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Tail Lights WORK ! Brake Lights Don't ? Battery Meter Flucuates when turn on A/C

2003 GMC - SIERRA 1500 / 5.3 L / 4X4 / Z71 Package

Installed NEW BATTERY and Alternator. Truck has fluctuating idle and battery meter when A/C turns on OR you lower all windows at one time.

Tail Lights work but Brake Lights DO NOT. Dual A/C = Left side only throws cold on COLD and room temp at HOT. / Left Side - Fluncuates on all temps goes back and forth with no movement mostly HOT or Warm even on COLD ?

ALL FUSES WORK … NO FUSES BLOWN

A/C , Truck Idle and Battery Meter went BAD after i had my Oil Pan Gasket replaced having to Lift Motor due to truck being 4X4 and mechanic let battery die for a couple days due to him leaving inside lights on…

Brake lights have been bad BEFORE ALL THIS HAPPENED…

Please help… All HELP APPRECIATED…

THANKYOU…

Sorry, your thought processis is too chaotic to follow.

You might have a bad ground.
Check if the ground straps are all tight. Maybe an engine ground was left loose after lifting the motor to replace that pan gasket.

how much is the idle flucuating? And does it flucuate even while in gear? just asking to see if this could be a comp. issue or wiring.

Check out what @db4690 suggests. A loose ground strap is a good possibility.

To check this out, use a booster cable - just the negative side and do not use the positive side - hook the black clamp on the negative post of battery and the other black clamp on a bare metal part of the engine.
If that fixes it, move the wire from the engine to frame, then from the frame to the battery. Whatever you bridge and fixes it is the part that has a bad connection.

The brake lights is likely related to a bad connection between the pedal and the brake light. I don’t think it is the switch mounted on the brake pedal because you won’t be able to get your transmission out of “P” when you press the brake pedal.

Brake lights? 3 possibilities:

  1. The brake filaments in the bulbs are burned out (There are 2 filaments - one for the brake lights and one for the tailights)

  2. The switch for the brake lights is not woking.

  3. There is a bad ground in the brake light circuit.

Voltage drop? That is to be expected. The alternator is supplying voltage that will vary according to load. Turn on the AC or operate the windows and the load goes up - ergo, the voltage will drop.