I am running into an issue with my brake lights. I have replaced the turn signal switch that all the rear wiring (except lights) goes through. The lights work (lower filament) on both sides, the left and right turn signals work (high filament). When I apply the brakes no lights come on. I removed the 1157 bulbs and there is 12v coming through the dark green and yellow wires (right and left turn wires) but as soon as there is any draw (I put an external light on the contact and grounded the light) but the light does not come on and the voltage drops to 0 on both the dark green and yellow wires. As soon as I take the external light off the circuit the power comes back to 12v on both wires. If one or both 1157 bulbs are put back in the voltage immediately goes down to 0 and returns to 12v when the bulbs are taken out. I have run new ground wires to both sockets to ensure good grounding but the problem persists. I can't think of what else to try to resolve the issue, any help would be much appreciated.

 

If voltage drops to zero when there is any draw, you have a resistive connection someplace in the circuit. The problem with low impedance voltmeters is that they measure voltage at nearly zero current. Resistance in the circuit will reduce current, which is what is required to heat up the filament in the lamp.

 

Check the brake switch wires and switch function. The brake lights are separate from the turn signals but do go thru the turn signal switch. Turn signal should stay on when brakes applied.

 

Thanks for directing me in the right direction Joe. I keep thinking that since the meter says there is 12v there that it also means that you have a good supply of power, not just enough to measure. It turns out that the connectors in the fuse box were rusty enough to interfere with the flow. Cleaned them up and everything works now. Thanks.