Also check voltage at coil's "+" when starter motor is turning engine, this can be a separate power pathway depending on the year of the vehicle.
TFI testing seen here:
http://www.therangerstation.com/tech_library/TFI_Diagnostic.shtml
A coil sparks when Ground is removed, that's what the old Points setup did, every time the points separated it cut the Ground "-" on the coil, when points connected again coil was "recharged' to spark again when Ground was cut.
Electronic ignitions do the same thing, they just do it without the need to adjust or replace the points all the time.
Sensor in distributor sees the shaft position as it rotates and relays that to spark control "module"(TFI in your case).
Module cuts the Ground to the coil to make it spark.
You can test a coil by powering it up and cutting the ground, nice blue/white spark is what you should see, yellow spark mean weak coil or low voltage at coil.
The voltage IN is multiplied by a set amount and becomes the voltage OUT, so lower voltage in = lower voltage out