TEST 1: Headlight Battery Power Circuit
The very first thing you and I need to do, is to make sure that the headlight switch is getting power.
This power comes in the form of battery voltage (which will range between 10 to 12 Volts DC) from a fusible link in the Junction Block (in the engine compartment).
In this test step, we'll check that the headlight circuit is getting battery power. In TEST 2 we'll check that the park lamp circuit is being fed with battery power.
You can test for power several ways. Here a few suggestions that may save you a headache:
- You can probe the front to the connector, but be careful since inserting anything into the front of the connector's female terminal, like the multimeter's lead, can permanently open it and thus damage it.
- It's best to back probe the connector or use a wire piercing probe instead of probing the front of the headlight switch connector.
OK, to get the troubleshooting started, this is what you'll need to do:
Remove and unplug the headlight switch from its electrical connector.
Set your multimeter set to Volts DC mode.
Probe the wire that corresponds to the circuit C (see photo in image viewer) with the red multimeter test lead.
NOTE: You should test this circuit by either back probing the connector or using a Wire Piercing Probe (the method I use). Or gently touching the multimeter probe to the front of the connector.
Connect the black multimeter test lead to Ground.
NOTE: It may be a bit hard to find a good Grounding point inside your pickup, so what I suggest you do is to use a battery jumper cable to Ground the multimeter test lead directly to the battery negative terminal.
Your multimeter should register battery voltage.
This circuit is hot (voltage present) all of the time, whether the key is On or not.
Let's take a look at what your multimeter test results mean:
CASE 1: If the multimeter registered 10 to 12 Volts. So far so good, since this tells you that the headlight switch is getting power to feed the headlight's circuit.
If you have a ‘no headlights’ condition but the park lights do come on, the next test should be test the headlight circuit inside the headlight switch. For this test, go to: TEST 3: Checking Headlight Circuit.
If the above does not apply to your specific situation, the next step is to confirm that the park lamps power circuit is getting juice. Go to: TEST 2: Park Lamps Battery Power Circuit.
CASE 2: The multimeter DID NOT register 10 to 12 Volts. Re-check your connections and re-test. If your multimeter still does not register battery voltage, then you've found the reason for the ‘no headlight’ condition.
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.
Although it's beyond the scope of this article to troubleshoot this lack of power, you now know that you need to find out why this battery voltage is missing since this will solve the ‘no headlights’ problem.
TEST 2: Park Lamps Battery Power Circuit
If you started from TEST 1, you have confirmed that the headlight circuit is getting juice (10 to 12 Volts DC).
In this test section, you'll check and confirm that the park lights circuit is getting power. This power also comes in the form of 10 to 12 Volts DC.
The circuit that supplies this voltage, is the one that's labeled with the letter E and is the Orange colored wire in the headlight switch's connector.
OK, this is what you'll need to do:
Disconnect the headlight switch from the its connector. Set your multimeter to Volts DC mode.
Probe the circuit labeled with the letter E shown in the photos in the image viewer with the red multimeter test lead.
Ground the black multimeter test lead.
Your multimeter should register battery voltage (10 to 12 Volts).
This circuit is always hot. So it should have voltage present at all times, whether the key is in the On position or not.
Let's take a look at what your test results mean:
CASE 1: 10 to 12 Volts did register on your multimeter- This tells you that the park lamp fuse is feeding power to the park lamp circuit of the headlight switch.
The next step is to test the park lamps circuit inside the headlight switch itself with your multimeter in Ohms mode. For this test go to: TEST 3: Checking Headlight Circuit.
CASE 2: 10 to 12 Volts DID NOT register on your multimeter. Re-check your connections and re-test.
If the multimeter still does not register 10 to 12 Volts DC, then you have found the cause of the ‘no park lamps’ problem on your GM pickup or SUV since without power, the headlight switch can not turn on the park lamps or the instrument cluster illumination.
Although it's beyond the scope of this test article to troubleshoot this lack of power (in this circuit), you at least now know that you need to find out why you're missing this voltage since this will solve the ‘no park lights’ problem on your GM vehicle.