brake light problem

[scooby3242]

I have a 1973 gmc truck. My driver side brake light does not work. I have changes bulbs and chased the wire. It appears that the passenger side uses a green wire that works and the drivers side uses a yellow wire that has no power when applying the brakes. I pulled the plug apart under the hood just before it goes into the firewall and no power coming out to the yellow wire from inside the cab. What should i check inside the cab? maybe the brake switch that has two wires?

[ray_mcavoy]

The brake light switch receives power via an orange wire from the "tail-stop-courtesy" fuse. When the brake pedal is pushed, the switch contacts close and send power to a white wire that leads to the turn signal switch. When the turn signal switch is in the center position, it connects the white wire to both the yellow and dark green wires that lead back to the left and right rear brake/turn lights.

Having the brake lights routed through the turn signal switch allows the turn signal to over-ride the brake light on one side or the other when signaling to make a turn. However, it can also prevent the brake light from working on one side like you are experiencing if the contacts inside the turn signal switch aren't making a good connection.

A quick test would be to unplug the turn signal switch connector (located along side the steering column just after it passes under the dash). With it unplugged, connect a temporary jumper wire between the white and yellow wires on the truck side of the connector. Check your brake lights again. If the turn signal switch was bad (and everything else is okay) you should now have a working brake light on the driver side. And the passenger side brake light won't be working anymore with this temporary test setup unless you also put a jumper wire between the white & dark green wires.

[scooby3242]

Thanks I’ll try that tomorrow