Short Circuits—Challenging Electrical Gremlins

Written by Don Stewart | 31 October 2008

Is your first generation Mustang driving you crazy with electrical gremlins? Examples might include directional signals that don’t work properly even though you replaced the bulbs, brake lights that don’t work right when the directional signals are turned on, or you could have a fuse that keeps blowing. Or maybe there’s some other aggravating electrical problem frustrating you.

This Pony Tricks column won’t go into all the possible sources of electrical malfunctions. There are just too many of them to cover in one column. What we’ll look at here are some probable reasons for blown fuses. They can be caused by such things as a failed device that has an internal short circuit, a defective or improperly wired light bulb socket, or a power conductor (wire) that has become grounded. Failed devices or grounded wires are the most common source but never discount bulb sockets either.

Just to cover the base, a short circuit occurs when the electricity in your car goes directly from positive to negative without energizing a device or a bulb along the way. The engine, the chassis, and most other metal parts of your car are grounded, meaning that when all is said and done, they’re connected to the negative side of the battery. So a short circuit receives electrical power from the positive side of the battery and then sends it to one of those grounds before it reaches the component that it’s supposed to power. It’s the same as touching both battery posts at the same time with a wrench or a pair of pliers. You probably saw the sparks fly if you ever did that. And if you were to do it with a piece of wire that wasn’t protected by a fuse, the wire could get hot enough to start a fire. So you can think of the blown fuse as a switch to keep a fire from starting.

While it may give you piece of mind to know that the blown fuse protects your prized pony, it doesn’t tell you where the problem is or how to fix it. So you have to find the culprit and repair it. You can do the job yourself or you can take the car to someone who will do it for you. Let’s assume for the sake of this column that you decide to do it yourself.

Step one, when preparing for the job, is to purchase a wiring diagram for your car from a place like NPD, Virginia Classic Mustang, or any one of several other sources. Next buy a self powered test light similar to the one pictured. Make sure that the tester has its own power (usually a AAA battery) and that it doesn’t depend on power from the car battery. Or you can use a VOM (Volt Ohm Meter) in place of the test light. Finally, disconnect both cables from your battery.

Before you do any testing, verify continuity between the engine and the chassis. Simply connect the test light alligator clip to a bare metal part on the engine block and touch the probe to a bare spot on the chassis. Paint can be an insulator so make sure you’re connected to bare spots. If the test light comes on, you have continuity. If it doesn’t come on, replace the short ground wire between the engine and the chassis and then verify that continuity has been restored. Now the fun begins.

For the circuit that’s blowing fuses, remove the fuse from its socket in the fuse block. Find that same socket on the wiring diagram and identify the wires connected to it. You’re looking for the wires coming out of the socket, not the power wires coming into it from the battery or the ignition switch. Each wire will be identified with a color code: blue, pink, orange with a green stripe, purple, whatever. Several wires will probably be bundled together in a wiring harness.

Starting at the fuse block, trace the wiring harness and then working only with the color codes you saw on the wiring diagram, remove all bulbs from their sockets and disconnect all devices. If any of the device connectors are bare terminals, wrap a piece of electrical tape around them to serve as temporary insulation. If there are secondary harnesses plugged into the primary harness, also remove their bulbs and disconnect their devices. Be aware that color codes sometimes change where a secondary harness is plugged into the primary harness. Consult your wiring diagram for that information.

When all of the bulbs have been removed and the devices disconnected, you’re ready to perform a global test. Fasten the alligator clip from your test light to a bare spot on the car’s chassis or engine block and then touch the probe to the output side of the fuse socket. The global test will check all wires and all bulb sockets at the same time in the primary harness and the secondary harnesses. The test light will illuminate if a wire or a socket is grounded. It won’t check any of the devices because they’re disconnected so, if necessary, we’ll get to them later.

If the test light came on during the global test, unplug a secondary harness and do another global test. Don’t plug the secondary harnesses back in at this time. Continue unplugging secondary harnesses one at a time and run the global test after each one is disconnected. If the light doesn’t come on after unplugging one of the secondaries, the ground is in that harness. Suspend the global testing, and begin testing the shorted harness for a grounded wire or a bad socket.

Test the bulb sockets first because they’re relatively easy to check. Centered in the floor of each socket, you’ll see the contact point that supplies power to the base of the bulb. Sockets for twin filament bulbs will have two contact points. Touch the test light probe to each contact point being sure to not touch the case of the socket, if it’s metal. The test light will come on if the socket itself is shorted, if its power lead is wired incorrectly, or if its power lead is shorted to the chassis at some point. Incorrect wiring has been found in some replacement sockets so don’t ignore the possibility. Replace any faulty sockets and retest the harness to see if the short circuit was eliminated. If it was , reconnect all secondary harnesses (no bulbs or devices yet) and do a global test to verify that all wiring and sockets are now ok. Even though devices haven’t been tested, it’s probably safe to assume that the short has been corrected if the test light doesn’t come on. Replace all of the bulbs, reconnect all of the devices, put a fuse in the fuse block, and reconnect the battery. You should be back in business.

If you didn’t find a bad socket, you have a grounded wire. To locate the short, you need to test each wire individually. Start by touching the test light probe to each wire terminal you disconnected from a device. Make sure you’re only testing wires with the correct color codes. Next move to the plastic connector where you separated the harness from the primary harness. Touch the probe in rotation to each individual pin that’s attached to one of the color coded wires you’re tracing. Test all such wires even if you’ve already found a shorted one because there may be more than one. You’ve located the offending wires whenever the test light comes on. Make a note of the color codes for wires that turned the light on.

Obviously, a shorted wire needs to be fixed. It’s a tedious job but if you spent the time and effort to locate the short in the first place, you’re certainly capable of making the repair. The proper repair is to replace the wiring harness and if you’re talking about a concours car, that’s what you should do.

But replacing a wiring harness can be a daunting task. It usually requires partial disassembly of the car. So if you’re working on a daily driver, you may be able to fix the short without replacing the harness or disassembling the car. With some luck, you might be able to visually see the short after you know what part of the harness to look at. If so, try to repair it by using electrical tape to replace damaged insulation or solder-in a short piece of wire to replace a damaged section. Insulate all solder joints with shrink tubing and electrical tape.

If you can’t see the damage or if the harness isn’t readily accessible for replacement, a reasonable repair might be to replace the offending wire by splicing-in a new piece of wire as a bypass around the short. To do that, clip the shorted wire out of the circuit at both ends, making sure to leave enough of a pigtail to solder-in the splice. Insulate the solder joints with shrink tubing and electrical tape. And at intervals along its length, tape the splice to the harness. If you choose to replace small damaged sections or splice completely around a short, use wire with the proper gauge. Gauge is the diameter of the conductor in the wire; the smaller the conductor, the higher the gauge number. If you can’t buy the right gauge, use the next larger (lower number) size.

When the shorted wire is repaired, check it with the test light to verify the success of the repair. Assuming all is well, reconnect all secondary harnesses to the primary harness and do one more global test. The test should be ok and you should be able to wrap up the job by replacing all bulbs, reconnecting all devices, installing a new fuse, and reconnecting the battery.

But what about a case where your initial series of global tests failed to identify a short in a secondary harness; in other words, the test light still came on when all secondary harnesses had been unplugged? If that’s what you found, the short is in the primary harness. The steps to locate it and repair it would be the same as described for the secondary harness. When the repair is complete, reconnect all of the secondary harnesses and do one more global test. The test should be ok and if it is, you can replace all bulbs, reconnect all devices, and wrap up the job.

We haven’t talked much about devices but if you didn’t find a bad socket or a shorted wire, the next step would be to begin checking each device one at a time. The service manual has test procedures for some devices and you may want to use those instructions. Or you may want to use what I call the trial and error method. It’s the method I prefer because it’s nearly fool proof.

Be cautious if you use trial and error. I know this has been stated before but make sure to wrap a piece of electrical tape around all unattached bare wire terminals to insulate them from metal parts in the vicinity. Don’t remove the tape until you’re ready to make a connection. Keep the battery disconnected except when the procedure says to connect it.

Start trial error by verifying that all secondary harnesses have been connected to the primary harness. Leave all bulbs out of their sockets. With the precautionary note in mind, untape one of the terminals and connect it to its device. Put a fuse in the fuse block and reconnect the car’s battery. Next do whatever you did in the past that caused the fuse to blow. If it blows, disconnect the battery and replace the device. If the fuse doesn’t blow, leave the first device connected, disconnect the battery, untape the terminal for the next device, connect it, and then reconnect the battery. Test the second device the same way you tested the first one. Keep repeating the steps until you locate the device that blows the fuse. Disconnect the battery and replace the faulty device. After the new device is in place, reconnect the battery and test one more time to verify that the fuse no longer blows. If the fuse is ok, your problem has been corrected. Disconnect the battery, reconnect the remaining devices, reinstall the bulbs in their sockets, and then reconnect the battery to complete the job.

Other than grounded wires, bad bulb sockets, and failed devices, some odds and ends that can cause a short circuit are corroded terminals, corroded connectors, or incorrect connections between two harnesses (typically, two harnesses that aren’t supposed to be connected to each other). As noted, there have also been cases of incorrectly assembled replacement parts such as light assemblies where the ground wire and the hot wire were reversed at the bulb socket.

To successfully locate a short circuit, be methodical and don’t discount any possibility, no matter how improbable it may seem. It could take a lot of time to find it but if you enjoy the challenge, finding and correcting a short can be very satisfying.

There’s one more comment. Strange directional signal activity was mentioned in the opening paragraph of this column. If you’re having that problem, the probable fix is to replace the directional signal switch. Refer to a shop manual or you can read the procedure in the December 2004 Pony Tricks column.