Difference between negative GND and positive GND
Pervious especially English cars had positive grounding systems, at the 60ties they changed to negative grounding. At that time the ignition coil was changed.
With a positive ground the ignition coil was marked with SW(ignition switch) and CB(circuit breaker, brake points). Internally in the ignition coil the high tension part was connected to the CB (brake points connector).
With the negative ground system the ignition coil was marked +(ignition switch) and –(brake points).
Internally the high tension coil was connected to +.
The way to check what type of ignition coil you have is to use a voltmeter.
Connect 12V across the ignition coil between +/SW and -/CB.
If you can read 12V on the voltmeter then your high tension coil is connected to +, if the reading is almost zero then the coil is connected to -, or CB.
When changing from positive GND to negative it is recommended to change the ignition coil to one made for negative GND, if that not can be done change the connection to the ignition coil so the CB will be connected to the battery and the SW to break points. In that case the internal connections will be the same as for the + and - marked ignition coil.
The following test is showing the differences between the positive ground systems and negative ground systems and why the ignition coil shall be connected the other way if changing from positive ground to negative ground.
Note that the test is done with an ignition coil marked + and -, it is initial done for a negative grounded systems but in this case it does not matter. A + and – marked ignition coil can be used in a positive grounded systems if it is wired the right way.
The difference between a SW/CB compare with a +/- ignition coil is the internal connection of the high tension coil. On a SW/CB the coil is connected to the connector for the brake points, on a +/- the coil is connected to the battery via ignition switch.
The spark plug high tension electrode or the ground electrode of the spark plug is cold or heated to show the differences depending of the wiring of the ignition coil.
On a spark plug it is the high tension electrode that became hot and not the ground electrode.
The test has been done with a special test fixture so the high tension electrode or ground electrode can be heated up.
The connection of the ignition coil is connected the same way as an ignition coil with marking SW and CB in a positive ground systems. The – is the same as SW, and the + is the same as CB.
Cold electrode
The spark plug ground electrode is heated.
No increase of the spark time.
The high tension electrode is heated.
An increase of the spark time is achieved.
This demonstrate what is happened if the ignition coil is wired the wrong way.
A cold electrode as reference.
The spark plug high tension electrode is hot.
No increase of the spark time. It is the same as the cold figure.
The spark plug ground electrode is heated up.
The spark time has increase.
A negative ground systems
When changing from positive ground to negative ground the ignition coil shall be wired with CB (marked + in picture) to +12V, and SW (marked – in picture) to the brake points.
The ignition coil can also be changed to a + - marked ignition coil.
A cold electrode or a hot spark plug ground electrode showing the same spark time.
With a hot high tension electrode the figures will be this.
The result is a significant longer spark time.
This showing that the ignition coil is wired the correct way.
The conclusion of this test shows that the connection of the ignition coil shall be changed or the ignition coil shall be replaced when changing from positive ground system to a negative ground.