Low beam headlight illumination

https://www.fixya.com/cars/t8706315-2003_gmc_yukon_xl_denali_low_beam

Ive changed the relay and the headlight switch on the dash .. still wont turn off headlights ? Just changing parts won't fix it , need to test . Hooking up a professional scan tool , one that can read codes in the BCM - body control module is the way to start to find the problem . Then familiarize yourself with how they work.

Headlamps
Major components of this system are the body control module (BCM), headlamp relay, beam select relay, ambient light sensor, headlamp and panel dimmer switch, turn signal/multifunction switch, park brake switch, and the low and high beam headlamps.
The headlamps may be turned ON in 2 different ways. First, when the driver places the headlamp switch in the ON position, normal operation occurs. Second, when the headlamp switch is placed in the AUTO position, automatic lamp control (ALC) operation occurs. During ALC control, the headlamps will be OFF in daylight conditions or in low/high beam operation in low light conditions.
The headlamp relay receives battery positive voltage directly from the battery. The BCM supplies a ground signal to the headlamp relay for normal headlight operation. The BCM also supplies voltage to the beam select relay coil for high beam headlight operation. When the driver places the headlamp switch in the ON position and the dimmer switch is in the low beam position, the headlamp relay supplies current flow through the left and right low beam headlamp fuses to both low and high beam headlamps. Both low beam headlights are grounded through the beam select relay. When activated, the dimmer switch sends a ground signal to the BCM in the high beam position and a momentary ground signal in the flash-to-pass (FTP) position from G106. The BCM then energizes the beam select relay which switches the headlamp grounds from the low beam headlight bulbs to the high beam headlight bulbs illuminating the high beam headlights. With the headlights in the low beam position, the high beams may be momentarily turned ON or flashed by activating the FTP portion of the switch.
The instrument panel cluster (IPC) illuminates the high beam indicator when the BCM detects that the high beams are requested. The IPC receives a class 2 message from the BCM requesting illumination.
The Headlamps On at Exit and Perimeter Lighting modes can be adjusted using the steering wheel controls to select the function as displayed in the driver information center (DIC). The Headlamps On at Exit selection will allow the driver to customize the period of time that the headlamps remain illuminated upon exiting the vehicle. This period can be adjusted from 0-180 seconds. The Perimeter Lighting feature allows the driver to select if they want the headlamps and backup lamps to illuminate when the door UNLOCK button on the key fob is activated. This feature can be turned ON or OFF by using the DIC steering wheel control buttons.

DTC B2575 Headlamp Control Circuit

Circuit Description
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a ground signal to the headlamp relay through the low beam headlamp relay control circuit. The headlamp relay will illuminate the low beam headlamps.

Install a scan tool.
Turn ON the ignition, with the engine OFF.
With a scan tool, command the headlamps ON and OFF.
Does the headlamp relay turn ON and OFF with each command?

Do you know what bi-directional control means ? Need a professional scan tool to do these type of test .

DTC B2602: Daytime Running Lamp Control Circuit Low or Open
DTC B2603: Daytime Running Lamp Control Circuit High

Without this type of info. from service repair manual an using a scan tool your missing the big picture .

The usual cause is a bad ground.
Check the ground with a test lamp.

Drl only work in daytime not at night ???? Daytime running lamps are only suppose to work in day light hours . Low beam & high beams ? none of these work at night ?
Headlamps
The headlamps may be turned on in two different ways. First, when the driver places the headlamp switch in the HEADLAMP position, for normal operation. Second, with the headlamp switch placed in the AUTO position, for automatic lamp control (ALC). During ALC the headlamps will be in day time running lamp (DRL) operation in daylight conditions, or headlamp operation in low light conditions.
Headlamp ON/OFF control is determined by the body control module (BCM) by a signal on the headlamp switch on input circuit when the headlamp switch is in the HEAD position. When the headlamp switch is in the AUTO position, the BCM determines headlamps ON/OFF by the voltage from the ambient light sensor.
If the headlight switch is left in the ON position, the inadvertent power control feature will turn off the headlights approximately 10 minutes after the ignition switch is turned to the OFF position. If the driver places the headlight switch in the ON position after the ignition switch has been turned OFF, or if the ignition switch is in the ACCY position, the headlights will remain on until turned off or the battery runs dead.
The high beam indicator is illuminated when the instrument cluster receives a class 2 serial data message from the BCM that the high beams are illuminated.

Do you know what the BCM is ? Do you know what DTC'S - diagnostic trouble code's are ?
DTC B2575
When the body control module (BCM) senses a request for low beam headlamp illumination, the BCM sends a ground signal to the low beam headlamp relay through the headlamp low beam relay control circuit. The headlamp low beam relay will apply voltage to illuminate the headlamps.

Low and High Beams Headlamps Inoperative
Step
Action
Yes
No
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics
Connector End View Reference: Lighting Systems Connector End Views or Body Control System Connector End Views in Body Control System.
1
Did you perform the Lighting System Diagnostic System Check?
YES - Go to Step 2
NO - Go to Diagnostic System Check - Lighting Systems
2
Place the headlamp switch in the ON position.
Change the dimmer switch position from the high to low beam positions several times.
YES - Does the headlamp system operate normally?
NO - Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
Go to Step 3
3
Are both high and low beam headlamps inoperative?
YES - Go to Step 4
NO - Refer to Headlamps Inoperative - Low Beams or Headlamps Inoperative - High Beams
4
With a scan tool, observe the headlamp switch parameter.
Does the scan tool indicate ACTIVE? YES - Go to Step 7
NO - Go to Step 5
5
Connect a test lamp between the ground circuit of the headlamp switch connector and battery positive voltage.
Does the test lamp illuminate?
YES - Go to Step 6
NO - Go to Step 9
6
Test the headlamps switch headlamps on signal circuit of the headlamp switch for a high resistance, an open, or a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 12
Go to Step 8
7
Inspect for poor connections to the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 12
Go to Step 10
8
Inspect for poor connections to the headlamp switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 12
Go to Step 11
9
Repair the ground circuit of the headlamp switch.
Did you complete the repair?
Go to Step 12
--
10
Important: Perform the setup procedure for the body control module.
Replace the body control module. Refer to Body Control Module Replacement
Did you complete the replacement?
Go to Step 12
--
11
Replace the headlamp switch. Refer to Headlamp Switch Replacement
Did you complete the replacement?
Go to Step 12
--
12
Operate the system in order to verify the repair.
Did you correct the condition?
System OK
Go to Step 3

Did you think of taking to a qualified repair shop ? The headlamp switch is a input to the BCM - body control module , the BCM then energizes the low beam relay . Could have DTC'S - diagnostic trouble code's stored . Also with scanner can view input data from the switch to the BCM an control circuit from BCM to the low beam relay . You people an just replacing parts . lol Your not seeing the whole picture .Your best bet , take it to a qualified repair shop .

For low beam operation, battery positive voltage is applied at all times to the coil and switch side of the low beam relay in the underhood fuse block. Ground is applied at all times from G202 to the headlamp switch. When the headlamp switch is placed in the HEAD position, ground is applied to the headlamps ON signal circuit to the body control module (BCM). When either the headlamp switch is placed in the HEAD position or ALC is enabled, the BCM applies a ground to the low beam relay control circuit. This energizes the relay and applies battery positive voltage to the LEFT and RIGHT low beam fuses in the underhood fuse block. Battery positive voltage is then applied from the fuses through the headlamp supply voltage circuit to the headlamps. Ground is applied at all times to the left headlamp from G101 and to the right headlamp from G100.

Headlamps Inoperative - Low Beams
Step
Action
Yes
No
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics
Connector End View Reference: Master Electrical Component Listin Wiring Systems.
1
Did you perform the Diagnostic System Check - Vehicle?
Go to Step 2
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2
Turn ON the headlamps.
Ensure that the headlamp dimmer switch is in the LOW position.
Do both low beam headlamps illuminate?
Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
Go to Step 3
3
Is only one low beam headlamp inoperative?
Go to Step 12
Go to Step 4
4
Disconnect the body control module (BCM) connector C1.
Connect a jumper between the headlamp low beam relay control circuit and ground.
Do the low beam headlamps illuminate?
Go to Step 5
Go to Step 7
5
Reconnect the BCM connector C1.
Disconnect the multifunction switch.
Install a jumper between the headlamp switch headlamps ON signal circuit and ground.
Do the low beam headlamps illuminate?
Go to Step 9
Go to Step 6
6
Test for a poor connection , open or short to battery voltage, in the headlamp switch headlamps ON signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 18
Go to Step 13
7
Remove the low beam headlamp relay.
Install a fused jumper between the battery positive voltage supply circuit and the switched low beam voltage supply circuit.
Do the low beam headlamps illuminate?
Go to Step 8
Go to Step 11
8
Test for a poor connection, open or a short to battery positive voltage in the low beam relay control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 19
Go to Step 10
9
Inspect for a poor connection at the harness connector of the headlamp dimmer switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 19
Go to Step 14
10
Inspect for a poor connection at the low beam relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 19
Go to Step 15
11
Inspect for a poor connection at the harness connector of the underhood fuse block. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 19
Go to Step 17
12
Inspect the inoperative low beam headlamp voltage supply and ground circuit for a high resistance, short to ground, or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 19
Go to Step 16
13
Test for a short to ground or a short to battery positive voltage in the exterior lamps OFF input circuit. Refer to Circuit Testingand Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
Go to Step 19
Go to Step 18
14
Replace the multifunction turn signal switch. Refer to Turn Signal Multifunction Switch Replacement in Steering Wheel and Column.
Did you complete the replacement?
Go to Step 19
--
15
Replace the low beam relay. Refer to Relay Replacement in Wiring Systems.
Did you complete the replacement?
Go to Step 19
--
16
Replace the inoperative low beam headlamp. Refer to Headlamp Assembly or Headlamp Bulb Replacement .
Did you complete the replacement?
Go to Step 19
--
17
Replace the underhood fuse block. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
Did you complete the replacement?
Go to Step 19
--
18
Replace the BCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.
Did you complete the replacement?
Go to Step 19
--
19
Operate the system in order to verify the repair.
Did you correct the condition?
System OK
Go to Step 3

Do you know anything about automotive electronics ? Electronic control modules .
Headlamp ON/OFF control is determined by the body control module (BCM) by a signal on the headlamp switch on input circuit when the headlamp switch is in the HEAD position. When the headlamp switch is in the AUTO position, the BCM determines headlamps ON/OFF by the voltage from the ambient light sensor.
If the headlight switch is left in the ON position, the inadvertent power control feature will turn OFF the headlights approximately 10 minutes after the ignition switch is turned to the OFF position. If the driver places the headlight switch in the ON position after the ignition switch has been turned OFF, or if the ignition switch is in the ACCY position, the headlights will remain ON until turned OFF or the battery runs dead.
The high beam indicator is illuminated when the instrument cluster receives a GMLAN serial data message from the BCM that the high beams are illuminated.
High Beams
The high beam headlamps receive voltage from the headlamp high relay located in the underhood fuse block. The BCM controls the relay on the headlamp high beam relay control circuit. High beam control is determined by a signal on the headlamp dimmer switch signal circuit from the multifunction switch. When the headlamp dimmer switch signal circuit is grounded, the BCM grounds the headlamp high beam relay control circuit, illuminating the high beam headlamps. The right and left high beam circuits are independent and are fused independently in the underhood fuse block.
Low Beams - Standard
The low beam headlamps receive voltage from the headlamp low beam relay located in the underhood fuse block. The BCM controls the relay on the headlamp low beam relay control circuit. Low beam control is determined by a signal on the headlamp dimmer switch signal circuit from the multifunction switch. When the headlamp dimmer switch signal circuit is open, the BCM grounds the headlamp low beam relay control circuit, illuminating the low beam headlamps. The right and left low beam circuits are independent are fused independently in the underhood fuse block.

Hooking up a factory scan tool an checking DTC'S - diagnostic trouble codes would be the very first step in trouble shooting this problem .
DTC B257A 00: Headlamp Switch Input Signal Mismatch
DTC 2575 00: Headlamp Control Circuit

Headlamps
You can use the headlamps two different ways.
?€¢
Place the headlamp switch in the HEAD position for normal operations

?€¢
Place the headlamp switch in the OFF position for automatic lamp control (ALC) operation. During ALC the low beam headlamps are ON for daytime running lamp (DRL) operation in daylight conditions at reduced intensity or low beam headlamps ON at full intensity with exterior park lamps ON in low light conditions. The ambient light sensor is a light-sensitive transistor that varies its voltage signal to the body control module (BCM) in response to changes in the outside (ambient) light level. When the BCM receives this signal the BCM turns ON just the low beam headlamps at reduced intensity for the daytime running lamp (DRL) operation. A resistor wire is used (CKT312) to reduced the voltage which reduces the light intensity for the DRL's function. The ambient light sensor can also turn ON the low beam headlamps and the exterior lamps for automatic lamp control (ALC). Any function or condition that turns ON the headlamps will cancel the DRL operation (this means no reduced voltage to the low beam lights). Thirty seconds after you start the engine with the headlamp switch in the OFF position, the DRL will perform one of two actions:
?€¢
Illuminate the low beam headlamps at reduced intensity in daylight conditions

?€¢
Illuminate the low beam headlamps and exterior park lamps for low light conditions

The DRL operates when the ignition switch is in the RUN position, the parking brake is not set and the transmission is not in the PARK position When these conditions are met and the DRL ambient light sensor indicates daylight conditions the DRL will illuminate
Should have BCM scanned for DTC'S - diagnostic trouble codes , could have DTC'S stored in memory !
DTC B2645 Ambient Light Circuit
The body control module (BCM) monitors the ambient light sensor signal circuit in order to determine if the daytime running lights (DRL) or the auto headlights (ALC) should be turned ON in the AUTO mode. When the BCM senses the ambient light sensor voltage is between 1.75-4.9 volts, either the DRL will be commanded ON or the ALC will be commanded ON.

1. Turn ON the headlamps.
2. Ensure that the headlamp dimmer switch is in the LOW position.

Do both low beam headlamps illuminate?
Is only one low beam headlamp inoperative? YES / NO
IF no

1. Disconnect the lamps switch assembly.
2. Install a jumper between the headlamp switch headlamps ON signal circuit and ground.

Do the low beam headlamps illuminate?
Do the low beam headlamps illuminate?
Go to Step 18
Go to Step 7

7

1. Remove the headlamp module.
2. Install a fused jumper between battery positive voltage and the switched low beam voltage supply circuit.

Do the low beam headlamps illuminate?
Go to Step 8
Go to Step 11

8

Test for the following in the headlamp module control circuit:
?€¢
An open

?€¢
A poor connection

?€¢
A short to ground

Refer to Circuit Testing and Wiring Repairs .
Did you find and correct the condition?
Go to Step 19
Go to Step 10

9

Inspect for a poor connection at the harness connector of the lamp switch assembly. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .
Did you find and correct the condition?
Go to Step 19
Go to Step 14

10

Inspect for a poor connection at the headlamp module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .
Did you find and correct the condition?
Go to Step 19
Go to Step 15

11

Test for a short to ground in the low beam voltage supply circuit. Refer to Circuit Testing and Wiring Repairs .
Did you find and correct the condition?
Go to Step 19
Go to Step 12

12

Inspect for a poor connection at the harness connector of the underhood fuse block. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .
Did you find and correct the condition?
Go to Step 19
Go to Step 17

13

Inspect the inoperative low beam headlamp voltage supply and ground circuit for a high resistance, a short to ground, or an open. Refer to Circuit Testing and Wiring Repairs .
Did you find and correct the condition?
Go to Step 19
Go to Step 16

14

Replace the lamp switch assembly. Refer to Headlamp Switch Replacement .
Did you complete the replacement?
Go to Step 19
--

15

Replace the headlamp module. Refer to Relay Replacement in Wiring Systems.
Did you complete the replacement?
Go to Step 19
--

16

Replace the inoperative low beam headlamp. Refer to Headlamp Assembly or Headlamp Bulb Replacement .
Did you complete the replacement?
Go to Step 19
--

17

Replace the underhood fuse block. Refer to Testing for Intermittent Conditions and Poor Connections .
Did you complete the replacement?
Go to Step 19
--

18

Replace the BCM. Refer to Control Module References for replacement, setup, and programming.
Did you complete the replacement?
Go to Step 19
--

19

Operate the system in order to verify the repair.
Did you correct the condition?
System OK
Go to Step 3

IF

1. I own a Protege and once the low beam bulb burns on one headlight the car electronics quickly cook the bulb on the other headlight as well. it is a poor design by the mfr. Pull both bulbs to make sure the low beam is not cooked.

if you have power to both headlight plugs for each low and high beam filaments and the ground is good as you say the high beams function normally then you either have bad low beam bulb filaments,poor or dirty connection or you have a daytime running headlight relay on the car and that is the problem with the low beam lights.yes the high beams will work but not low beams if it is the relay.

there is a bad connection in the switch or the multifunction switch is bad