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My problem

The 10 amp fuse for the right headlight on my 2001 Ford Escort keeps blowing so I presume I have a short to ground somewhere between the fuse block and headlight.

What I have tried

I purchased this cheap short finder off Amazon: SG Tool Aid 25100 Short Tester

I have followed the instructions that came with the device by:

  1. tearing apart a burned out fuse and putting the blades back where the fuse was in the fuse block
  2. connecting both leads of the short finding device to the ends of the fuse blades.

The instructions say that at this point the load in the circuit should be inoperative, but my headlight operates just fine using the short finding device in place of the fuse. I tried disconnecting the headlamp, but I could not follow the wire with the analog magnetic sensor. Maybe because of the metal it is under.

What steps should I take from here to fix the over-current problem I have that has burned out a few fuses already? I want to try replacing it with a cycling circuit breaker (one that would auto-reset after current spike). Good idea?

Measurments

  • 5 amps between fuse terminals
  • 0.6 Ohms between either fuse terminal and ground

Wiring diagrams (click to view larger)

  • Left: main fuse to headlamp fuses
  • Right: headlamp fuses to headlamps

  • Does the fuse blow right away, or does it take a while? If it takes a while is it seconds or something much longer? – dlu Dec 30 '16 at 0:44
  • @dlu Last time it blew right away. – Web_Designer Dec 30 '16 at 0:54
  • Probably a high resistance (partial) short then, or a way to big bulb. If you can follow the wire harness to the lamp the short finder may be able to help you. Otherwise look for anyplace where the wires appear to be damaged or where they might be rubbing against metal. – dlu Dec 30 '16 at 1:02
  • @dlu I just measured 5 amps between the fuse terminals, so I tried another fuse which lasted at least a minute, until I tried to back up the car. – Web_Designer Jan 29 '17 at 0:34
  • Chances are the wire going to the bulb, goes through a hole in the bodywork somewhere. At those places, a grommet is used. If the grommet has broken and fell off, the wire is hanging over the edge of the hole in the body. Because of vibrations, that relatively sharp edge of the body hole will cut the wire's isolation, shorting it to the ground. Because of the bad contact, it's only a partially short, the ones that you discover not so quickly. – Bart Jan 29 '17 at 14:18
2

Fuses blow because they get too hot and they melt. So, either you have:

  • A short – which you don't because the light works, although you might have a high resistance short, check that by disconnecting the bulb and measuring between the fuse terminal and ground, the resistance should be infinite. Another sign of a high resistance short is that the bulb may not be as bright is the one on the other side.
  • Too large a bulb in the light, you can check this by hooking up your meter where the short finder was, you should see about 5 Amps. If the draw is up around 10 Amps, then that would be the cause, check the bulb it may be the wrong wattage or have internal damage that behaves like a high resistance short.
  • Fuse terminals that are making poor contact, that could cause the fuse to heat up and blow – if the fuse takes a long time to blow (more than a few seconds) and the current drawn by the bulb is reasonable, then overheating is the most likely cause. If the fuse block terminals are making poor contact you may see evidence of melting on the fuse or around it.
  • An environment around the fuse that is be too hot and that combined with the heating of the fuse due to the current drawn by the headlight may be enough to blow the fuse.
  • This is my slow reply, thanks for the help though. I made these observations with the lamp still connected: The resistance between either fuse terminal and ground is only 0.6 Ohms. The current through the fuse location was around only 5 amps. – Web_Designer Jan 29 '17 at 0:13
1

There are two kinds of shorts, dead shorts and partial shorts. With a dead short the load device will stop working with the tool connected because all the current shunts to ground.

With a partial short only some of the current is shunted. This will cause an over current but with the tool connected, which controls the current, the load will work.

Disconnect the head light. Connect the tool as directed and continue diagnosing as directed.

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