Low voltage when at a stop light voltage drops

Depending on your vehicle, when the battery voltage drops the vehicle sensors cannot function properly due to low voltage going to the sensors. The ac compressor will shut off, the vehicle will go into "limp in" mode. You may be having an alternator that is beginning to go out or have weak battery connections.

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.


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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 .

High resistance = low currant flow = dim lights ! The way a professional automotive Tech goes about finding the problem is by using voltage drop testing to see which side of the circuit has the problem , B+ or ground ! Mastering Voltage Drop Testing with Pete Meier and Jerry Truglia

Low voltage would mean that there is corrosion further up the circuit. Corrosion equals resistance. Resistance equals voltage drop. Keep searching the wiring and connection further up. You're bound to find some green fuzzy corrosion at some point

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It sounds like a loose cable that is vibration sensitive. I am not exactly following you but if it stops when the car is turned off then get the alternator and or regulator tested. Your battery should hold about 13.8v when running.

Once voltage drops below 9 volts, many things will stop working. After voltage drops below 8 volts, most likely everything will shut down. If you have a low voltage condition, you need to have the battery/alternator charging system checked for problems. Once you get the voltage issue fixed, most likely all the other issues will disappear.

You might need to remove the alternator and carry it down to your partstore so they can bench test it and determine if it is charging properly? It could be the voltage regulator in the back of the alternator which held by four screws?

Buy a two pack of the low beam bulbs and change both had lights. the voltage drop will burn one out since they have different resistance.

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Have the alternator output checked at idle and if the idle is too low incrtease it to spec idle revs. It appears that the voltage in you electrical system is too low for the gauges Check when at the lights take you foot of the brake and put tranny out of gear and see if there is a difference

fix the idle speed to 850-900 rpm
check the alternator and voltage regulator as the voltage should read 14.5-14.8 volts dc at 1000rpm or higher regardless of what is running
have it bench tested by an accredited auto electrician