On Tue, 23 Aug 1994, Will Zehring wrote:
> I am finishing up the wiring on the '63 B and have a question regarding the
> coil. In the Haynes manual it identifies the two plug connectors at the top
> of the coil as "sw" and "cb." When I look at the coil I see only a (+) and
> a (-). Keeping in mind that this is going to remain a (+) earth car, I
> suppose that the lead that goes from the coil to the plug on the body of the
> distributor is (-)? Yes/No?
No. The original coil was labeled "SW" (ignition switch) and "CB"
(contact breaker) to indicate which wire went to what (but see last
paragraphs below to confuse matters). On a positive ground car, the
ignition switch is the negative (-) side of the circuit, the contact
breaker (points) completes the circuit to ground (+). If your coil is a
replacement marked "-" and "+", connect "+" to the points and "-" to the
switch.
The car will probably run if the connections are reversed, but you won't
get as hot a spark. You'll notice it under load and at high speeds.
In most of our cars the original coil is long gone, and wiring up a
replacement isn't always as straightforward as it might seem to be. If
your coil terminals are labeled "+" and "-", make sure the terminal
corresponding to the grounded battery terminal is connected to the
points. In other words, for a positive ground system the "+" coil
terminal is always connected to the points, and for a negative ground
system the "-" terminal always connects to the points.
It gets trickier when the coil's terminals are marked "SW" and "CB",
because not all "SW/CB" coils are created equal. Some are intended for
positive ground systems, others for negative ground systems.
Sometimes (but not always) the coil will be labeled "Positive Earth" or
"Negative Earth", which makes things easy. If your coil matches the
electrical system (i.e. positive earth coil on a positive ground system or
negative earth coil on a negative ground system), then connect CB to the
points and SW to the switch and you'll always be right. If you use a
positive earth coil on a negative ground system (or vice versa), reverse
the connections.
If a "SW/CB" coil doesn't say whether it is for positive or negative
ground, start by assuming it is for positive ground. That's usually a
safe assumption since most unlabeled coils are pretty old, dating from the
days when most BritCars used positive earth systems. A well-equipped shop
can test for correct polarity, but you can do it yourself with a common
lead (actually graphite) pencil. Pull off a plug wire and start the
engine. Hold the plug wire close enough to the plug for the spark to jump
the gap, and hold the pencil point in between. You should see a flare
from the pencil point toward the plug. If the flare is from the pencil
point toward the plug wire, then your coil connections are backwards.
Oh yeah, you might want to wear thick rubber gloves while doing this...
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Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
feold@umd5.umd.edu 1962 Triumph TR4 CT3154LO (daily transportation)
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