Helping a friend on this truck...1989 silverado 2500 4x4...had a intermittent brake light coming on issue..if you wiggled the brake lever light went out...was a bad brake light switch. Now i have all lights BUT no brake when lights are truned on. All else works great...its a colorado no rust/corossion truck....my question... is there 2 brake light circuits??..the middle bulb is the brake light 2 element bulb...will only light up when lights are off...anyone know this circuit?

 

I will also add that the cruise control does not work so I also tried replacing the brake light switch again and the headlight switch both of those replaced before I switched them back and it did not change so I know those parts are OK and check the voltage at the back I’m not getting 12 V when the headlights are on for the brake only

 

What about turn signal operation with the lights on? Is it normal? What about when you’re pressing the brake pedal? The brake light and turn signals use the same bulb/circuit, so a problem with the turn signals as well indicates the turn signal switch or flasher could have a fault that’s affecting brake light operation as well with the lights on.

A lack of rust/corrosion on the undercarriage doesn’t mean there’s no wiring problems. Wiring insulation cracks and shrinks with heat and age, and can rub through due to the vibrations of driving. Dirt and debris can collect on the frame and get inside the harness loom. Thorough inspection of the wiring should be done to find obvious problems - the steering column, section near the gas tank where the harness splits off, and rear section of wiring (between the rearmost crossmember and bumper) are known problem areas regardless of truck condition. If there’s any trailer wiring, cut it out for diagnostic purposes, especially if it’s been installed with scotch locks.

Try removing one of the brake light bulbs and see if the other one works and vice versa, indicating a short on that branch of the circuit.

 

[QUOTE=Gumby22;463741]What about turn signal operation with the lights on? Is it normal? What about when you’re pressing the brake pedal? The brake light and turn signals use the same bulb/circuit, so a problem with the turn signals as well indicates the turn signal switch or flasher could have a fault that’s affecting brake light operation as well with the lights on.

A lack of rust/corrosion on the undercarriage doesn’t mean there’s no wiring problems. Wiring insulation cracks and shrinks with heat and age, and can rub through due to the vibrations of driving. Dirt and debris can collect on the frame and get inside the harness loom. Thorough inspection of the wiring should be done to find obvious problems - the steering column, section near the gas tank where the harness splits off, and rear section of wiring (between the rearmost crossmember and bumper) are known problem areas regardless of truck condition. If there’s any trailer wiring, cut it out for diagnostic purposes, especially if it’s been installed with scotch locks.

Try removing one of the brake light bulbs and see if the other one works and vice versa, indicating a short on that branch of the circuit.[....Thanks for the reply turn signals with lights on is normal when pressing brake turn signals are normal I will check the harness but all things on there look perfect no debris no trailer wiring I will try to remove one brake bulb and check to see if the other is working that’s a good idea/QUOTE]