First of all I was completely wrong with no brake lights with light switch on. Problem is I have brake lights with light switch on. I disconnected harness at taillight harness to harness from firewall with light switch on and have 12 volts to all three wires. I then went inside cab and disconnected cab harness to firewall taillight harness. First checked 12 volts at brake switch and 12 volts at each yellow and green brake light wire and they check ok. Checked for 12 volts at each wire for turn signals and that checks ok. I then checked for 12 volts at each brake light wire at cab harness plug with light switch on. Did not get 12 volts at either wire. Thats telling me there is short in the plugs or wires in the harness from firewall to taillight harness. Does that make sense? This harness is new but could be bad if I shorted something out. Seems to be one brake wire to taillight wire is connected, as the turn signal lights work differently depending on which side is engaged.
Electrical is the worst via phone or by keyboard. That said. When a brake pedal is giving voltage to the tail lights it does not go directly to the back of the vehicle. The voltage from the brake switch (1 wire) travels to the turn signal switch where is divided into (2 wire) L & R. This is the simplest I can explain it. Meaning, you may have a signal switch issue???
If you're saying you have 1 wire that has power at the plug on the firewall bulkhead with the harness to the tail lights unplugged and you have power at all 3 wires at the tail light harness plug WITH the tail light harness plugged into the firewall bulkhead. Then you can guess you either have some wires touching inside the tail light harness, a problem with the tail light harness plug (either end) or a problem with the firewall bulkhead connector.
I found my '64 wiring diagram if this will help. It's dark as my free editing software isn't supported here.
I thought it was the turn signal switch until I checked at cab harness. Lights on have power to brown taillight wire NOT yellow and green brake light wires. Makes sense it is in the firewall to taillight harness, which is where I have power on all three wires before the taillight harness. Thank You
Checked ohm reading on firewall to tail harness. Brown (taillight) to each yellow and green (brake) No reading on one and a low reading on the other.
Lets call the harness that runs down the length of the truck (from firewall bulk head to tail light plug) the intermediate harness and the harness to the tail lights the tail light harness. If you ohm the intermediate harness separately (unhooked at both ends), you should have continuity on the brown itself only and the same with the yellow and dark green wires (should be 3 individual wires). The brown (running light) wire doesn't split until the plug to the tail light harness (on the tail light side). You have to unhook BOTH ends of the harness to check it, if you don't you're including the unhooked circuit into your test. If the intermediate harness tests ok unhooked at both ends, go down further and test the tail light harness.
First test at bulb socket filament's light switch on and had 12 volts to all contacts. I eliminated sockets and taillight harness by disconnecting taillight harness to intermediate harness and had 12 volts to all three wires on intermediate harness. That moved me to front cab to intermediate harness. The unplugged cab 3 wire tested ok. Lights on have 12 volts to brown taillight wire. NO VOLTAGE to either yellow or green brake light wires. That leads to short in intermediate harness only, as my turn signal circuit's (brake and turn) only key on checked ok. Now I have both ends of intermediate harness disconnected. I did the ohm test out of curiosity to see if brown wire (power) was connected to one wire or both wires (green &yellow) I only get a reading on one not the other. So, when I tear into harness I should find a short on brown and one of the yellow or green wires. (Make sense?) Hopefully it is wires out of connector so I can repair. One good thing is I learned allot about wiring with help from everyone here. So Thank You all for the help. Now into the harness I hope!!
You've pretty much isolated the problem to the intermediate harness. Now just find it and fix it like you're doing. Before you total open up the harness check both connectors and make sure nothing inside or outside is touching, if it's ok then open the harness. Let us know what you find and good job.
AND... how old are the sockets? I generally ****-can them at the 1st sign of troubles like you are having. Get new bulbs and a couple of spares.
If I remember correctly, the brown wire goes to both taillights and supplies power to the tailight/parklight circuit which is the dim filament. The green and yellow are the turn/ brake light circuits. Step on the brakes and test the white wire at the turn signal switch. It should have power when you do this. With the turn signal switch in the neutral position, you should have power to both the green and yellow wires with the brakes on. Test for this at the turn signal switch. With the turn signal switch in the left turn position, power goes from the white wire to only one of the wires and I can't remember which one and vice versa for right turn. If that is all good, the problem is in the harness to the back of the truck or a bad ground. The tailight/ parklight is a separate function and has nothing to do with the brake lights.
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