Coil + and - @ CB and SW

Coil + and - @ CB and SW

Postby EastEndJohn » Sat Dec 08, 2018 1:03 pm

I have a spare coil , which has + and - on the nose end . The original coil markings are: CB and SW (eg Con. breaker / Switch .)
Am I right in thinking , that the minus - sign , is the SW terminal : and the plus + sign goes on the CB terminal ? I hope this makes sense ??

( ps . my 1948 P3 had no battery when purchased ) I believe the following is the case if there is any advise please:
• It depends on whether you are still +ve earth or have converted to -ve earth.
• If +ve earth then the CB = +ve, SW = -ve.
• If -ve earth then CB and SW will (or should) have been reversed.
• So +ve earth use + to contact breaker,
• If -ve earth use + to the ignition switch.

so to confirm : I have re built the original Dynamo and flash sparked the F post on it with the battery in the Pos Earth position therefor the car is now Pos earth and so on the coil pos earth CB = + ( plus ) and is connected to the distributor leaving SW or - ( minus)connected to the switch.

I hope I have not made this more complex than needs be!

many thanks
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Re: Coil + and - @ CB and SW

Postby GOY189 » Sat Dec 08, 2018 7:28 pm

There is a very good guide at

This says
Definition #l Coil Polarity (in relation to battery)
The polarity of the coil should match that of the battery by connecting it so (+) goes to (+) and (-) connects to (-). But don’t worry about which way you install the battery (positive or negative ground) or which way you install the coil (regardless of coil markings) it will automatically adjust itself. The coil will work efficiently and put out the same voltage either way it is hooked up, but the spark plugs are more sensitive when it comes to polarity, hence our second and more important definition.

Definition#2 Coil Polarity (In relation to spark plugs)
Coil polarity should be such so as to provide negative polarity to the spark plugs center electrode.

It has been found that it takes approximately 15% less voltage to form an arc at the plugs, if the hotter center electrode is negative, and the cooler (by comparison) ground electrode is positive. The center electrode is hotter since heat transfer from the tip must make its way through the porcelain insulator past the sealing gaskets to the shell block and then to the water jackets.

If your center electrode is positive, your car will probably still run fine until, with its 15% handicap, it exceeds the coil output. If you live where temperatures dip down to 0° you may not get your car started. Driving with a full load and accelerating hard up a hill may cause an ignition miss. If your ignition system is well-worn to where you have various voltage losses, you could get a miss.

Correct coil polarity won’t eliminate these problems, just put them off by 15%.
If your coil has – & + markings by the primary terminals, you will be pretty safe by hooking it up by those marks, but test it for correct polarity anyway, using one of the tests listed further on. If your coil has CB & SW or BAT & DIST markings, there is no way of telling if the coil was marked in relation to a positive or negative ground car, and the only sure way to tell if the coil is installed right is to test it out.

You test for correct polarity by hooking up a voltmeter with the negative lead to the plug terminal (which should be of negative polarity) and the positive lead to the block (which should be of positive polarity). Set the meter on the highest volt range. These connections remain the same whether you have a positive ground or negative ground electrical system. The secondary winding’s polarity which we are testing is determined by the combined hookup of the battery and primary windings, so it may or may not match the battery’s ground.

Cranking the engine over (you don’t have to start it) should show an upward swing of the voltmeter needle (don’t be concerned with taking a reading). If the needle swings down off the scale, your coil is hooked up wrong. To correct, reverse coil primary leads. Do not worry about the coil markings (refer to definition #1.)

If you don’t have a voltmeter, test by removing a plug wire from a plug and hold a plain lead pencil point in the path of the arc. A flair (hard to see) towards the plug shows correct polarity while a flair towards the coil shows reversed polarity."
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Re: Coil + and - @ CB and SW

Postby luli » Sat Dec 08, 2018 8:25 pm

For many years Lucas system had been positive ground. Therefore in old lucas coils SW was the (-) and CB was the (+). If you use an old Lucas coil and negative ground system, you should reverse the connection ie CB on the coil to the switch etc. See pictures here
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD
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Re: Coil + and - @ CB and SW

Postby EastEndJohn » Sun Dec 09, 2018 1:32 pm

You two guys are a treasure, Nice to see there are still those that care enough to help those in need. So many thanks and do keep them coming. You have both given me lots to think about.
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