There we were, strapped in and ready to hit the dirt. A turn of the key yielded only silence at a time when we should have heard the throaty V-8 roar to life. A second attempt had the same results. Stripping the harnesses off, we climbed out of the truck and started nosing around under the hood. After a few minutes we found the cause of our electrical malfunction. The ignition wire for the starter was not properly secured and the push-on connector had loosened up, leading to a poor connection.
Electrics and autos have gone hand-in-hand for many decades and the infusion of electrical components into new 4WDs is increasing daily. Whether you may be adding a new accessory or simply having to troubleshoot or repair an electrical problem, some basic knowledge of electrical theory and practice can be handy.
We see it time and time again, where people have electrical troubles ranging from just a buzzing noise in the stereo to some connection failure that grossly affects the starting or running of the vehicle. Basic auto electrics don't have to be all that complicated and some careful attention up front when installing components and routing wiring can save you big headaches down the road.
Most vehicles we drive are equipped with, and operate from, a 12-volt battery. Along with the alternator, this source provides all the electrical energy needed on our vehicle. The battery is used for starting purposes and can supply electrical power when the engine is not running. Once the engine has been started, the alternator can be actuated by the voltage regulator to supply the electrical requirements of the vehicle, and recharge the battery as needed.
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 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 a lesser 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. As an example, this becomes especially important when trying to get that last pound of winch pull when you're stuck in the mud.
An electrical circuit consists of a complete loop or path. That is to say that in an operating circuit, current flows from the positive side of the battery to the device being powered, and then back to the negative side of the battery. If this path is interrupted at any point, the circuit is broken and the device will not function.
This brings up two points concerning how electrical circuits are wired. First, note that a switch can be placed inline to engage or disconnect the device of interest. Such a switch is generally placed on the positive wire leading to the device, such that the downstream wiring is left unenergized when the switch is off.
As current flows through a wire it is subjected to a slight resistance in the wire. This resistance causes some voltage loss, or drop, as the current travels through it to the end where your electrical device resides. If you use too small a wire, the voltage drop across the wire will be significant and the voltage available at the device will be somewhat less than the full battery voltage. Performance of that device can suffer.
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 is 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.
Whenever high power electrical devices are added, they need large wires to carry the high current loads to the device. In such a case, it's best to minimize the length of wire run if possible. We can run our heavy power wires more directly from the battery to the powered device and switch this path using a relay to make or break this path.
Two good examples of the use of relays (or solenoids) are the solenoid on the starter motor and the solenoids used to actuate a winch motor. In both cases, the relay or solenoid is used to switch large current loads remotely. This setup allows you to engage the starter motor from the ignition switch or command the winch motor remotely, without having to route the heavy power cables to the switch location.
Once you understand the basic function of a relay, they are fairly simple to use and wire when adding high-power accessories. As shown here on a common Bosch style relay, there are two terminals connected to the relay coil: one to the control switch (85) and one to ground (86). The other two relay terminals are simply wired inline to switch the current flow to your device. When 12 volts (battery and ground) is applied to the relay coil, an electromagnetic switch connects the two high-power contacts and allows current to flow to the device under control.
Whenever an electrical device is added to your vehicle, it's good practice to add a protective fuse or circuit breaker to the positive power line. Many accessories often come with the proper inline fuse included. If this is the case, you can simply wire the device to your battery or other source of 12V power. Other electrical components, such as lighting, may require you to add your own fusing. electrified!
Fuses are chosen such that their rating exceeds the expected current draw of the device (by at least 20 to 30 percent), but with a rating that is below the maximum current rating for the wire size you are using. If the manufacturer does not provide the current draw for the device, we can calculate it from the specified power consumption, using this formula:
Current (amps) = Power (watts) / 12 volts
For instance, a 100-watt light would draw 8.3 amps, according to this formula. If we add two lights on one circuit, our total is 16.6 amps. At minimum, we should chose a wire size to support a 20-amp current flow and fuse our power line with a 20-amp fuse.
Wiring & Connections
The auto environment is a harsh one. Temperatures extremes are wide, and vibration and aging can also contribute to possible electrical problems. It pays to make electrical connections carefully and protect them from rubbing on metal edges and flopping around. Simple bundling with tie-wraps can help increase the reliability of your wiring.
Even using the finest equipment and materials won't provide you with a successful install if you make unreliable wiring connections and fail to adequately protect them.
Non-conductive silicone dielectric grease can be applied in connectors to keep water out and prevent corrosion that can lead to poor conductivity across the connector pins. This is especially true in places where your wiring is exposed to high-humidity or submersion conditions.
Soldering wire connections provides a solid, reliable connection that can be more immune to temperature extremes and vibration. Connections are best insulated with heat-shrink tubing. Properly soldered wires provide the most secure connection when wiring auto electrics, but quality crimp connections can also provide reliable joints that can withstand heat, cold, vibration, and moisture. Any joined wire should be carefully stripped and attached to the crimp connector with the proper crimping tool for best results.
It's inevitable that at some point you'll experience some vehicle electrical issue whether you're adding some new accessory to your rig or have some bit of troubleshooting to do. Having some basic electrical knowledge can go a long ways toward making the task easier. Clean, organized routing will also help keep your wiring decipherable and make any future troubleshooting easier. A little preventive care can go a long way to ensuring you're not stuck in the rain on a dark night trying to get some faulty electrical circuit working.