Brake light issue I can't get smart to

[papapavs1]

Hello, '78 K25, and I don't have brake lights anymore. I did a quite a bit of work to the truck, but a few things that may be of interest are: replaced the ignition switch, pulled the bed off for a while to paint it, and had the steering wheel pulled to replace ignition lock cylinder. Went back together and now theres no brake lights. All other lights and signals work. I tested that I had 12v to the brake switch, which I did. Tested continuity on the switch, which I had with the pedal pressed, and opened the circuit with the pedal not pressed. Checked at the block on the firewall, and the green and yellow wires each had 12v on them with the brake applied, down to zero when not applied. But then when I go back to the plug at the back of the box, I get 0v on either wire with the brake applied. I put a signal on, and I get 12v on the corresponding wire, so I know the wires aren't bad. What am I missing here?

 

[PrairieDrifter]

Hello, '78 K25, and I don't have brake lights anymore. I did a quite a bit of work to the truck, but a few things that may be of interest are: replaced the ignition switch, pulled the bed off for a while to paint it, and had the steering wheel pulled to replace ignition lock cylinder. Went back together and now theres no brake lights. All other lights and signals work. I tested that I had 12v to the brake switch, which I did. Tested continuity on the switch, which I had with the pedal pressed, and opened the circuit with the pedal not pressed. Checked at the block on the firewall, and the green and yellow wires each had 12v on them with the brake applied, down to zero when not applied. But then when I go back to the plug at the back of the box, I get 0v on either wire with the brake applied. I put a signal on, and I get 12v on the corresponding wire, so I know the wires aren't bad. What am I missing here?

Are you sure the wires aren't bad? Sounds like a pinched wire.

 

[papapavs1]

Signals work fine, which is the same wire as the brakes so I don't think theres a pinched wire. I also looked and didn't see anything wrong

 

[Ricko1966]

Hello, '78 K25, and I don't have brake lights anymore. I did a quite a bit of work to the truck, but a few things that may be of interest are: replaced the ignition switch, pulled the bed off for a while to paint it, and had the steering wheel pulled to replace ignition lock cylinder. Went back together and now theres no brake lights. All other lights and signals work. I tested that I had 12v to the brake switch, which I did. Tested continuity on the switch, which I had with the pedal pressed, and opened the circuit with the pedal not pressed. Checked at the block on the firewall, and the green and yellow wires each had 12v on them with the brake applied, down to zero when not applied. But then when I go back to the plug at the back of the box, I get 0v on either wire with the brake applied. I put a signal on, and I get 12v on the corresponding wire, so I know the wires aren't bad. What am I missing here?

Did you mess with the turnsignal switch,at all? It breaks contact to the brake lights when the turn signal is on.

 

[papapavs1]

I had to pull it out of place and reinstall it for the lock cylinder. I don't have any brake lights even when not signaling either. I checked the block of terminals on the firewall and I had voltage to both of the signal wires with the brake applied, and I believe that is farther down the line than the signal switch

 

[PrairieDrifter]

If I remember correctly brakes and signals are not the same wire. I may be wrong though.

 

[Jgonick]

If I remember correctly brakes and signals are not the same wire. I may be wrong though.

yep- same wire (DK GRN & YELL) from lights to turn switch- Turn signal switch feeds brake wires from different sources depending on braking (WHT), hazards (BRN), or turn signal (PPL). If signals/ hazards work back lights, I would be looking at brake switch (orange & white wires) and turn signal switch. Might not be getting enough amps -bad connection (maybe??)

Edited to make it clearer-

 

[Jgonick]

Wiring diagram of taillights