yo,
If your coil is already off the Bronco, you can bench test your coil. To set up the test, remove one spark plug wire from its plug, then remove the spark plug using a spark plug socket. Next put the spark plug back into the spark plug wire. Be careful not to let anything drop into the empty spark plug hole.
Holding the plug wire with insulated pliers, find a spot on the engine that is a good and easily accessible grounding point. Pretty much any exposed metal, including the engine itself, will do.
Holding the spark plug wire with your pliers, touch the threaded portion of the spark plug to the grounding point. Have somebody crank the engine with the key, and look for a bright blue spark to jump across the spark plug gap. If you see a nice, bright spark (clearly visible in daylight) your coil is doing its job.
and/or;
Make a spark tester for that test;
W/ an old spark plug and cut off the L-shaped tip,& clean up the center electrode and surrounding metal with a bit of sandpaper
Drill a small hole in the thickest part of the metal (where the wrench goes on). Use a metal screw to attach a length of wire, a couple of feet, to the body of the spark plug. To the other end of the wire, solder or otherwise attach a big alligator clip.
Since the tip is cut off, the spark will have to be stronger to jump the wider gap. It is harder for a spark to form in a compressed cylinder so it more accurately represents the spark performance under running conditions. With the engine off, remove the spark plug wire you wish to test from the spark plug. Attach the spark tester to the plug wire and connect the alligator clip to a good ground. Remember that the coil output can give you a very nasty shock, so make sure to wear thick gloves or use a rag to hold the tester, and not to touch the body of the car when the system is energized. Hold the tester away from the car body and use your remote starter switch or have an assistant start the car (or simply turn it over). You should get a healthy bluish white spark from the center post to the side of the spark tester.
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Were the Ign Module & coil color codes in both diagrams correct? If not, try and see what colurs they are on your Bronco.
Also, verify this wiring diagram by SeattleFSB
Power is supplied to the BATT or + coil terminal in the same fashion as the older breaker point ignition. Under normal "run" conditions, power comes from the key switch through a length of resistance wire (0.8 to 1.6 ohms). During cranking, this resistance is bypassed by a wire from the "I" terminal on the starter relay.
You may have tested that wire w/ built-in Resistance
Fuse Block Diagram in a 78
Source: by highhorse78 (High Horse) at
http://fullsizebronco.com/forum/showthread.php?t=146214