Another "My brake lights don't work" thread

[strikk]

Please educate me, I am stumped...

1998 k1500, 4.3l, eclb.

Symptom: When I press my brake pedal, only the third brake light lights up.

So far I have:

• New ground at rear end for white+black wires
• New brake light switch at brake pedal
• The entire turn signal stalk from a parts truck and swapped that out, same results
• Swapped out head light switch from a parts truck (made my cluster lights come back to life)
• Replaced, reseated, jiggled, and swapped
  • Brake light fuze #18, in cab
  • Brake light relay under hood
• I have taken the entire rear tail light and fuel pump harness from a parts truck, same result
• I have swapped out tail light housings + boards from a parts truck, same result

Tail lights work under normal condition, blinkers and backup lights no issues. just non functioning.

Before I pull the entire apart to trace wires between the column and firewall, what the heck else can I test?

 

[strikk]

I hope to find some time this week to dig deeper under the dash. Driving around with only the third brake lamp has me on edge.

The wiring diagram in my chilton manual is great at telling me the brown wire carries the signal for the brake pedal but I have not found any extensive wire diagrams for under the dash that include the brake lamp switch on the brake pedal.I don't believe that one was never published

 

[Eveready]

Have you subbed out a junkyard tail light just to see if the board on the tail light is at fault?
Also since most lights work by completing a ground it might be instructive to run a jumper from a solid ground to the tail light assembly and complete the ground locally and see if it lights up. If it does then you have established that the assembly is functional and the problem is between the light and the brake light switch.

1. Determine if the light itself is functional ... if not, replace.

2. If it IS functional then chase down the break between the back of the truck and the brake light switch. I am inclined toward that being the problem since both lights are out. Good luck and please report what it turns out to be.

 

[Urambo Tauro]

Not sure I would trust a junkyard tail lamp any more than I trust the "parts truck" turn signal stalk. Really need to break out a test lamp or multimeter and trace power before any more parts get thrown at it.

If all of the blinkers are working (which I assume includes the hazard lamps), then the circuitry going from the cab to the tail lamps must be intact. The brake switch is apparently working, since it lights up the CHMSL... I suspect the problem is going to be somewhere in the multifunction switch or wiring under the dash.

 

[Eveready]

Not sure I would trust a junkyard tail lamp any more than I trust the "parts truck" turn signal stalk. Really need to break out a test lamp or multimeter and trace power before any more parts get thrown at it.

If all of the blinkers are working (which I assume includes the hazard lamps), then the circuitry going from the cab to the tail lamps must be intact. The brake switch is apparently working, since it lights up the CHMSL... I suspect the problem is going to be somewhere in the multifunction switch or wiring under the dash.

After thinking about it more I agree with you and you are of course right about the junkyard tail light given the reliability (or lack of it ) of those boards. Most likely a broken wire. I wonder if the circuit doesn't run between the 3rd brake light (which works) and the tails. It would be logical that it does. I would check connection at the 3rd brake light as it most a wire from there that provides ground to one side or the other of the tails which in turn are likely jumpered together.

 

[delta_p]

go to the hatzie thread and look in '98 volume 1 lighting systems. There's a full schem. in there. https://www.gmt400.com/threads/88-95-service-manuals.43575/

It may be in the turn hazard switch or the wire feeding it the stop lamp power (the one from the underhood relay to the turn haz switch. The turn signal flash get it power through the flash relay and supporting fuse.

I had a problem once and couldn't figure out the the turn hazard switch even from the schematic so i tried to draw my own.

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[BiG_rEd98_5spd]

Please educate me, I am stumped...

1998 k1500, 4.3l, eclb.

Symptom: When I press my brake pedal, only the third brake light lights up.

So far I have:

• New ground at rear end for white+black wires
• New brake light switch at brake pedal
• The entire turn signal stalk from a parts truck and swapped that out, same results
• Swapped out head light switch from a parts truck (made my cluster lights come back to life)
• Replaced, reseated, jiggled, and swapped
  • Brake light fuze #18, in cab
  • Brake light relay under hood
• I have taken the entire rear tail light and fuel pump harness from a parts truck, same result
• I have swapped out tail light housings + boards from a parts truck, same result

Tail lights work under normal condition, blinkers and backup lights no issues. just non functioning.

Before I pull the entire apart to trace wires between the column and firewall, what the heck else can I test?

I had an issue with my brake lights flickering and cruise not working. Brakes came on with a light press to pedal, harder pressure lights would turn off. I replaced light housings, light bulb boards, lights pig tails to the lights, all ground throughout the truck, new third brake light lights and wiring. Only to find out that the clip plugging into the brake light switch at pedal also clipped into the harness under dash, had frayed and severed wires. Pulled it, ran new wires and pins to connector and bam. Problem gone. So I might have went over kill, but this 20 year old truck got Perty stuff in it now

 

[strikk]

Closing the loop on this old thread...

Symptoms: Functioning rear tail lights for hazards, turn signals, running lights, backup lights and fully functioning third brake light. Problem: Rear tail brake lamp would not activate when depressing the pedal.
Cause: Warn power feed wire to relay under hood. This wire break was located 5" inches or so back from the relay under the driver side relay box. The wire loom was warn through causing this wire to rub against the fender liner and cause the break

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[Erik the Awful]

That's a tricky find. Good work.

 

[Eveready]

Thanks for letting us know. That would be a hard one to chase down and very glad you managed to find it. I wonder how many new tail light boards have been installed on trucks over the years when something like this was the culprit? I hope you saved your old parts, many of them may be good!