuit
halfway (approximately) between your LAST point (where he wasn't ) and you
present point. (Where he is.) It's kind of like calling in artillery fire -
you bracket the target and walk it in.
Hope that helps!
Mark van der Hoek
Houston, for now.
--
"They that can give up essential
liberty to obtain a little temporary
safety deserve neither liberty nor
safety."
-- Benjamin Franklin
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