TECH - Basic battery and electrical checks
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Now that you have found the circuit that is shorting, use the wiring schematic and follow along where that color coded wire runs in the harness from the component connector to the fuse that is blowing. In this test scenario, you would follow along where the green wire runs and find where it is touching metal and repair the area. Sometimes the section of wire where it is shorting should be replaced. In this case, cut out the bad portion and install a length of replacement wire of at least the same gauge. The ideal repair would be soldering in a section with heat shrink tubing on the two ends where the new section attaches. If you have had to unwrap the harness to get to the wire, be sure to rewrap the area to further protect the wires.
Faulty electrical components can cause a direct short. In one instance, I had a stoplight switch that mounts to the brake pedal that was shorting. When I connected the battery positive cable for power, the voltage regulator started buzzing. After disconnecting and checking components, I found the stoplight switch was causing the regulator problem. I removed the switch and tested it. It actually tested okay but when I reinstalled it and plugged it back in, I had the same problem. I then replaced the switch and the problem went away. The faulty stoplight switch was shorting internally and grounding against the brake pedal but would test okay out of the car.
I have also come across some bad reproduction turn signal housings. When they were made, the signal wire for the turn signal and the wire for ground were swapped. Whenever I used the park lights I was okay but when I turned on the turn signals it would blow the fuse. I traced the problem to the housings. When I checked the housings I found out they had been wired wrong. Everything was fine after I replaced the housings with some new ones that were wired properly.
Don't assume things are plugged in correctly either. I was working on a Cougar that was blowing the fuse when you opened the door and the courtesy lights came on. I traced the problem to where someone had plugged two connectors into each other because it looked right since they were both two wire connectors. The wires color codes however didn't match and power to one component was connected to power to another component. When the courtesy lights came on, 12v met 12v with no ground and the fuse would blow.
A blown fuse is not always an indication of an electrical short. Occasionally, a fuse will blow if a component or it's connector is corroded. This happens often with dash lights. The headlight switch has a ceramic wheel that turns when you brighten or dim the dash lights. The contacts on this wheel can get green with corrosion or rust and this makes the resistance higher than normal and increases the load on the circuit. Over time, the fuse will get too hot trying to handle the load and will blow. Always make sure that all connectors are clean and dry inside before you plug them into the components. The same holds true for the holders of the fuses in the fuse panel. Over time, these can get rusty and that makes the fuses easier to blow.
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#2
The only thing I would add/mention is carefull attention to earth (ground)points. Often earth wires, connectors and terminals look OK but are defective in some way. Clean, tight and secure is the standard to aim for. Poor earths can give really weird faults.
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