The Ford EDIS (Electronic Distributor-less Ignition System) uses a set of coil packs to fire the spark plugs. Like the more traditional e-core and oil filled coils, this coil pack works on the principle of magnetic induction. Simply put, when a current is passed through the primary winding of the coil, a magnetic field develops around the coil. When the current is turned off, the magnetic field collapses and induces a high voltage spike into the secondary winding. This high voltage spike is the spark that fires the air fuel mixture by jumping the gap on the spark plug.
Refer to the EDIS ignition system wiring diagram located in the service manual. Following that diagram, identify the power and ground wires for the individual coils in the coil pack.
Test for power, at the coil's wiring harness connector, with the automotive circuit tester. Both the power and the ground wires should test positive for power with the key on and the engine not running. If power is not supplied to the coil, check the fuses for the ignition system located in the under-hood power distribution block (fuse block). If power is present at the coil on the power wires, but not on the ground wires, the coil pack has an open circuit. Replace it as needed.
Remove the electrical connector and test each coil for resistance with the digital volt/ohm meter. Compare the readings of the resistance tests to the specifications in the service manual. Resistance specifications vary from year to year, and engine to engine. The service manual for your specific Ford vehicle will list the correct specifications for your engine and year model. Any readings that fall out of the specified resistance range indicate a faulty coil. Replace as needed.
Test each of the coil secondary circuits for short circuits by setting the digital volt/ohm meter to "Continuity Test" and touching each of the plug wire sockets with one lead, while holding the other lead against the metal coil bracket. An indication of continuity proves the coil secondary is shorted to ground. Replace as needed.