Here are some expected readings:
Meter on coil + with key in "run." : You should see somwhere between 6--10V and not 12V. The lower voltage indicates the coil is properly drawing current through the box
Meter on coil -- with key in "run": You should see a low voltage perhaps .5--1 or 2V, the lower the better. If you read full battery, or in any case way higher than 2V, the box is not grounding the coil NEG and properly causing it to conduct
Meter on coil + cranking to start USING THE KEY: Should be close to "same as battery" when cranking, IE higher than 10.5. Compare by connecting meter to battery + and crank and read meter. Battery should be above 10.5 minimum. Below 11 indicates battery is low on charge or old and getting weak, or just plain too small (cranking amps) for the application.
Once again, to try and separate a harness/ car body wiring problem from the ignition system itself, jumper battery power direct to coil + and try check spark. Do not leave this attached any longer than to test, it's hard on the coil
