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Tips & Tricks For Vehicle Electrical Troubleshooting

Posted in How To: Electrical on May 25, 2015
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From the earliest days in vehicle history, electrical energy has been used for powertrain, lighting, and a multitude of accessories. Today, modern vehicles use multiple microprocessors onboard and all manner of electronics from a current hungry starter down to the tiniest chips that communicate when to fire an airbag during a collision.

If you’re like us, you’ve added lighting, radio gear, or some other electrical gadgets to your rig and maybe struggled with the installation or been haunted by some electrical woe that you have to troubleshoot. Generally, vehicle wiring is reliable, but it must withstand the abuses of vibration, temperature extremes, humidity, dust, and other environmental factors. As such, solid electrical connections that can survive these long-term conditions will make your life safer and easier.

Soldering automotive wiring is a good way to ensure neither vibration nor corrosion will compromise the electrical joint. Use of a good quality rosin-core solder with good heat flow will usually offer a solid, low-resistant connection. Top the joint with heat-shrink tubing to further seal it from contaminants.

When building project vehicles, wiring is often one of the last pieces of the project. As such, many builders rush through the wiring process with poor routing or sloppy connections. We’ve seen numerous vehicles that were beautiful builds, but lacked good attention to detail when it came to the wiring. We’ve also seen a good number of vehicles suffer electrical issues as a result.

Whether you’re adding a new accessory, or simply trying to troubleshoot an electrical problem, some basic knowledge of electrical theory and practice can be handy. There are two basic properties we deal with when discussing electrical systems: voltage and current. Voltage (volts) is the electrical potential, and can be analogous to the water pressure in a tank. The greater the voltage, the more energy we can typically supply. Thus, a fully charged battery or properly operating alternator will provide brighter lights than will an ailing electrical source. Current (amps) is a measure of electrical flow through a conductor and can be compared to water flow in a pipe. Increased flow capability means that greater energy is available at the far end of our wiring path.

An appropriately sized fuse should be placed on the positive supply wire when connecting an accessory. Place it as close as possible to the battery or point of power connection. Fuse size (in amps) should be chosen to match the current capability of the supply wire, and exceed the expected current draw of the device being powered by at least 20 to 30 percent. Modern “blade” fuses are readily available up to a 40 amp rating. Beyond that you can use the larger Littlefuse MEGA fuses that are available up to a 500 amp rating.

Since an electrical circuit consists of a complete loop or path, current must flow from the positive side of the battery to the device being powered, and then back to the negative side of the battery. An interruption along any point in the path will prevent the device from functioning. Since the same current flows around this entire loop, both the positive and negative (ground) wires must be of sufficient size to carry the needed current.

Wire size should be chosen based on the current draw of the device in question. The table below shows the American Wire Gauge (AWG) sizes recommended for various current load requirements. These sizes are valid for runs up to about 15 feet. For longer runs, it’s best to upgrade to a larger size (lower gauge number) wire. If in doubt, always go with the larger wire to ensure you have plenty of current capacity, and minimize voltage drop over the length of the wire.

Need a relay? When running high-current electrical items such as driving lights, you’ll want to remotely switch the high current supply line from the cab. To minimize wire length to the lights, we would run the positive supply wire directly from the battery to the lights, putting a relay inline to switch this path with a control wire from our cab-mounted switch. In this way we don’t have to run the heavy supply wires into and out of the cab to the lights.

Power for added accessories can be tapped from a number of locations. For power hungry items such as winches, high-intensity lighting, and powerful audio amps it’s often best to tap power from the positive battery terminal or a large secondary terminal close to the battery. It may also be possible to tap 12V power from a power stud in the factory fuse block or other studded distribution block under the hood. Just be mindful of the amount of current you require and make sure the source you’re choosing is up to the task of supplying the extra load.

Wire Size (AWG) Current Capacity (amps)
20 5
18 8
16 12
14 18
12 24
10 40

Troubleshooting
When hunting down electrical problems there are several common culprits to consider first. Along with the heat, cold, and vibration in a vehicle, electrical components are stressed and exposed to contaminants such as water, salt, and dust. Aging mechanical connections are often the common weak point. Many electrical problems can be traced to poor connections.

Always take care when making connections and check them first when troubleshooting a problem. Sometimes simply unplugging and plugging a connector back together can help break through contact surface corrosion that may be causing high resistivity at the connection. Electrical contact cleaner can be used if needed.

Troubleshooting tools need not be very expensive for the DIYer. Simple test lights may help you determine if you’re getting voltage to some point. However, we prefer to use a multimeter for troubleshooting. They allow you to take actual voltage or current measurements, plus an ohmmeter can be used to check for continuity. This seven-function meter from Harbor Freight (www.harbor freight.com) is quite inexpensive. However, do not use a test light or inexpensive meter to check engine computer circuits as these can damage such modules.

On a vehicle, the chassis serves as a common ground and is grounded to the battery. The negative side of many electrical devices is simply wired to a metal body panel or some other metal ground source. Loss of ground can also be a common failure point, so check for a solid ground connection when you’re experiencing a failure. Loss of the ground connection is also a very common failure mode when dealing with malfunctioning trailer lights.

Electrical circuitry can seem confusing, and for many the tediousness of a quality wiring job can drag on and on. However, time spent up front carefully routing and making connections can mean little or no future electrical issues that may leave you frustrated, stranded, or sparking a vehicle fire.

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Parts Guide
Electrical Stuff For Your Ride
If you’re doing some installation work, you might find some of these components handy to help you do the job right. We’ll share some of the electrical items we’ve found work well on some of our projects.

Del City offers a variety of electrical components, including quality butt splice connectors for connecting wires of like size using a crimp connection. They offer regular insulated, heat-shrink, and Cool-Seal butt connectors. The heat-shrink versions provide a solid crimp connection and use the surrounding insulation to completely seal the terminal when heated. Their Cool-Seal connectors seal without heat. When wires are inserted, adhesive surrounds the wires and forms a moisture-tight connection.
Del City
800.654.4757, www.delcity.com

Posi-Lock offers solder-free, reusable connectors in both regular and watertight styles. You simply strip the wires to be connected, slide each into the connector ends, and then screw the connector together by hand. No other tooling is required. They also produce their Posi-Tap that allows you to tap a connection to a wire without cutting it.
Posi-Products
www.posi-products.com

When needing to route large battery cables through the firewall or some other panel, it’s possible to protect the cable with a rubber grommet. However, a better solution is to use a Thru Panel Battery Connector such as this one from Moroso. The large plastic feedthru can be secured to the metal panel, providing a solid terminal connection on both sides of the panel.
Moroso Performance Products
203.453.6571, www.moroso.com

We’ve used inexpensive plier-type crimpers and can usually get decent results. However, if you want to improve the quality of your crimp connections, or have a lot to do, we can recommend this ratchet type crimper from Del City. It provides mechanical advantage to your grip and has interchangeable jaws to accommodate a wide range of crimp connectors. This tool can provide you with fast and very reliable crimp connections.
Del City
800.654.4757, www.delcity.com

Vehicle manufacturers today encase their harness wires in convoluted plastic tubing. The split and ribbed tubing keeps all the separate wires neatly bundled and protected from chafing and abrasion. It’s also a great material to use when adding electrical wiring to your rig. Taylor Tubing is a popular brand available in multiple colors from Summit Racing.
Summit Racing Equipment
800.230.3030, www.summitracing.com

Looking for in-cab winch controls, custom switch panels, or other in-cab control and monitoring panels? 12VoltGuy has many generic switch panels, some vehicle specific panels, and can do full custom to order. Panels are cleanly engrave-labeled and can include fuses and status LEDs.
12VoltGuy
530.532.4529, www.12voltguy.com

We’ve found building our own custom battery or winch cables to be easy and economical. We crimp copper battery lugs using a Temco manual crimper that accepts wire sizes from 8 to 4/0 gauge. Using a press, bench vise, or a couple of hammer hits on the crimper completes the crimp. The lug collapses onto the stranded wire until any free space is consumed and the crimping action ceases. It’s quick and simple to use. We like using flexible welding cable for making high current cables.
Temco Industrial Power Supply
877.474.8209, www.temcoindustrialpower.com

Should you need to rewire a classic vehicle, perform an EFI engine swap, or simply building a scratch-built rig, then a pre-made vehicle harness may help. They are generally not the least expensive route, but quality harnesses can save you tons of time from building or modifying your own harness. Painless Wiring is a company with a quarter century of providing just such harnesses.
Painless Performance
817.244.6212, www.painlessperformance.com

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