Coil "polarity"

[MrWillys]

I have a Lucas coil. It does not have + or -, but SW and CB markings at the contact points. The SW contact is connected to the ignition side, and the distributor is connected to CB.
A coil is not really pluss or minus, so it is possible to connect it in any way, but it will affect spark intensity if you do it wrong. So far, so good.
I have been Googeling a bit, and it may be that my set up is incorrect, that the way it is connected is for a positive ground car? and for a negative ground car (mine), I should connect the ignition side to CB and the distributor to SW.
Am I right in saying SW = todays positive and CB = todays negative contact?
So guru Wes, what say you? 6 volt system negative ground, 43 Willys.

[wesk]

The negative on the US coil refers to the direction to ground. Which is the distributor. I do not know what the internal wiring is on a Lucas coil.

[evanso1975]

Check out the second bottom paragraph on this page:

http://mgaguru.com/mgtech/ignition/ig109.htm

[wesk]

Owen,
Excellent reference. Let's put it here to keep the thread in tact.

The following quote is from the MGA Guru web site:

IGNITION COILS Variety - IG-109
These are examples of ignition coils you may find on an MGA.



The coil on the left is original style with plain metal case and female threaded screw in high tension connector. See diagram at left for the connector style. This one originally had a "Lucas" sticker on it. The primary terminals would be screw posts for 1500 and early 1600 cars, but may have Lucar push on terminals like this one for late 1600 and 1600-MK-II cars. The bracket is a more modern replacement type with a tab in the center which might be used to attach a condenser (capacitor) for RF noise suppression, or a bracket for a ballast resistor (in other applications).

The coil in the center is a period accessory style Lucas High Energy Coil. This has it's own unique bracket with the Lucas logo in blue. The high tension connector is original screw in type. Primary terminals are original screw posts.

The coil on the right is a modern Lucas Sport Coil (40,000 volts capacity). The gold color case is standard for this item as well as the bracket with the black sticker. The high tension terminal has the more modern push in type connector. The primary terminals have screwed on Lucar connectors which can be removed if you want to use early style ring lugs for the wire connectors. This model has been available since at least the early 1990's, and maybe longer.

If you just need a replacement coil to keep you running, you can pick one up at almost any local auto parts store. I recently bought one at NAPA as Echlin number IC64. You may expect these to be painted black, and they may have push on terminals which may be either riveted in place or removable to expose screw posts. The key feature is that you need a coil designed for 12 volts non-ballasted ignition system. This will have primary coil resistance of about 3.2 ohms. A coil for ballasted ignition system (like late model MGB) will have primary resistance of about 1.6 ohms. You should take an ohm meter with you when you go to buy a coil, and look for about 3.2 ohms primary resistance for the MGA. A high energy coil may have slightly lower primary resistance, perhaps around 2.7 ohms.

Ignition coils are available under many brand names such as Bosh, Crane, Mallory, and a great variety of house brands, and the many available colors are irrelevant. Many of the well known brands will feature high energy or high voltage, while the house brands may be either high energy or lower cost standard coils.

There is nothing magic about a high energy coil, but it may help the engine start a little easier in cold weather or adverse conditions (like fouled spark plugs). When parts of the ignition system are older and somewhat deteriorated the higher voltage may help fire the spark through high resistance plugs or wires. It might also cause arcing and misfire if the high tension wire insulation is breaking down, or the end boots are deteriorating.

A final note about terminal markings: Later production ignition coils were marked "+" and "-" on the primary terminals, which makes hook up pretty much self-evident. The primary terminals of early coils for positive earth setup were marked "SW" for Switch, and "CB" for Contact Breaker". In the positive earth system the ignition switch supplies power from the Negative battery post, and the contact points ground the other side of the coil to the Positive battery post. Therefore "SW" = "-" and "CB" = "+". This bit of knowledge and logic can avoid confusion if you change your vehicle from positive earth to negative earth. In that case you reverse the coil wires, and the "CB" terminal connects to the positive Switch wire while the "SW" terminal gets grounded to negative through the Contact Breaker. If you're still confused, buy a new coil with the "+" and "-" markings.
On 6/23/2011, Fletcher R.M. wrote:
"Coil shown at left is not original MGA. That is a Sports coil, 60s era more or less. Will be dated by WWYY, Marked SA12 = Model (Sports, 12V), ribbed red insulator, slightly larger OD than stock HA12, which has smooth black insulator. Coils for Morris etc are LA12 - 20Kv, "high performance" cars like MG are HA12 - 30Kv, Sports are SA12 - 40Kv. Not sure when the ribbed case ones were made, but every one I ever saw (in 60s) was on a 40s-55 car and had obviously been there for a while. It may be dated too". - FRM

[MrWillys]

The primary terminals of early coils for positive earth setup were marked "SW" for Switch, and "CB" for Contact Breaker". In the positive earth system the ignition switch supplies power from the Negative battery post, and the contact points ground the other side of the coil to the Positive battery post. Therefore "SW" = "-" and "CB" = "+".

I'm getting a head ache, but this says that on my car with negative ground, I should connect the ignition to CB and the distributor to SW? That is not the way it is wired at the moment.

[wesk]

As the Lucas/MGA Guru says, yes.