Trying to get my truck inspected this week but I have an issue. My brake lights are stuck on.
The truck is a 1994 gmc 1500. I’ve replaced the turn signal switch in the column, and the brake light switch. Also tried new circuit boards in the rear and no change.
All other lights work on the truck. Signal lights, hazard lights, headlamps, and back up lights.
Do you need to adjust the brake light switch at the pedal?
That is where I would concentrate most of my efforts.
Slippery said:Do you need to adjust the brake light switch at the pedal?
That is where I would concentrate most of my efforts.
Have the switch removed from the pedal bracket. When manually actuated I have 3rd brake lights but nothing at the tail lights.
Do the brake lights go out when you step on the brake pedal? Do they change if you put the truck in reverse?
I think it’s time to get out the multimeter.
Remove the bulbs and check for 12v on the ground side of the brake light bulb. It sounds like there may be something back feeding the brake lights.
If you have power on the ground side of the bulb, It could be a short or a broken ground on another circuit using the brake lights as a ground when the pedal isn’t pressed,
So I am only getting power to the brake light bulbs when the park lights are on.
shut the lights off and all the bulbs go out.
But either way brake lights don’t respond to the switch. except the third brake light which is functioning normally.
You should check between the leads and the chassis as well, just checking across the plug will not show if the power is coming from the ground or power side of the plug (except if the reading is positive or negative).
Showing .04 volts on the yellow and green wires when not powered.
Black wire for main rear lights was replaced to a nice clean new ground on the box.
There isn't much crossover area from brake to tail lights. The high mount light is on a separate circuit. By far, the most likely spot for trouble like that is in the wiring under the rear of the truck. Anything that doesn't look factory- Tape on the harness? Trailer plug? Modified factory harness plug? Climb under the truck with a good light and follow the harness.
Does it have trailer wiring? Look there first.
In reply to Streetwiseguy :
Already ripped out a poorly installed trailer harness. Nothing aftermarket left.
Pleading my ignorance here but does the brake light switch control the ground to the second circuit on the brake/turn bulb? Or does it feed power?
So, is the brake lite stuck on or is it not coming on?
In reply to Dr. Hess :
When park lights are on both filaments on the central bulb are illuminated.
When park lights are off neither is illuminated.
That sounds like the classic bad ground at the light socket or bulbs crammed in the socket backwards symptom. A 12v test light is a better tool for troubleshooting lighting than a multimeter. The meter will show 12v through a bad connection that won't carry current.
Did it ever work since you've had the truck?
Try unplugging the turn/hazard switch at the steering column and see if anything changes. Inspect the harness running forward, particularly that point that's labeled on the schematic "in trailer harness at rear crossmember." Look for something shorted there, and in the whole trailer light harness.
In reply to Dr. Hess :
I bought the truck as a non runner, like any dumb hot rodder I focused entirely on getting horsepower in the engine bay. Now that’s that part is completed I need to deal with making it legal.
As of last week it didn’t have low beams, or signal lights. After hitting it with the parts cannon it now has everything but functional brake lights.
Will try unplugging the signal light/headlight switch and see if there are any changes.
I am sure that this is a long shot, but I had a front turn / running light (1157) break the outside filament, which proceed to flip over and arc weld itself to the post of the inside filament. Pretty sure the brake lights wouldn't turn off and am absolutely sure the cruise control wouldn't disengage when I tapped the brake pedal.
Of course, it happened while I was on the freeway doing 70+ mph. Luckily, I had enough time to turn off the cruise before I took someone out. 1981 Olds Delta 88 Wagon.
Unplugged the turn signal switch. No change.
Also I had pulled all the bulbs in the system one by one to see if it was a bad bulb. No change.
Is there a better schematic? That one seems to be missing some stuff. I can't see the wire from the left brake light to wherever it connects, or anything connecting to the stop light switch. The wires might have been in color and the B&W copy dropped them out.
From what I can see, if you pull the connector to the turn signal, you should have nothing at the tail lights at all. At this point, I am starting to suspect something wrong in the trailer light circuit.
Just for the hell of it, try disconnecting the hazard flasher.
I'm still leaning towards a short in the trailer circuit or plug.
Hazard switch is part of the turn signal/wiper control switch.
When drhess says disconnect the hazard flasher he means pull the flasher unit itself, not the switch. I get it, he might be on track. It is in the partly in the circuit even when the switch is removed.
In reply to TurnerX19 :
Ohhh, will try this when I get home.
Pulled the flasher unit. Both filaments still lit with it removed.
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