How To Test The Low-High Beam Headlight Dimmer Switch

This tutorial will help you to test the low-high beam headlight dimmer switch on your GM pick up, van, mini-van or SUV.

Specifically, you'll be able to troubleshoot and diagnose a No Low-Beams or a No High-Beams headlight problem.

As you might already be aware this low-high beam headlight dimmer switch is part of the turn signal switch.

In this article, I'll show exactly what you need to do to test it. Now, if you need to just test the headlight switch, you can find the test article here: Headlight Switch Test.

Symptoms Of A Bad Low-High Beam Headlight Dimmer Switch

The most common (and well, probably the only one) is usually the low beam headlights don't work.

In my personal experience, when the low-high beam headlight dimmer switch has failed, you'll only be able to roll around using the high beams only.

Well, in this article, I'll show you how to test it in a step-by-step way. You'll be able to say that low-beam headlight dimmer switch is indeed bad or that it's not.

What Tools Do I Need To Test The Low-High Beam Headlight Dimmer Switch?

You'll need some basic hand tools to remove the steering wheel covers and some of the dash panels, to access the low-high beam headlight dimmer switch wires.

You'll also need:

1. Multimeter.
   • The multimeter can either be a digital one or an analog one.
   • If you need to upgrade or buy a multimeter, check out my recommendation: Buying A Digital Multimeter For Automotive Diagnostic Testing (found at: easyautodiagnostics.com).
2. Wire piercing probe.
   • Although this tool is not an absolute must, if you do buy one, you'll realize just how easy it makes testing the voltages inside the wires.
   • If you need to see what this tool looks like, you can see it here: Wire Piercing Probe.

As you can see from the list above, it's nothing that's gonna' break the bank.

Circuit Descriptions

As mentioned at the beginning of the article, the low-high beam headlight dimmer switch is part of the turn signal assembly.

If you have already taking the steering column covers off, you've noticed that the turn signal switch has a lot of wires connecting to it.

All of these wires connect to one of two connectors. One connector is a gray one and the other is black.

Luckily, we only have to worry about testing 3 wires and these three wires are the ones that are part of the gray connector (see image 1 of 3)

If you disconnect the gray connector from the instrument panel harness, you'll notice that the gray connector has the numbers 1 through 13 embossed on one side (see image 2 and 3).

The three wires that we need to worry about, are the ones that connect to pins 11, 12 and 13 of the gray connector.

Here are the circuit descriptions:

• Circuit 11: High Beam Headlight Feed.
• Circuit 12: Low Beam Headlight Feed.
• Circuit 13: Input from headlight switch.

You'll notice that there's no mention of the colors of the wires and this is intentional, since the colors on your specific vehicle may be different.

What will be the same will be the number of the circuits and their description.

Alright, let's turn the page and get testing.

TEST 1: Checking Headlight Switch Input

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.
   • Find the circuit labeled with the number 13.
   • 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.
   • 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.
   • 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: 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: Low Beam Output.

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.
2. That the headlight switch is bad.
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.