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DIY Auto Service: Automotive Lighting Diagnosis and Repair

Updated on April 30, 2014
LED lights are replacing the once standard incandescent lights.
LED lights are replacing the once standard incandescent lights.

Types of Lighting

The lighting system on today’s cars and trucks has become much more involved with body computers controlling most of the electrical functions on the truck and trailer. It used to be when you turned the light switch on, you completed an electrical circuit to the lights. On most vehicles today the light switch is an input into a computer. The computer will turn the lights on.

A variety of incandescent bulbs have been used to light all the lighting requirements. The trucking industry today has changed, with the addition of light emitting diode type or LED lighting. The LED consumes much less energy (amps) than the incandescent bulbs it has replaced. The other factors that this will affect are; wiring size and total amperage needed to power a tractor-trailer as well as alternator output.

See my Hubs "DIY Auto Service: Basic Electrical and Electronic Testing" for basic electrical theory and "DIY Auto Service: Basic Digital Volt Ohm Meter (DVOM) Electrical and Electronics Testing" for meter usage.

Truck and Trailer Systems Textbook

Lighting Circuits

Lighting circuits can be identified by their location and function.

Interior lights operate inside the cab of the vehicle. The interior lights consist of; dash and panel lights, dome lights, mirror lights and rear bunk lights.

Exterior lights operate on the outside of the vehicle. They include parking or running lights, clearance lights, turn/ stop lights, back-up light and identification lights. Since the exterior lights operate outside the vehicle they are exposed to the weather as is the wiring. Exterior lighting and wiring are also more exposed to damage. Poorly repaired exterior wiring is a common occurrence and cause of ongoing lighting problems.

Bulb Types

Halogen single and dual element headlight bulbs.
Halogen single and dual element headlight bulbs.

Bulb Types

Incandescent bulbs have been the staple for many years. The filament is heated up until it glows. The gas surrounding the filament controls the temperature and disintegration of the filament. The filament is sensitive to vibration and impact. Many of the tail and marker lights have been replaced by LED or Light Emitting Diodes. LED headlights are also on the market but very expensive.

Headlight Bulbs

Headlight bulbs can be a single filament for either low or high beam in a 4 headlight system. For a two headlight system each headlight bulb has two filaments, one for high beam and the other for low beam.

The most popular headlight bulb is the halogen bulb. The halogen bulb uses halogen gas and a tungsten filament. The halogen bulb is more expensive but is brighter (more candle power) than the standard headlight bulb.

Service Tip:

Do not touch the glass on a halogen bulb. The oils from your finger can etch the glass. The extreme glass temperature could cause the glass to crack.

Park, Turn, Stop and Tail lights

Park, Turn, Stop and Tail lights are used in a designated location, color and brightness. For many years the bulb used for the stop turn taillight bulb was an 1157, 2157 and 3157. This designates it is a two filament bulb with a common ground. When energized, the bulb has a lower light emission from the Park or Taillight filament and a brighter emission from the Turn or Brake filament.

Running, Identification and Clearance Lights

Running, Identification and Clearance Lights were a one filament bulb typically an 1156, 2156 or 3156 for the full sized bulbs and a 194 for the bayonet or smaller bulb. One filament means it has only one brightness level and requires only two wires, power and ground.

LED or Light Emitting Diodes

LED or Light Emitting Diodes are replacing many of the common uses for incandescent lights. The cost of LED’s are about 5 to 10 times the price of an incandescent bulb but, the reliability of 30,000 hours plus of service and the amperage draw is only about 10 to 20% of the standard bulb has made them very popular.

Pickup Truck Headlight

Pickup truck LED Headlight.
Pickup truck LED Headlight.


The headlight circuit may be 2 bulb or 4 bulb systems. The Headlights have 2 circuits; one for low beam and one for high beam. The main control for the headlights is the Headlight switch, which also controls the parking or running lights. A dimmer switch controls the high or low beam headlight selection. Many vehicles use a stalk switch located in the steering column to control the lights and the Dimmer Switch.

  • Power is supplied to the headlights and parking lights thru 2 separate fuses or circuit breakers (DOT Requirement on trucks).

  • Ground for each circuit usually use a chassis or body ground to complete the circuit back to the batteries.

Service Tip:

Fiberglass hoods on trucks do not conduct electricity. The ground circuit for the lights on the hood will use a connector to ground to the vehicle chassis. There may be one ground for all the lights, left side and right side grounds or several ground wires.

Headlights and Parking Light Switch Circuit

The headlight switch has two separate electrical circuits it controls. It is a ganged switch that moves two contacts with one movement. The headlights are on one circuit and the parking lights are on the other. Headlight switches usually have three positions;

  • Off,
  • Parking Lights only
  • Parking Lights/Headlights together.

Two Bulb headlights have high and low beam filaments in each bulb. Three wires usually connect to the bulb.

  1. Low Beam Power.

  2. High Beam Power

  3. Shared Ground.

Note: Four bulb headlights have a separate high beam light bulb with a two wire connection.

Lighting Voltage Drops

Perform the Voltage Drop on the positive side. Is this reading above .25Volts?
Perform the Voltage Drop on the positive side. Is this reading above .25Volts?
Perform the Voltage Drop on the negative side.
Perform the Voltage Drop on the negative side.

Lighting Circuit Voltage Drops

A voltage drop in a wiring circuit can show up as a dim light or a light that will not light. A typical lighting problem would one parking light dim with the rest at their normal brightness. Because all these lights are on a parallel circuit, the excessive resistance or bad connection is confined to that one bulbs wiring. If the problem was with the power, ground or light switch which controls the entire circuit, then all the bulbs would have the problem.

Lighting Circuit Voltage Drop

Performing the lighting circuit voltage drop is the same as previously outlined for starters and alternators. The circuit must be active to perform the test. An accurate voltmeter and jumper leads are needed to perform the testing.

The steps for performing the voltage drop are to:

  1. Turn the light circuit on.

  2. For the positive side check, hook the meter lead to the battery positive post or fuse for the circuit.

  3. Hook the other lead to the positive connection at the bulb.

  4. Read the meter and write down the reading. (_______V)

  5. For the negative side check, hook the meter lead to the negative post.

  6. Hook the other lead to the negative connection at the bulb.

  7. Read the meter and write down the reading. (_______V)

  8. Add the two readings together. Remember .5V is the maximum allowed. If either the positive or negative reading is over .25V this is where a problem lies.

  9. Repair the condition and retest the circuit.

Bad Lighting Voltage Drop

The problem with this circuit is on the positive side after the splice. Repair the wire.
The problem with this circuit is on the positive side after the splice. Repair the wire.

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