A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.
For intermittent problems like yours the bulb may be dim already. Watch what takes place when it gets bright. That's when the short is occurring. It could be due to the rocking of the engine when you shift between reverse and drive. It could be due to the body flexing when you drive over bumps in the road. The bulb limits current to a safe value when the short occurs, in this case about one amp. With only one amp of current split between all the brake light bulbs, when the short is not occurring, those brake lights won't be very bright. If you want to be safer when driving in traffic, use a head light bulb instead of a brake light bulb in place of the fuse. The low beam filament will limit current to five amps, and the high beam filament will allow about six amps to flow. The head light bulb won't be very bright when you press on the brake pedal, but the truck's brake lights will be nearly full brightness. When the short occurs, the head light bulb will become full brightness. The goal, if possible, is to keep that short occurring until you come to a stop and can get out and move things around to see what makes the short go away.
I made a test harness for this purpose out of a right rear tail light harness from a Viper. I could plug in one, two, or three bulbs with a total of five filaments, depending on the circuit I was working in. In one case in particular, I draped it over the rear-view mirror in a full-size van so I could see the bulbs from anywhere around the vehicle. It allowed just enough current to flow that I could barely see the brake lights glowing dimly, then, I found that when I pushed on the left rear lens just right, the dim brake lights would go out and the test bulbs got bright, indicating I was making the intermittent short occur. Turns out someone went in there to splice in a trailer harness, and they got the wire pinched under the lens and one of the screws had rubbed through the insulation. Quick fix that could have taken days to figure out.
Sunday, August 4th, 2019 AT 9:39 PM
