Need help!

Shorted ignition coil, fuel injector, alternator, oxygen sensor heater, etc. What have you checked so far?

Plug in a pair of spade terminals in place of the fuse and use a pair of jumper wires to connect them to a 12 volt light bulb. To prevent having to crank the engine, bypass the automatic shutdown relay with a stretched out paper clip in the relay socket, terminals 30 to 87. The bulb will be full brightness when the short is in the circuit. When you unplug something or move something to remove the short, the bulb will go out or get dim. Since the fuel pump might be in that circuit and he likes to draw quite a bit of current, a head light bulb will work nicely. That will limit current to 5 or 6 amps. You can try a brake light bulb too, but he will probably still be fairly bright even when the short is gone.

Spade terminals are just any type of wire connector that looks like the two flat terminals on a fuse. The only reason for using them is to allow wires to be easily connected to the sockets in the fuse box. I soldered a pair of wires to the top of a blown fuse after grinding off some of the plastic but I don't expect you to do that. First of all, it's hard to get the solder to stick before the plastic melts and the fuse falls apart, and my reason for doing this was I built a tool that I could quickly grab and use many times over and over. When customers are paying by the hour, it takes too long to rummage around for terminals to plug in and hunt for a light bulb and wires. Since you're only doing this once, piecing this bulb and wires together is sufficient.

There's two reasons for using a light bulb. First, it has resistance to current flow. That is what limits current in the wires to a safe level. Every circuit has to have resistance. When it doesn't, we say it is shorted, and when the current is not limited, the fuse blows.

Think of a municipal water tower. Water flowing out of it goes through a four-foot diameter pipe into many one-foot diameter pipes, and eventually to your house and out the garden hose. The reason the tower would not empty out instantly is because of the resistance to water flow there is in the pipes. That resistance is a result of their small diameter, the kink in the hose, and the nozzle on the end of the hose. If you break that four-foot pipe under the tower, nothing will limit the water flow and the tower will empty too fast. The only difference is while you can "see" that imaginary break in the pipe, we don't know where that short is on your truck.

The other difference is you can see the excessive water flowing from a broken pipe. We can't see current flow. We can only see the RESULT of excessive current flow in the form of a melted wire or blown fuse. The light bulb gives us a means to see and gauge how much current is flowing in the circuit. When the short that you're searching for is in the circuit, your bulb will act exactly the same as the regular head light or brake light on the truck. Adding any resistance at all reduces the current flow and the bulb will get dimmer. The desired resistance in this circuit is the normal resistance of the ignition coil, the injectors, the alternator field, oxygen sensor heaters, and possibly the fuel pump. Those are the equivalent of the water pipes going into all the houses in your neighborhood.

When it's determined that too much water is flowing from the water tower, someone might turn off the valves in the four different pipes leading away from it. When the north, south, and east side valves are turned off, the excessive water flow continues, but when the west valve is turned off, water flow decreases to a normal level. Now you know which pipe to follow. A mile down the road that pipe branches off into six smaller pipes. Here again you turn off one valve at a time to determine which one has the excessive water flow.

The excessive electrical current is approached the same way but instead of having valves, (switches), we have to look for things to unplug. Sometimes it's necessary to cut wires but that should be a last resort because doing so makes more work fixing them after the problem is found.

What you're looking for is the bulb to dim when you do something that isolates part of the circuit. I can add another method of doing the same thing. Sorry I didn't think of this sooner. You can install a new fuse, then install the light bulb terminals in the relay socket, terminals 30 and 87. It doesn't matter where the bulb and its resistance is in the circuit as long as it is before the short. If you have the paper clip bypassing the relay now, that's perfectly fine. Either way will work.

Previously you already mentioned that you unplugged the ignition coil and a new fuse still blew. That is one potential problem that you have already eliminated. In the past people have found a shorted oxygen sensor and the wiring harness for the sensor has fallen down onto the hot exhaust manifold and some of the wires melted until they touched the exhaust pipe. Your job now is to check the wires to those things and wiggle them and unplug things until the light dims.

Am I to understand you're using a 12 volt test light to check for voltage on the coil's secondary / output terminal? I never tried that because I assumed the high voltage would burn out the bulb. Regardless, a better test is to pull the coil wire out of the distributor cap and hold it 1/4" away from the engine while a helper cranks the engine. If you get a nice sharp spark there, put that wire back and try the same thing on a spark plug wire. You'll need to stick a screwdriver into the spark plug boot and hold it close to the engine so you can watch for spark. Sorry if I'm telling you stuff you already know.

Because of the way an ignition coil is wired internally, you will have 12 volts on the high voltage wire. That CAN be measured when the engine is not running but it just means 12 volts is getting to the coil and electrically the coil is okay. There still could be a no-spark condition.

Happy to hear you found the short. When students were troubleshooting my "bugged" cars, along with finding the defect, they had to tell me the typical most reliable and economical fix. They learned real fast that I would not accept "run a new wire". At first that might make sense to a do-it-yourselfer but my rationale was first of all, if the old wire wasn't cut out, the short was still there, but more importantly, if they don't find exactly where it is grounded, how long will it be before the next wire in the harness rubs through and shorts? You just avoided that second wire by finding the location of the short. Well done.

Now is the time to retest at the end of the coil wire and you should still find the test light is dim. The coil secondary terminal is tied to the 12 volt supply through the primary winding so the test light will light up at the coil but it will be dim at the end of the coil wire. That is because there is a lot of resistance in the plug wires. Remember that the voltage will only be there for one second after turning on the ignition switch and during cranking.

The spark voltage could burn out your test light. If it does not, it's because that really high voltage is only there for a few microseconds. Over time, the average voltage will still be very low.

Happy to hear the problem is solved.