Test Scenario A: In this illustration,...
Test Scenario A: In this illustration, the direct short is at the power wire of component B. With the battery disconnected, fuse removed, and components unplugged, the self-power test light will illuminate only at the green wire because it completes the circuit where it shouldn't.
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 the power wire feeds. If the test probe illuminates when it touches the power wire, then that wire is shorting direct to ground somewhere and you've found your problem. There should never be any direct connection between the power wire and the chassis or body. Trace the wire from the connector back to the fuse panel. Chances are, the wire has rubbed against something and the insulation is chafed or broken, exposing the bare wire, or there is something improperly connected. If the test probe doesn't light up after testing the power wires of the components, then the culprit may be a component that's internally shorted. In that case, try replacing it to see if that eliminates the short.
Another method some use to check for shorts is to have the battery connected and use a circuit breaker, which can be a turn-signal flasher. In Test Scenario B, connect the battery 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 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's shorting, use the wiring schematic to follow where that color-coded wire runs in the harness from the component connector to the fuse that is blowing. In Test Scenario A, you would follow where the green wire runs and find where it's touching metal, then repair the area. Sometimes the section of wire needs to 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. If you had to unwrap the harness to get to the wire, be sure to rewrap the area to further protect the wires.
Test Scenario B: To check...
Test Scenario B: To check for a short with the battery connected, remove the fuse and connect a turn-signal flasher to the fuse holder clips. A short will keep the flasher unit "clicking" until it is found and corrected.
Faulty electrical components can also cause a direct short. In one instance, I discovered a shorting brake light switch, which mounts to the brake pedal. When I connected the battery positive cable for power, the voltage regulator started buzzing. After disconnecting and checking components, I found the brake light switch was causing the regulator problem. I removed the switch and tested it. It tested OK, but when I reinstalled it and plugged it back in, I had the same problem. I replaced the switch and the problem went away. Although the faulty brake light switch tested OK out of the car, it was shorting internally and grounding against the brake pedal.
I've also come across a handful of defective reproduction turn-signal housings. When they were made, the signal wire for the turn signal and the wire for ground were swapped. The parking lights worked, but switching on the turn signals would blow the fuse. I traced the problem to the housings. When I checked them, I discovered they had been wired incorrectly. Everything was fine after I replaced the housings with new ones that were wired properly.
Don't assume everything is plugged in correctly, either. I once worked on a Cougar that was blowing the fuse when the doors were opened and the courtesy lights came on. I traced the problem to where someone had plugged two connectors into each other because they were both two-wire connectors. However, the color codes didn't match and power to one component was connected to power for another component. When the courtesy lights came on, 12-volt met 12-volt with no ground and the fuse would blow.
A blown fuse isn't always an indication of an electrical short. Occasionally, a fuse will blow if a component or its connector is corroded. This happens often with dash lights. The headlight switch has a ceramic wheel that turns to brighten or dim the dash lights. The contacts on this wheel can turn green with corrosion or rust, making resistance higher than normal to increase 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, making the fuses easier to blow.
Editor's Note: For the past 30 years, Lance Morgan has specialized in electrical diagnosis and repair for '65-'73 Mustangs at his shop in Edmund, Oklahoma. He occasionally helps Mustang owners with their electrical problems by e-mail at webmaster@1970mgr.org.