How to Tell if My Headlight Switch Is Bad

Headlight switches vary by vehicle make and model -- some models have a stand alone switch that operates only the headlight and parking lights, while others are built into the turn signal switch. What is more important, however, is that you understand exactly how the circuit for your vehicle's headlights work. On some models, the switch transmits a low-current, 12-volt signal that turns on the headlight relay or relays. Others might provide ground to the headlight relay, or a 5-volt signal to your vehicle's body control module. Your car's configuration determines exactly how you test the switch and what your results should be.

12-Volt Supply System

On this type of system the switch receives a low-current, 12-volt power supply. When the parking lights, or headlights are turned on, the switch sends this power supply to the controlled side of the headlight relay. The relay turns on and power is sent to the headlights on a higher-current circuit. This design helps prevent heat and premature failure of the headlight switch.

Determine which wires to test

Review your vehicles wiring diagram and determine which color wire is the power supply wire. You'll also need to know which wire supplies voltage to the parking lights, which wire supplies voltage to the headlight relay and which wire supplies to the high-beam relay.

Test for voltage at the switch

If your wiring diagram shows a ground wire in the wiring harness, probe that wire with the negative lead of the multimeter. If there isn't a ground connection at the switch, locate one within close proximity to the switch.

Set the multimeter to "DC" current on the 20-volt scale. Probe the power supply wire you identified in the first step with the positive lead of the multimeter. Touch the negative lead to ground. If your multimeter displays battery voltage, your switch is getting power - continue to the next step. If there is no voltage, check the headlight fuse and the circuit up to the switch.

Test the switch for continuity

Set the multimeter to "Ohms." If your meter has an audible continuity alarm, follow its manufacturer's instructions to turn on that feature. Disconnect the wiring harness from the headlight switch.

Touch the positive lead of the multimeter to the pin on the switch that receives power. Touch the negative lead of the multimeter to the pin to which the output wire for the parking lights connects. Turn the headlight switch to the parking light position and observe your meter. If you read anything over a few Ohms, or don't get an audible alert, replace the switch. Move the negative lead to the pin that the headlight relay output wire connects to. Turn the switch to the "On" position for the headlights. If there is more than a few Ohms, or infinite Ohms, replace the switch. If the switch falls within the aforementioned specifications, there is a problem somewhere in the circuit after the headlight switch.

Ground Supply System

On this type of circuit, the switch simply supplies the path to ground for the headlight relay. If your vehicle's headlight circuit is of this design, you'll notice that on the wiring diagram that the headlight relay has two constant power supplies, and the ground side of the relay is routed to the headlight switch.

Identify the ground and output wires

Take a look at your wiring diagram and determine which wire from the switch goes directly to a body ground -- symbolized as three dashes at the end of the wire. Also determine which wire goes to the parking lamps and the headlight relay.

Check for constant ground

Locate a source of power on your wiring diagram and probe that wire with the positive lead of the multimeter. Probe the constantly grounded wire with the negative lead of the multimeter. Set the meter to the 20-volt DC scale. You should get a voltage reading -- either a 5-volt reference signal or battery voltage, depending on which circuit you choose. If you don't see a voltage reading, repair the constant ground position to the switch and check for proper headlight operation.

Test the switch for continuity

This is almost the same as checking continuity on a headlight switch that transmits 12 volts to the headlight relay except you can leave the wiring harness connected and probe the wires at the harness. Set the multimeter to test for resistance, or to the continuity setting. Probe the positive lead of your meter to the constant ground wire. Probe the negative lead into the output wire to the parking lights. Turn the switch to the parking lights setting and observe the meter. You should see zero or no more than a few Ohms of resistance. Move the negative lead to the output wire for the headlight relay, then turn the switch to the headlights setting and check the meter. Replace the switch if there is excessive resistance in either position.

Computer Controlled Systems

On this type of system, the body control module does most of the work. The switch receives a 5-volt reference from the BCM. When the switch is turned to the parking light or headlight position, the signal is sent back to the BCM. The BCM then acts as the either a control for the headlight relay, or controls the headlights directly. Testing this system is similar to testing a 12-volt system. This is what makes automatic headlights possible, and prevents the need for the use of external capacitors for delayed-off functionality.

Identify the reference and return wires

Review your diagram. Identify which wire delivers the 5-volt reference signal to the switch, and which wire wires feed that signal back to the BCM for parking light and headlight control.

Locate a good source of ground

If there is a ground wire that goes to the headlight switch, or steering column on the wiring diagram, you can use that for your source of ground. Otherwise you'll need to locate a good body ground on the firewall, driver side kick panel or frame of the dashboard.

Test for reference signal

Cycle the ignition key on and off once to wake up the body control module. Set the multimeter to DC current on the 10 or 20 volt scale. Probe the wire that delivers the 5-volt reference signal to the switch with the positive lead. Touch the negative lead of the meter to a good ground source, or probe a ground wire, if available. Check the reading on the meter. If there is no 5-volt reference signal, there is either a problem in the circuit between the BCM and the switch, or the BCM has failed. If you do read five volts, proceed to the next step.

Test output from the switch

Use the same settings from the previous step. Probe the parking light output wire with the positive lead of the meter. Probe a ground wire in the harness with the negative lead, or touch a good source of ground. Turn the switch to the parking light setting and observe the meter. If you don't see between four and five volts with the switch turned on, replace the switch.

Move the positive lead to the output wire for the headlights, then turn the switch to the headlights position. Verify that the reference signal passes through the switch with a voltage drop of no more than one volt. If you don't get a reading, or get less than four volts, replace the switch.