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Re: MGA 1500: Plug wire/cap resistance?

To: Steve Morris <>, "MG List" <>
Subject: Re: MGA 1500: Plug wire/cap resistance?
From: Barney Gaylord <>
Date: Thu, 14 Jun 2001 22:29:27 -0500

At 09:53 PM 6/14/2001 -0400, you wrote:
>.... The point gap was .006 with a worn rubbing pad, so I got a new one
and a new condenser.
>.... adjusted to exactly .015 gap. I've got the new condenser in, but I've
since checked the old one, and it was OK, too (.2 mF.) I've replaced the
questionable internal wiring,

You did too many things at once.  Now you may have to start from scratch
with the debugging.

.... I've gone through the 9-step low tension wiring check, and I've
checked the coil for internal grounding and correct resistance (~3.5 Ohm.)
I get spark at the coil (jumps about 3/8", but I don't know how bright or
"snappy" it should be.)

Blue spark is better than orange, but 3/8" should be hot enough to fire the
plugs anyway.  What did you do to get the spark at the coil?  Was this done
by cranking on the engine, or by disconnecting and touching a wire, or
manually opening the points, or what?  If it was by cranking the engine,
then you can figure the problem has to be in the high tension circuit
between the coil and the plugs.  If you got the spark by some other method,
we gotta talk.

>.... I am not getting any spark at all at any plug! I checked for
continuity though the cap's center electrode to the end of the coil wire,
so that's good.

A crack in the rotor can allow the spark to find ground on the dizzy center

>So I just pulled each spark plug wire and checked for continuity. Nothin'.
Nada. Zip.  All four wire show no continuity from the bare end to the
snap-on boot at the plug end.

If you happen to have carbon core wires you should have a wire resistance
of about 5000 ohms per foot of length.  This is enough resistance to look
like an open circuit to a test light, but easy to measure with an ohm
meter, and a good spark will go through a wire with up to 20,000 ohms
resistance and fire the plug.  Solid core wires should have virtually no
resistance showing between the end connectors.

>Further, I've poked the digital VOM's probe though the wire sheath at the
plug end, and checked back to the bare end, and I get continuity, and a
reading of .3 Ohm on my lowest (highest) 200 Ohm scale.

Uh, which bare end? The near end will show low resistance even with carbon

>I get no reading or continuity from the wire immediately behind the
snap-on plug boot through the boot itself (I'm talking about the molded
phenolic cap/boot that snaps over the spark plug.) Is there a resistor in

No.  But you obviously have no direct electrical connection there, and you
should have.  The original type MGA spark plug connectors have a screw
thread in the center to penetrate the end of the wire to make the contact.
You can grip the wire firmly near the end and unscrew the connector from
the wire.  Then cut about 1/2" off the end of the wire and screw the
connector back on.  A little soapy water or spit on the insulation jacket
makes it easier going.

If you are lacking continuity to the HT wire connector at the distributor
end, then you need to remove that terminal, strip back the insulation to
leave about 3/8" of the center wire exposed, lay the wire back over the
side of the insulation jacket, and crimp the terminal back on the end such
that the terminal makes direct contect with the wire conductor.

If your plug wires have rubber covered angle connectors on the output end,
refer to paragraph immediately above.

For carbon core wires you need to use a short piece of solid wire (like a
paper clip for instance), poke that deep into the center of the HT wire
(axially from the end in) to make contact with the carbon core, then bend
the wire back over the insulation jacket and crimp on the terminal.  If you
leave any physical gap between the end terminal and the carbon core the
continuing spark as it runs will in short order erode away the carbon core
until the gap is so large that the spark can no longer jump the gap.

>Is it so high a resistance that it would show as an open circuit with no
continuity on a digital VOM?

Yes.  If you have a small gap with no direct connection the ohm meter will
show open circuit (maybe 50 mega ohm), and the spark may or may not jump
the gap depenging on how large the gap.

>If not, how in the world could I have failed all four at once? ....

Remove the dizzy cap.  Pull the H/T coil wire from the cap.  Hold the coil
wire near the center of the rotor and crank the engine.  If the spark jumps
from the coil wire to the rotor you have a defective rotor that is allowing
the spark to find ground on the center shaft.  Replace the rotor.

Barney Gaylord
1958 MGA with an attitude

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