Decreased headlamp intensity.

Probably the HDM - headlamp driver module .
The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
• Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).
• Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).
• Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.

HDM is a relay located in the under hood fuse box . #46

The headlamps may be turned on in 2 different ways. First, when the driver places the headlamp switch in the HEADLAMP position, normal operation occurs. Second, with the headlamp switch is placed in the AUTO position, automatic lamp control (ALC) occurs. During ALC control, the headlamps will be in daytime running lamp (DRL) operation in daylight conditions, or low beam operation in low light conditions. Headlamps
The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
• Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).
• Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).
• Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.

The HDM - is just a relay in the under hood fuse box . That is probably your problem . HDM relay #46 , fuse's HDM fuse's #53 for 4.2 l engine or fuse # 57 for 5.3 l engine . both are 15 amp.

Do you know anything about automotive electrical circuit testing ? Using voltage drop testing to find which part of the circuit has the problem , Voltage or ground side ? There is no common problem with both of these systems. After looking at a wiring diagram ,they have different grounds an B+ voltage supply's .
The STOP fuse in the engine wiring harness junction block supplies battery positive voltage to the normally open stop lamp switch. When the driver presses the brake pedal, the switch contacts close and battery positive voltage is supplied to both the VEHICLE STOP fuse and the VEHICLE CHMSL fuse. The current flow is now to the stop lamps which are grounded at G401 and G402. The center high mounted stop lamp (CHMSL) is grounded at G401.
For trailer wiring, a separate stop lamp circuit is connected through the GMSF to the trailer wiring harness.

Headlamps
The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
• Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).
• Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).
• Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.

DTC B2575
When the BCM receives a ground signal from the headlamp switch requesting the low beam headlamps, the BCM will energize the HDM low beam relay by grounding the low beam relay control circuit.
For the brake light problem ,without testing , i would say there is a ground problem . Voltage drop testing the circuit would show what is wrong.


Mastering Voltage Drop Testing with Pete Meier and Jerry Truglia
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Do you know what a wiring diagram is ? You need to look at one to see which wires to test . Free wiring diagrams here http://www.bbbind.com/free_tsb.html Enter vehicle info. year,make , model ,engine ! Under system click on lighting ,under subsystem click on exterior lighting . Click the search button ,then the blue links ! The fourth one down is the brake lights . You see the fuse's for the brake lights in the rear fuse box ? Do you know where that is located ? Fuse Block - Rear
Beneath the left rear seat.
Know as for your headlamps , probably the headlamp driver module is bad ! The headlamp switch is a input to the BCM - body control module . Hooking up a factory scan tool to view DTC'S - diagnostic trouble codes , an view input data to the BCM . To see if it is getting the signal to turn on the headlamps an see if it send PWM signal to the HDM .
Your best bet ,take it to the dealer .

No on the fuse . The LH HDLP and RH HDLP fuse in the fuse block - under hood, supply battery positive voltage to both low and high beam lamps at all times. For manual operation, when the headlamp switch is in the ON position, the headlamp switch supplies ground directly to the headlamps and to the body control module (BCM) on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. The BCM uses this signal to determine if the conditions are present to illuminate the daytime running lamps or park lamps.
The headlamps may be turned on 2 different ways. First, when the driver places the headlamp switch in the ON position, for manual operation. Second, with the headlamp switch placed in the OFF position activating automatic lamp control (ALC).For ALC operation, the BCM receives a signal from the ambient light sensor indicating a low or high ambient, outside, light level. Under low light conditions the BCM grounds the headlamp relay control circuit energizing the headlamp control relay. The now closed relay supplies ground through the headlamp switch to the headlamps and the BCM on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. When the BCM grounds the headlamp relay control circuit it also grounds the park lamp relay control circuit illuminating the park lamps. The headlamp relay is grounded at G203.
Daytime Running Lamps (DRL)
The low beam headlamps operate at reduced intensity when in daytime running lamps (DRL) mode. The ground circuit to the switch side of the DRL relay has a resistor in-line to reduce the intensity of the low beam headlamps for DRL operation. The body control module (BCM) receives a signal from the ambient light sensor indicating a low or high ambient, outside, light level. Under daylight conditions the BCM grounds the DRL relay control circuit energizing the DRL control relay. The now closed relay supplies ground directly to the low beam headlamps and to the BCM on the headlamp low beam signal circuit. The low beam headlamps now illuminate with reduced intensity. The DRL relay is grounded at G203. Any time the headlamps are illuminated the DRL will not illuminate. Manual park lamps do not effect DRL operation.

The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
?€¢
Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).

?€¢
Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).

?€¢
Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.

The normal battery voltage input to the HDM must be between 6 and 16 volts DC to operate. Any deviation to these values could cause improper operation.
Major components of this system are the BCM, HDM, ambient light sensor, multifunction switch, park brake switch and low beam headlamps.
DTC B2575 Headlamp Control Circuit
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a pulse width modulated (PWM) ground signal to the low beam headlamp driver module through the low beam headlamp driver module control circuit. The low beam headlamp driver module will illuminate the headlamps, adjusting the intensity based on the PWM signal received
You best bet is to take your vehicle to a ASE certified repair shop or the dealer >

Daytime Running Lamps (DRL) NOTE: The smart junction box (SJB) must be configured when installing a new module. For additional information, refer to Section 418-01 .
The DRL system operates the low beam headlamps at a reduced intensity. The SJB supplies pulse width modulated (PWM) voltage to illuminate the low beam headlamps at a reduced intensity when the following conditions are met:

• The headlamp switch is not in the HEADLAMPS ON position.
• The ignition switch is in the RUN position.
• The transmission is not in PARK (P).

This dealer only repair ,unless you have the ford IDS (factory scan tool ) .

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

Maybe bad sensor as seen on the following video



And see how he troubleshoot it

Did you check the fuse's an HDM relay #46 in the under hood fuse / relay box ?
Headlamps
The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
?€¢
Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).

?€¢
Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).

?€¢
Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.

The daytime running lamps in your 2001 Chevrolet Impala are controlled by the Body Control Module (BCM) Here is a description of how the system works:

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 either the DRL or the headlamp relay for auto headlamp operation. Any function or condition that turns ON the headlamps will cancel the DRL operation.
Thirty seconds after you start the ignition with the headlamp switch set in the OFF position, the DRL will perform one of two actions:

Illuminate the high beam headlights at a reduced intensity in daylight conditions
Illuminate the low beam headlights in low light conditions
The DRL/EXT lamps fuse in the top of the engine fuse block supplies battery positive voltage to both the DRL switch contacts and to the DRL relay coil. When the BCM energizes the DRL relay coil, current flows through the right high beam lamp, to the left hand high beam lamp, to the energized DRL relay switch contacts, to ground G101.
The high beam headlamps are now in series, and split the voltage so that the headlamps illuminate at halt intensity. The DRL operates when the ignition switch is in the RUN position, and the parking brake is not set or the transmission is not in park. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate.