TEST 1: Checking Headlight Switch Input

How To Test The Low-High Beam Headlight Dimmer Switch

The very first thing we need to do, is to make sure that the Headlight Switch is actually sending an input signal.

This input signal is sent only when you turn the Headlight Switch knob to ‘Headlights On’ position.

Now, in case you need to test the Headlight Switch itself, you can find the testing instructions here: How To Test The Headlight Switch.

This is what you'll need to do:

  1. Locate the Turn Signal Switch's Gray connector.
    1. Find the circuit labeled with the number 13.
    2. If you had to disconnect the Gray connector find this wire, reconnect the Gray connector now to the Instrument Panel wiring harness connector.
  2. Turn the Headlight Switch to the On position.
  3. Set your multimeter to Volts DC mode.
  4. Probe the wire that corresponds to the number 13 with the red multimeter test lead.
    1. You can accomplish this by using a Wire Piercing Probe (To see what this tool looks like, click here: Wire Piercing Probe).
  5. Ground the black multimeter test lead on a good Ground point.
    1. Inside your vehicle, it may be a bit hard to find a Ground point to Ground the multimeter test lead, so I suggest using a battery jump start cable to Ground the black multimeter test lead directly to the battery negative terminal.
  6. You're multimeter will register one of two things: Battery Voltage (12 Volts) or No voltage at all.
  7. When done, turn off the Headlight Switch.

Let's analyze your test results:

CASE 1: If the multimeter registered 12 Volts. This is good, since it means that the Headlight Switch is working.

The next step is to check that the Low-High Beam Headlight Dimmer Switch is redirecting this voltage to the low beam headlights on circuit number 12. Go to: TEST 2.

CASE 2: The multimeter DID NOT register 12 Volts. Re-check your connections and repeat the test.

If your multimeter still does not register battery voltage, then this tells one of three things:

  1. That the Low-Beam Headlight Dimmer Switch is NOT bad and...
  2. That the Headlight Switch is bad or...
  3. That there's a short in the wiring between the Headlight Switch and the Low-High Beam Headlight Dimmer Switch (although this would be extremely rare).

You can find the Headlight Light Switch test here: How To Test The Headlight Switch.

TEST 2: Low Beam Output

How To Test The Low-High Beam Headlight Dimmer Switch

So far, you have confirmed that the Low-High Beam Headlight Dimmer Switch is getting the Headlight Switch input signal (TEST 1).

The next step is to see if the Low-High Beam Headlight Dimmer Switch is able to turn the 12 Volt signal around and redirect it to the low beam headlights.

This is another pretty easy test that you'll perform with your multimeter.

OK, this is what you'll need to do:

  1. Identify the circuit labeled with the number 12 of the Gray connector.
    1. The Gray connector must remain connected to the instrument panel harness connector at all times during this test.
  2. Turn the Headlight Switch to the On position.
  3. With your multimeter to Volts DC mode:
    1. Probe the wire that corresponds to the number 12 with the red multimeter test lead.
      1. The best way to do this is using a Wire Piercing Probe (To see what this tool looks like, click here: Wire Piercing Probe).
  4. Ground the black multimeter test lead on a good Ground point.
    1. Inside your vehicle, it may be a bit hard to find a Ground point to Ground the multimeter test lead, so I suggest using a battery jump start cable to Ground the black multimeter test lead directly to the battery negative terminal.
  5. Move the turn signal lever so that you are in low beam mode.
    1. In low beam mode, your multimeter should register 10 to 12 Volts.
    2. Remember, you can move the lever back and forth to select either low or high beams.
  6. Move the turn signal lever so that you are in high beam mode.
    1. In high beam mode, your multimeter should register 0 Volts.
  7. Cycle between low and high beam mode several times. Each time, you should see 10 to 12 Volts in low beam mode and 0 Volts in high beam mode.
  8. When done, turn off the Headlight Switch.

Let's take a look at what your test results mean:

CASE 1: Your multimeter registered voltage (10 to 12 Volts) in low beam mode- This is the normal and expected test result.

This also tells you that the Low-High Beam Headlight Dimmer Switch is doing what it's supposed to do for the low beam headlights. The next step is to check that the Low-High Beam Headlight Dimmer Switch is outputting the correct voltage for the High Beam Headlights... for this, go to: TEST 3.

CASE 2: Your multimeter DID NOT register voltage (10 to 12 Volts) in low beam mode. Re-check all of your multimeter connections and repeat the test one more time...

If your multimeter still did not register any voltage in low beam mode, then the Low-High Beam Headlight Dimmer Switch is bad and needs to be replaced.



Cadillac Vehicles:

  • Escalade
    • 1999, 2000

Chevrolet Vehicles:

  • Astro 4.3L
    • 1996, 1997, 1998, 1999
  • C1500, C2500, C3500 Silverado
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002

Chevrolet Vehicles:

  • C1500, C2500 Suburban
    • 1995, 1996, 1997, 1998, 1999
  • Express Van 1500, 2500, 3500
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • K1500, K2500, K3500 Pick Up
    • 1995, 1996, 1997, 1998, 1999, 2000
  • K1500, K2500 Suburban
    • 1995, 1996, 1997, 1998, 1999

Chevrolet Vehicles:

  • P30 Van
    • 1997, 1998
  • Tahoe
    • 1995, 1996, 1997, 1998, 1999, 2000

GMC Vehicles:

  • Sierra C1500, C2500, C3500
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2003
  • Suburban C1500, C2500
    • 1995, 1996, 1997, 1998, 1999

GMC Vehicles:

  • K1500, K2500, K3500 Sierra
    • 1995, 1996, 1997, 1998, 1999, 2000
  • P3500 Van
    • 1997, 1998
  • Safari
    • 1996, 1997, 1998, 1999

GMC Vehicles:

  • Savana Van 1500, 2500, 3500
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Yukon
    • 1995, 1996, 1997, 1998, 1999, 2000