On Mon, 25 Aug 1997 14:20:59 -0400 AdrianJones@compuserve.com wrote:
>..... Also checked voltage in and out of coil - about a steady 13 going
in and a steady 6 volts coming out. Is that OK?
NO! Absolutely not! You may have found the problem.
>..... I have a feeling it's something simple .....
Yup, for sure, something simple in the end. Bear with me here.
Check this first, just to be sure. The coil primary resistance should
about 3 ohms. My original non-balasted coil reads 3.2 ohms. My Lucas
40,000 volt Sport Coil reads 2.6 ohms.
Without a ballast resistor it should be battery voltage going into the
coil, about 13 volts is good. With a ballast resistor the input to the
coil should be about half as high, but should go to full voltage when
cranking on the starter. When the contact points are open, the
distributor side of the coil primary circuit should have about the same
voltage as the input side, since no current is flowing. When the contact
points are closed, the voltage on the distributor side of the coil should
be ZERO (referenced to engine ground), or something very close to zero.
The contact points should provide a near perfect ground to complete the
coil primary circuit. Then the voltage across the coil primary terminals
should be about 13 volts with no resistor, about 7 volts with a resistor.
It sounds like you're not getting a good ground through the contact
points and the distributor. The coil wire goes to a connector on the
side of the dizzy. Inside, a wire goes from there to the points. The
points are mounted (electrically grounded) on the moveable vacuum advance
plate. Drawing a vacuum on the diaphragm should make the plate move.
Between the moveable plate and the body of the dizzy there is a short
piece of finely stranded flexible wire, usually with a screw eye terminal
on both ends, sometimes welded to the moveable plate. That wire provides
a reliable ground for the moveable plate to the dizzy housing. The dizzy
housing is grounded on the engine block.
You have here several places to find a bad connection. Anything more
than 1 ohm total resistance between the dizzy side terminal and the
engine block is bad news. Something less than 0.25 ohms is good. I hope
you have a good ohm meter with a low reading scale. You want to be able
to read resistance to a tenth of an ohm. Check and clean all of these
connection points.
It is common for the dizzy to have a bad ground on the engine block. You
may have to remove the dizzy to clean the base mounting collar. Also
clean the clamping plate and be sure it clamps securely on the bottom of
the dizzy housing. You have to get a good contact here to the engine
block. Paint on the engine block is an insulator, but you should get a
good contact through the clamp and bolt anyway. Try for zero ohms
between the dizzy housing and the engine block.
Another common problem, VERY common, is that the small flex wire between
the movable plate and the dizzy housing can be broken inside of the
insulation where it's not obvious. This can make the connection
intermittent. It may have a perfectly good connection when at rest, then
when the vacuum advance kicks in and moves the plate at higher engine
speed, the broken wire moves and looses the connection. This one is a
particular pain because it's so hard to find. Remove the wire, or at
least one end of it. Put the ohm meter across the wire, stretch the wire
out to full length, and tug on it a little. If the conductor is broken
inside of the insulation, you will loose the connection.
If this circuit is lost when the engine is running, the ground path
becomes very unreliable, having to go through the moving center bushing
of the movable plate. The result can be much the same as turning the
ignition switch repeatedly off and on. It is a very effective rev
limiter by way of ignition cut-out, and you will get backfiring in the
exhaust when the current comes back on.
Betcha money? Simple cars. You should have it running well in less than
an hour.
Barney Gaylord
1958 MGA with an attitude
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