[Top] [All Lists]

Re: MGA 1500: Plug wire/cap resistance?

To: "Barney Gaylord" <>, "MG List"
Subject: Re: MGA 1500: Plug wire/cap resistance?
From: Steve Morris <>
Date: Fri, 15 Jun 2001 00:58:23 -0400
On 6/14/01 11:29 PM, Barney Gaylord  <> wrote:


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

Yep, probably. I put the original condenser back in, so that was one 
variable I eliminated.

>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.

Sorry, not enough info... I checked by pulling the coil end of the high 
tension wire off of the coil and cranking over the engine with the 
ignition on. The spark was between the coil and the wire. I have since 
taken the dizzy end of the HT coil wire off, and held it to ground. Good 
blue spark about 1/2 to 5/8 long or even more.

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

Looks like new. (Later: Oops! See below)

>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.

Solid core wires. See below.

>>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

Sorry. One VOM probe on the end of the plug wire that was bare because I 
took it out of the distributor cap, leaving a bare end. The other probe 
is sharp, and I poked through the wire insulation right behind the plug 
end boot, and got .3 Ohm.

>>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.

I didn't know how these spark plug connectors came off, so I didn't start 
pulling and twisting. I have theoriginal type, not a traditional rubber 
boot. End to end, the wires show a reading of .3

The connectors themselves, when checked from the screw that goes into the 
wire to the plug friction connector, do not show any continuity, or 
reading on any scale of my VOM, on any of the four of them.

>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.

That's it! I just tried this, and a big blue spark jumps from the HT coil 
wire to the rotor (which of course is the one item I did not purchase yet 
as a replacement, nor a dizzy cap.) Even I know that shouldn't happen, 
but it didn't show up as a short under VOM continuity check, because of 
the low voltage/current.

Thanks, Barney! You saved the day, again. I still don't understand why 
the spark plug connectors do not show continuity. There must be resistors 
molded inside with more resistance than my meter can read, and shows as 
an open.

Thanks again,

Steve Morris     Avon, Ohio
1958 MGA 1500    Red/Black
NAMGAR #5987     BuckAyes Ohio Chapter
LoCo Brits       <mailto:>

/// mailing list
///  (If they are dupes, this trailer may also catch them.)

<Prev in Thread] Current Thread [Next in Thread>