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coil? square box on fender? help

9441 Views 11 Replies 5 Participants Last post by  cartmanstwin
so finishing up the rebuild, the bronc was driving down the road like a champ!! however, no taillights or brake lights. I work on the wiring and amazing I have tailights and brake lights. I then go to start up the bronco and take it down the road with all the lights and everything. will not start, is not even firing. ive checked all my fuses and all the plugs I unpluged (cuz i had some issues and I unplugged all the plugs from under the hood) but it will not start!! could the coil have just gone out, when it has started fine for the last 30 starts? what about the square box on the side of the fender. When I check power with a test light, there is power coming out of the box, but it seems dim. also the other plug that goes over to the coil, the power is intermittent (flashing on the light). I know that the stuff I did to the tailights is on a different circuit and all the fuses are good. HElP...
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Devil - how about filling in your Bronco info under your name so we can help without having to guess too much?
thanks for all the responses and miesk5 I was also thinking that, Im hoping to get out to the shop within a next couple of days and will give that a try thanks for that amazing diagrams
I keep a Duraspark spare in every Ford I drive. :whiteflag
thanks for all the responses and miesk5 I was also thinking that, Im hoping to get out to the shop within a next couple of days and will give that a try thanks for that amazing diagrams
yo, YW,
And the thanks goes to Indybronco for taking time to find and post it up.
Ok, so I got a used dura spark from the junk yard, still sounds like no spark! I am gonna buy a new one just to make sure not bad as well. When I had the test light on the coil as I turned it over, the light was very dim. Is it my coil? All my fuses show good except the top right one, which I get nothing, which I assume is nothing because it's the only one not working like normal
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.

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.

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
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I found a wire coming through the firewall which had bare wires. the wire had power when the key was on, but I couldn't see where it was to go. after a week long and new coil and module, I found a plug which had three wires going in from the firewall and three wires out. but after looking further, I could seee the plug from firewall, someone had put two wires in one hole. I put the wire in and tried it. Walah!!!!! had spark. :twak yeah I know DUMB!!!!!!
yo devilwmn,

Never know what a prvious owner or sho[ hacked-up until the botched-uo stuff vibrates loose or shorts! or worse; when the Bronco catches fire!
true... thanks for the help anyway though! The diagrams will defineately come in handy later!!
hot wire the coil. just run a wire from the battery to the positive side of your coil. if it starts. replace the duraspark unit. if that doesn't work. plan on a few days of tracing and find the power wire that comes from the ignition switch.
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