TEST 1: Checking Power (12 Volts) Headlight Circuit

How To Test The Headlight Switch (4.3L, 5.0L, 5.7L)

This section (TEST 1 and TEST 2) will help you diagnose a ‘no head lights’ problem.

The very first thing we'll do is to make sure that the circuit that supplies the circuit that controls the headlights (inside the headlight switch) is getting power.

This power comes in the form of 12 Volts and they are always present (which means that even if the ignition key is removed from the ignition switch you still have power).

It's very important that you keep several things in mind when testing for these 12 Volts:

  1. You can probe the front to the connector, but be careful and avoid damaging the metal female terminal with the multimeter test lead.
  2. It's best to back probe the connector or use a wire piercing probe instead of probing the front of the headlight switch connector.

This is what you'll need to do:

  1. 1

    Remove and unplug the headlight switch from its electrical harness connector.

  2. 2

    With your multimeter set it in Volts DC mode probe the wire that corresponds to the circuit C (see photo in image viewer) with the red multimeter test lead.

    You can accomplish this by:

    1. Back probing the connector.
    2. Using a wire piercing probe (the method I use).
    3. Or gently touching the multimeter probe to the front of the connector.
  3. 3

    Ground the black multimeter test lead on a good Ground point.

    NOTE: Since you're working inside the vehicle, you may not have a handy 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.

  4. 4

    Your multimeter will register one of two things: battery voltage (12 Volts) or no voltage at all.

    Voltage should be present whether the key is in the On position (in the ignition switch) or not.

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

CASE 1: If the multimeter registered 12 Volts. This is the correct and expected test result and tells you that the circuit that controls the headlights (inside the headlamp dimmer switch does have power).

The next step is bypass the headlight switch with a jumper wire and see if the headlights come on. For this test, go to: TEST 2: Fused Jumper Wire (Headlight Circuit).

CASE 2: The multimeter DID NOT register 12 Volts. Re-check your connections and re-test. If your multimeter still does not register battery voltage, then this tells you that the reason that the headlights are not coming On when is a lack of Power.

This test result also tells you that the headlight switch is not bad since without power in this circuit, it can not turn on the headlights.

TEST 2: Fused Jumper Wire (Headlight Circuit)

How To Test The Headlight Switch (4.3L, 5.0L, 5.7L)

So far in TEST 1, you have confirmed that there's battery voltage (10 to 12 Volts DC) in circuit C (circuit C is the red wire).

The next step now is to use a fused jumper wire to and manually jumper circuit G and circuit C together.

circuit G is the Yellow wire sticking out of the headlight switch connector.

I recommend that you make your own fused jumper wire to use in this section but you don't have to. You can use just a piece of wire.

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

  1. 1

    Disconnect the headlight switch from its electrical harness connector.

  2. 2

    Locate terminals C and G of the headlight switch's connector (see the image in the image viewer).

  3. 3

    Using a jumper wire, jumper terminals C and G together.

    NOTE: Whatever you use to jumper these two female terminals together, be careful and do not damage them. By damage, I mean using something that will permanently open up the terminals and resulting in a false contact later with the male spade terminals of the headlight switch.

  4. 4

    The result that you should see is that the headlights should come On.

Let's see what your test results mean:

CASE 1: The headlights came On- This tells you that the headlight switch is bad and needs to be replaced.

Here's why:

  1. So far you have verified that when you turn On the headlights (via the headlight switch), the headlights do not come On.
  2. In TEST 1, you confirmed that the headlight switch is getting power in circuit C.
  3. In this test step, when you manually jumpered circuits C and G the headlights came On.

Taking the 3 results above into account, they tell you beyond a shadow of a doubt that the headlight switch is bad and needs to be replaced. If you'd like to save some bucks on the headlight switch, go to: Where To Buy The Headlight Switch And Save.

CASE 2: The headlights DID NOT come On. Re-check your connections and re-test.

If the headlights did not come On, then you have eliminated the headlight switch as bad.

Although it's beyond the scope of this article to test for a short in the wiring between the headlight switch and the headlights, this result tells you that there is an open-circuit problem in circuit G (yellow wire).



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