This is how to test an ignition coil for the 1980-1984 Yamaha xs650. This method will work with any model, however the numbers shown for the correct operating range in this guide are for the xs650 only, your model could have different ohm requirements, so check your shop manual for the correct readings for your model.

The ignition coil will need to be unplugged from the wiring harness at the point where the smaller wires connect to the coil. Once those are un-plugged from the coil, use a ohm-meter to test the primary and secondary windings.

To test the primary windings, connect the ohm-meter to the points on the coil where the wiring harness connected to the coil, put the red lead, or positive, of the ohm-meter to the  red or white side of the coil connection, and then connect the negative lead of the ohm-meter to the gray or orange side of the coil connection. The primary resistance should be 2.25 to about 2.75 ohms.

To test the secondary windings in the coil, put one lead from the ohm-meter to each end of the spark plug wire connections. The secondary winding resistance should be 10,500 to about 15,500 ohms.

If the readings you find are different, then you will need a new coil, if they are not, then the problem is elsewhere in the electrical system.