Electrical basics for race cars

Voltage is the electrical push, measured in volts. Current is the flow of electricity, measured in amps. Resistance is what slows the flow, measured in ohms.

Ohm's law ties the three together. Voltage equals current times resistance. Put another way, current equals voltage divided by resistance. A 12 volt source across 6 ohms of resistance pushes 2 amps.

This explains most electrical trouble. A corroded or loose connection adds resistance. More resistance lets less current reach the part, and the bad spot turns energy into heat. A fuel pump on a bad connection runs slower and moves less fuel.

To read voltage, set the meter to DC volts. Put the black lead on ground and the red lead on the point you are testing. You measure voltage with the circuit on. A fully charged 12 volt lead acid battery at rest reads about 12.6 volts.

To check continuity, turn the circuit off first. Touch the leads to both ends of the wire or switch. A beep means there is a complete path for current. No beep means the path is open.

To read resistance, turn the circuit off and isolate the part. Touch the leads together first to see what the leads themselves read. Then measure the part and subtract the lead reading. Never measure resistance on a live circuit, because it gives a false reading and can damage the meter.

A continuity or resistance test uses only the tiny current from the meter. A wire with only a few strands left can pass that test and still fail when a pump or starter pulls real current.

A voltage drop test finds those problems. Turn the circuit on so it is working under load. Put one meter lead on each end of the section you are testing, such as the battery post and the pump terminal on the same wire. The meter shows the voltage lost in that section.

A good wire or connection loses very little. Look at a fuel pump feed or its ground. A drop of more than half a volt there points to damaged wire, corrosion or a bad connector. Test the ground side as well as the power side. Bad grounds are a common source of trouble.

Use the terminal made for your wire size and the crimp tool made for that terminal. A ratcheting crimper with the right die makes the same crimp every time. Pliers crush the terminal and make a weak joint.

Strip only as much insulation as the terminal calls for, and do not nick or cut the strands. All the strands go into the barrel. The strand ends should show just past the crimp without reaching the contact area. The insulation belongs in the insulation grip, not under the wire crimp.

A bad crimp is loose, has strands outside the barrel, has cut strands or has insulation caught under the wire crimp. Too much crimp is bad too. Over-crimping damages strands and raises resistance.

Pull test every crimp. Hold the terminal and give the wire a firm tug. The wire must not move or pull out. If it moves, cut it off and crimp a new terminal.

A fuse or circuit breaker is there to protect the wire. If a wire shorts to ground, current spikes, the wire heats up and the insulation can catch fire. The fuse opens the circuit before that happens.

Size the fuse to the wire. The fuse rating must not be higher than the wire can safely carry. If the part needs a bigger fuse than the wire allows, use a bigger wire.

Put the fuse as close to the power source as you can, because wire between the source and the fuse has no protection. A breaker can be reset after it trips. A fuse must be replaced.

A blown fuse is a warning. Find the short or overload before you replace it. Never put in a bigger fuse to keep it from blowing.

The master disconnect switch shuts off all electrical power in the car. It lets the driver or a safety worker kill everything fast in a crash or a fire. Cutting power stops the fuel pump from feeding a fire and stops sparks from damaged wires.

Many race cars must have one, such as cars with the battery moved to the trunk. NHRA rules put the switch on the positive side of the electrical system. Turning it off must stop every electrical function. That includes shutting off a running engine. Wire it so an alternator cannot keep the engine running after the switch is off. Where the rules call for it, the switch mounts at the rear of the car, clearly labeled, so a safety worker can reach it from outside. Check your class rules for the exact setup.

Keep wires away from headers, exhaust and turbos, because heat hardens and melts insulation. Keep wires clear of anything that moves, like the driveshaft, the steering and the suspension.

Use a rubber grommet anywhere a wire passes through sheet metal. Cover wires with loom or sleeve where they could rub. Support harnesses with cushioned clamps so they cannot vibrate against an edge. A chafed wire can short to the chassis.