I am checking about 100 coils I have to see how many I can rebuild. I have some that the ohm reading is below 3.oo and some above. what is each of these readings telling me?
Between the two terminals on the side you should measure about 3250 ohms (High tension coil). Between the terminal in the bottom and the nuts that holds the upper trembler you should measure something like 1/3 ohm (low tension coil).
Fun Projects makes a HT tester, that easily determine if the HT coil and connections are OK and the couls is rebuildable or better used as fuel in the workshop heater :-)
I know how to test good from bad. what does a reading below 3250 indicate and what does a reading above 3250 indicate?
It depends on how far below and how far above. If it's a little below, it's probably a later KW coil. These are fine for rebuilding. (Most later type KW coils will show a reading that is lower, around 2700.) Earlier ones are closer to 3000 and some go a bit over. Ford coils tend to run in the 3000-3500 range if they are good. A few will go as high as 4000. All the lower readings mean is that there are probably a few shorts in the high tension windings, a minor concern. The lower the reading the greater the number of shorts. Personally I don't like to rebuild any Ford coils with a reading of less than 2850, or any KW coil with a reading of less than 2500. YMMV. A very high reading, say near or over 6000, probably means a broken wire in the high tension with a bit of conductive substance like carbon in the break, allowing current to flow but at very high resistance. One particular modern make of inexpensive high tension winding made for the T has a resistance of around 7000 due to their use of finer wire than Ford and KW used.