How To Test The Low-High Beam Headlight Dimmer Switch

TEST 2: Low Beam Output

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.
   • 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:
   • Probe the wire that corresponds to the number 12 with the red multimeter test lead.
     • 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.
   • 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.
   • In low beam mode, your multimeter should register 10 to 12 Volts.
   • 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.
   • 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 outputs the correct voltage for the high-beam headlights. For this, go to: TEST 3: High Beam Headlight Output.

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.

TEST 3: High Beam Headlight Output

You've reached this point because you have confirmed that the low-high beam headlight dimmer switch is getting the headlight switch input signal (TEST 1) and that it's also returning a low beam signal to the low beam headlights (TEST 2).

In this test step, you're gonna' verify that the internal circuit that feeds the high beams is working or not.

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 13 of the gray connector.
   • 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. Once again, your multimeter must be set to Volts DC mode:
   • Probe the wire that corresponds to the number 13 with the red multimeter test lead.
     • Using a wire piercing probe is the best way to do this (to see what this tool looks like, click here: Wire Piercing Probe).
4. Ground the black multimeter test lead on a good Ground point.
   • I suggest using a battery jump start cable to Ground the black multimeter test lead directly to the battery negative (-) terminal.
5. Activate the turn signal lever so that you are in high beam mode.
   • In high beam mode, your multimeter should register 10 to 12 Volts.
6. Activate the turn signal lever so that you are in low beam mode.
   • When the switch goes into low beam mode, your multimeter should read 0 Volts.
7. Cycle between high and low beam mode a few 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 interpret your test results:

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

This result also tells you that the low-high beam headlight dimmer switch is OK and not the cause of the 'No High-Beam' or 'No Low-Beam' problem and here's why:

• In TEST 1, you confirmed that the low-high beam headlight dimmer switch is getting the headlight switch's input signal when you turn on the headlights.
• In TEST 2, you confirmed that the low-high beam headlight dimmer switch is activating the low beam headlights.
• In this TEST 3, you have confirmed that the low-high beam headlight dimmer switch is activating the high beam headlights.

All of the above test results confirm that the low-high beam headlight dimmer switch is NOT faulty or defective.

CASE 2: Your multimeter DID NOT register voltage (10 to 12 Volts) in high 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 high beam mode, then the low-high beam headlight dimmer switch is bad and needs to be replaced.

Test Summary

Hopefully this article helped you to find the repair solution to your 'No Low Beam Headlights' or 'No High Beam Headlights' problem on your GM pickup, van, mini-van, or SUV.

I know that this article is pretty narrow in its focus and doesn't cover all possible malfunctions and so, I invite you to share your own troubleshooting and repair experience with the rest of this web page's readers. This not only improves the article, but will help anyone else with a similar issue or problem. You can use the contact form below.

Feel free to also check out all of the 'How To Test' article found here:

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