All Lights Go Out When Brakes Lights Should Be On

Posted on Feb 7, 2014

had to rewire a boat trailer due to damaged wiring and lights. I`m using a standard 4 pole flat connection to my truck. The trailer is simple - two rear lights and nbo side markers. Both rear lights are grounded to the harness to the truck. Almost everything works fine. Turn on the headlights and the trailer lights are both on. Both

All Lights Go Out When Brakes Lights Should Be On
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turn signals work. When I have my truck lights off, the brake lights work fine on the trailer. This is a very common and classic problem. There is too much resistance in the ground or return circuit. The total current of the running iamps is small enough to keep the voltage drop across the ground resistance low enough to sustain the lamp illumination. As soon as the cold filaments of the brake lights are supplied with current, it drives the voltage drop across the return portion of the circuit so high all the lamps extinguish. The running lamps are illuminated. The resistance in the ground circuit is higher than normal. Let us figure the current for the lamp is 1-Ampere. The ground resistance is 3-ohms. This mean there is a 3-volt drop across the ground resistance. The lamps are receiving only 9-volts, however they do illuminate at this voltage, although at reduced luminance. So the trailer lamps are burning. Ww step on the brake. The brake lamps are designed to be brighter than the running lamps, so they draw more current. This means their filament resistance is lower, and when the brake lamps are off, their filament resistance is very low, practically a dead short. When the brake switch closes, the cold brake lamp filaments act more like wire than a lamp, and they drive the voltage drop across the ground resistance much higher, let`s say to about 9-volts. The voltage drop across the cold lamps is only about 3-volts. This...

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