Over time, many things can cause electrical problems with your Mustang. Components will get old and worn or a wire can rub against something and expose the wire to metal. I have seen where a body shop will inadvertently crimp a wire when they install a body panel. Sometimes, when mechanical repair is done, the wiring just gets in their way. This is unfortunate because the wiring of your Mustang is it's nervous system. If it isn't in good shape, then things won't work properly. In the worst case, this can cause an electrical fire. One condition that can cause problems is an electrical short. Let's examine what a short is and then how to find it.
Current normally flows from the battery positive cable, to the starter solenoid, through the main wiring harness back into the dash, through a fuse, then to an electrical device, and then to ground, which is either the metal body somewhere or a ground wire in the wiring harness that attaches to the body. An electrical short is where current flows from the battery, and then somewhere directly to the body chassis or metal which is a direct ground and full unchecked amperage. Since current flows from the battery then direct to ground, it has taken a "short" cut to ground thus creating an electrical "short circuit".
If a wire is shorting to ground without going through a fuse, it will get extremely hot and burn through if current flows long enough, sometimes taking only a few seconds. If a short exists on a fused circuit, then the fuse gets too hot and quickly melts the element inside, thus breaking the circuit and protecting the wires of that circuit. When a fuse blows for any reason, then obviously there is a problem. Be happy that the fuse is doing it's job and giving up it's life for the cause, otherwise wires can melt or, even worse, an electrical fire can occur.
Whenever a circuit is blowing a fuse, DO NOT use anything like a fuse wrapped in metal gum wrapper or other technique that would keep current flowing through that circuit. This would cause the wire to get too hot and possibly take some other wire with it when it melts and cause more problems than you originally started with. If a fuse is blowing, there is a reason and the cause needs to be found and corrected.
To locate a short, the best tools are a wiring schematic of your year of Mustang and a self-powered test light, or continuity tester. A self-powered test light is similar to a test light to check voltages but is used with battery power disconnected. It has it's own small battery and light and lights up when it detects a wire that is touching ground somewhere. It has a sharp probe on one end to pierce wire insulation to get to the wire if needed and an alligator clip on a wire at the other end. The alligator clip attaches to the metal body or dash somewhere to ground the test light.
Here is the method I use and prefer because battery power is disconnected. The first thing is disconnect both battery cables at the battery and remove the fuse that is blowing. Using the wiring schematics, determine what components are on the circuit that fuse protects. Each component will have a color coded wire for power and in most cases a black wire for ground. Unplug the connectors from each component. If one of the components is a light bulb then remove the bulb from the socket. Ground the test light by attaching the alligator clip to a bare metal part of the body or chassis. At each component connector or socket, use the test light to check the power wire or connector contact that power wire feeds. If the test probe lights when it touches the power wire, then that wire is shorting direct to ground somewhere and you have found your problem. There should never be any direct connection between the power wire and the chassis or body. Now just trace that wire from the connector back to the fuse panel. Chances are the wire has rubbed against something and the insulation is gone exposing the bare wire, or there is something improperly connected. If, after testing the power wires of the components, the test probe does not light up then the culprit may be a component itself that is internally shorted. In that case, try using a replacement component to see if that eliminates the short.
Another method some mechanics use to check for shorts, is to have the battery connected, use a circuit breaker instead of the fuse, which could even be a turn signal flasher, and connect it to the holder clips of the fuse that keeps blowing . This safely puts a load on the circuit. Disconnect components one by one. When the circuit breaker clicking either slows or stops, then the last component you removed is on the circuit causing the problem. In place of a circuit breaker, you can also use a single automotive bulb in a socket and connect it to the fuse holding clips. This method has the battery connected for normal operation so be sure you don't have an unfused wire somewhere getting too hot while testing.
Now that you have found the circuit that is shorting, use the wiring schematic and follow along where that color coded wire runs in the harness from the component connector to the fuse that is blowing. In this test scenario, you would follow along where the green wire runs and find where it is touching metal and repair the area. Sometimes the section of wire where it is shorting should be replaced. In this case, cut out the bad portion and install a length of replacement wire of at least the same gauge. The ideal repair would be soldering in a section with heat shrink tubing on the two ends where the new section attaches. If you have had to unwrap the harness to get to the wire, be sure to rewrap the area to further protect the wires.
Faulty electrical components can cause a direct short. In one instance, I had a stoplight switch that mounts to the brake pedal that was shorting. When I connected the battery positive cable for power, the voltage regulator started buzzing. After disconnecting and checking components, I found the stoplight switch was causing the regulator problem. I removed the switch and tested it. It actually tested okay but when I reinstalled it and plugged it back in, I had the same problem. I then replaced the switch and the problem went away. The faulty stoplight switch was shorting internally and grounding against the brake pedal but would test okay out of the car.
I have also come across some bad reproduction turn signal housings. When they were made, the signal wire for the turn signal and the wire for ground were swapped. Whenever I used the park lights I was okay but when I turned on the turn signals it would blow the fuse. I traced the problem to the housings. When I checked the housings I found out they had been wired wrong. Everything was fine after I replaced the housings with some new ones that were wired properly.
Don't assume things are plugged in correctly either. I was working on a Cougar that was blowing the fuse when you opened the door and the courtesy lights came on. I traced the problem to where someone had plugged two connectors into each other because it looked right since they were both two wire connectors. The wires color codes however didn't match and power to one component was connected to power to another component. When the courtesy lights came on, 12v met 12v with no ground and the fuse would blow.
A blown fuse is not always an indication of an electrical short. Occasionally, a fuse will blow if a component or it's connector is corroded. This happens often with dash lights. The headlight switch has a ceramic wheel that turns when you brighten or dim the dash lights. The contacts on this wheel can get green with corrosion or rust and this makes the resistance higher than normal and increases the load on the circuit. Over time, the fuse will get too hot trying to handle the load and will blow. Always make sure that all connectors are clean and dry inside before you plug them into the components. The same holds true for the holders of the fuses in the fuse panel. Over time, these can get rusty and that makes the fuses easier to blow.
I occasionally help Mustang owners with some of their electrical problems via email. If you would like me to troubleshoot a problem you are having, email me at the Mustang Grabber Registry at webmaster@1970mgr.org and I will see if I can help.