What To Know About Your Car’s Lights, Flashers and Electrical Systems

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Powered by the electrical system, lighting and signaling devices increase vehicle visibility and driver and pedestrian safety.

Your vehicle runs as much on electricity as it does on gasoline. According to statista.com, by 2030 electronics will account for 50 percent of a vehicle’s cost. This is a far cry from the 1930s, when hazard warning (four-way) flashers first became available as a safety feature.

Here’s what you need to know about these important parts of your car.

Bulbs and Sockets

There are several light bulbs and light bulb sockets in every car. By activating a switch — headlamp, brake light turn signal, etc. — a wire harness carries power from the battery through a fuse, circuit breaker or relay, through the switch, and then to a socket. That’s where the bulb contacts terminals that energize and light it.

It’s an easy DIY job to replace a burned-out bulb in your car. Remove the socket from the lamp housing and then remove the bulb. Lighting circuits are designed for a specific load (the bulb). Always use the exact same bulb size, rating, color and shape. However, you may need a new design flasher if you’re upgrading to LED bulbs.

Before installing a new bulb, check that the socket is free of corrosion and the terminals are not damaged. Clean corroded sockets with a socket brush and electrical contact cleaner. Place a drop of grease on the new bulb to prevent corrosion.

Turn Signal Flasher

Usually there are two flashers, one for the turn signals and one for the hazard warning lights. Flashers use contacts made from bimetal strips (two unlike metals that expand at different rates). Power flowing through the bimetal strips turns the bulbs on.

The power also heats and warps one bimetal strip, causing it to deflect, disconnecting it from the other strip, turning the lights off. The clicking sound a flasher makes is from the contacts on the bimetal strips opening and closing when cool. A burned-out bulb will make the turn signals blink fast.

Flashers can be found under the dash, in the fuse box or relay cluster. Check your owner’s manual for the exact location.

Fuses

Car Fuse BoxCherdchanok Treevanchai/Getty Images

Fuses protect electrical circuits, wires and devices in your vehicle against damage caused by a short circuit, low-resistance circuits or high-current devices. (Short circuits are called “short to ground” when a wire carrying voltage goes to ground before reaching the intended device.) Fuses have a small wire between two terminals that melts from high-current loads, causing the circuit to open.

Always check the fuses first if one or more devices stop working. Your owner’s manual will indicate where the fuse box is. Hold the suspect fuse up to the light to see if the wire has melted, or use an inexpensive automotive circuit tester to identify bad fuses.

Replace fuses with the same exact amperage rating. If you replace a bad fuse with one with a higher amperage rating, the circuit can overheat and possibly melt the wire harness — or worse, start a fire. If a new fuse “pops” after installation, or continually pops while driving, your vehicle has a shorted electrical component or a short circuit.

Circuit Breakers

A circuit breaker carries out the same functions as a fuse, but performs like a flasher. If the current draw of a device is too high, the breaker “trips,” stopping current flow. Once the circuit breaker cools down, the contacts close and current begins flowing. This means you won’t have to replace any fuses.

Circuit breakers come in two designs: manual-reset and automatic-reset. Manual-reset circuit breakers need to be reset by hand and are commonly used in the automotive aftermarket for high-powered audio systems or add-on lighting kits.

Automatic-reset breakers protect high-current devices such as cooling fans, blower motors and air conditioning compressors. Your vehicle has a short circuit or defective device if an automatic-reset breaker is constantly resetting itself.

Relays

Relays permit a small amount of current to control a high-current circuit. When you turn on a switch (the input circuit), current activates an electromagnet inside the relay, closing the relay’s contacts. This allows high current flow (the output circuit) from a separate circuit to activate high-current devices — a horn or electric fuel pump, for example.

Because relays are powered by separate circuits, there may be separate fuses, one for the input side and one for the output side. Without relays, huge switches and large-gauge wiring would be necessary to carry high-current loads from switches and on-board computers (ECM) to output devices.

Grounds

Grounds complete an electrical circuit. Often overlooked, even by the pros, check for bad grounds when you are experiencing strange or erratic electrical issues. Things like one headlight dimming, lights coming on when lowering a power window, or the ECM storing more than one trouble code.

Ground connections can be found all over your vehicle. Locate the grounds using a service manual (different from your owner’s manual). Bad grounds are caused by loose or damaged ground wires or terminals, corroded connections or poor repairs, especially after major body work.

For a DIY fix, remove the fastener holding the ground wire or cable from your car’s engine, body or frame. Scrub the ground wire’s mounting lug and where it’s attached to the vehicle with sandpaper to remove corrosion, so that the metal is shiny and free of paint. Reinstall the ground connections and firmly secure in place. Cover with a small drop of silicone paste to prevent problems in the future. Perform a voltage drop test (see DVOM below) to check the repair.

Connectors

Connectors are another often overlooked part of the electrical system. Connectors are usually plastic housings with metal terminals for joining wires. Today’s vehicles use dozens of connectors in many configurations, held together with a plastic latch and sometimes a locking pin. Many connectors incorporate seals or gaskets to prevent terminals from corroding due to moisture and road salts.

If a device is not working properly, disengage and separate the suspect connector and inspect the terminals. Determine if they have backed out, are bent, damaged or corroded. Clean corroded terminals with electrical contact cleaner. Leave repairing factory connector terminals to the pros.

However, you can repair a damaged wire using crimp connectors. Always solder the new connection with rosin core solder — NEVER acid core — and seal the repair from the elements using electrical tape or heat shrink tubing.

Safety note: Always disconnect the negative battery cable before making any automotive repairs.

Buying and Using a Digital Volt Ohm Meter

A digital volt ohm meter (DVOM) is a great tool to help troubleshoot electrical systems and sensitive car electronics. To protect automotive electronics from voltage spikes or high current damage, purchase a DVOM rated at 10 million ohms of resistance.

DVOMs measure voltage, amperage (current) and resistance (ohms), as well as testing diodes and performing other functions. Voltage is measured “live” in series (like holiday lights, where if you remove one bulb and all the bulbs go out) across two terminals.

Amperage is also measured live, but in parallel (where if you remove one bulb, all remaining bulbs stay lit) with the meter becoming part of the circuit. Resistance is measured when a device is off, preferably when removed from the vehicle.

To protect the meter’s internal fuses, always set the meter to the correct scale (ohms, voltage, etc.) and the highest value setting before using. Check the maximum amp rating of your meter. Testing a 30-amp blower motor with a meter rated for 10-amps can quickly ruin your meter. The most basic tests you can perform with a DVOM are voltage readings and voltage drop tests. Here are the steps for a simple voltage drop test:

  • Set the meter to DC volts.
  • Connect the black (negative) lead to a clean piece of metal on the engine.
  • Connect the red (positive) lead to the negative side of the battery.
  • Be sure all wires, tools, etc. are clear of moving parts and start the engine.
  • The reading should be 0.1 volts or less. If higher, check the ground wires and ground connections between the engine and battery ground. You can follow this process when diagnosing most electrical problems.
  • Reverse the leads. The reading should be the same.

Bob Lacivita
Bob Lacivita is an award-winning ASE and General Motors auto technician, educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics. His work has been featured in The Family Handyman, a Reader's Digest book and Classic Bike Rider magazine. He has been a career and technical educator for 25 years teaching automotive technology, as well as writing state, federal and organizational foundation grants. He also helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into career and technical education.